although lipid analyst tend to have a firm understanding of what is meant by the term

8/3/2019 Although Lipid Analyst Tend to Have a Firm Understanding of What is Meant by the Term

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Although lipid analyst tend to have a firm understanding of what is meant by the term

"lipid", there is no widely-accepted definition. General text books usually describe lipids

in woolly terms as a group of naturally occurring compounds, which have in common aready solubility in such organic solvents as hydrocarbons, chloroform, benzene, ethers

and alcohols. They include a diverse range of compounds, like fatty acids and their

derivatives, carotenoids, terpenes, steroids and bile acids. It should be apparent that manyof these compounds have little by way of structure or function to relate them. In fact, a

definition of this kind is positively misleading, since many of the substances that are now

widely regarded as lipids may be almost as soluble in water as in organic solvents.

While the international bodies that usually decide such matters have shirked the

task, a more specific definition of lipids than one based simply on solubility is necessary,and most scientist active in this field would happily restrict the use of "lipid" to fatty

acids and their naturally-occurring derivatives (esters or amides). The definition could be

stretched to include compounds related closely to fatty acid derivatives throughbiosynthetic pathways (e.g. aliphatic ethers or alcohols) or by their biochemical or

functional properties (e.g. cholesterol) . My definition is -

Lipids are fatty acids and their derivatives, and substances related biosynthetically or

functionally to these compounds.

This treats cholesterol (and plant sterols) as a lipid, and could be interpreted to includebile acids, tocopherols and certain other compounds. It also enables classification of such

compounds as gangliosides as lipids, although they are more soluble in water than in

organic solvents. However, it does not include such natural substances as steroidalhormones, petroleum products, some fat-soluble vitamins, most polyketides, carotenoids

or simple terpenes, except in rare circumstances.

If "lipids" are defined in this way, fatty acids must be defined also. They are compounds

synthesised in nature via condensation of malonyl coenzyme A units by a fatty acidsynthase complex. They usually contain even numbers of carbon atoms in straight chains

(commonly C14 to C24), and may be saturated or unsaturated; they can also contain othersubstituent groups.

Fahy et al. (J. Lipid Res., 46, 839-862 (2005)) have developed a classification system forlipids that holds promise (see our page onNomenclature). While their definition of a lipid

is too broad for my taste, it is based on sound scientific principles (although these may

not mean much to non-biochemists), i.e.
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Lipids are hydrophobic or amphipathic small molecules that may originate entirely or in

part by carbanion-based condensations of thioesters (fatty acids, polyketides, etc.) and/or

by carbocation-based condensations of isoprene units (prenols, sterols, etc.).

The most common lipid classes in nature consist of fatty acids linked by an ester bond to

the trihydric alcohol - glycerol, or to other alcohols such as cholesterol, or by amidebonds to sphingoid bases, or on occasion to other amines. In addition, they may contain

alkyl moieties other than fatty acids, phosphoric acid, organic bases, carbohydrates andmany more components, which can be released by various hydrolytic procedures.

A further subdivision into two broad classes is convenient for chromatography purposes

especially. Simple lipids are defined as those that on hydrolysis yield at most two types of

primary product per mole; complex lipids yield three or more primary hydrolysisproducts per mole. Alternatively, the terms "neutral" and "polar" lipids respectively are

used to define these groups, but are less exact.

The complex lipids for many purposes are best considered in terms of either theglycerophospholipids (or simply if less accurately as phospholipids), which contain a

polar phosphorus moiety and a glycerol backbone, or the glycolipids (both

glycoglycerolipids and glycosphingolipids), which contain a polar carbohydrate moiety,

as these are more easily analysed separately. The picture is further complicated by theexistence of phosphoglycolipids and sphingophospholipids (e.g. sphingomyelin). See also

our page on Lipid Nomenclature.

Simple Lipids

Triacylglycerols: Nearly all the commercially important fats and oils of animal and plant

origin consist almost exclusively of the simple lipid class triacylglycerols (termed"triglycerides" in the older literature). They consist of a glycerol moiety with each

hydroxyl group esterified to a fatty acid. In nature, they are synthesised by enzyme

systems, which determine that a centre of asymmetry is created about carbon-2 of theglycerol backbone, so they exist in enantiomeric forms, i.e. with different fatty acids in

each position.
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A stereospecific numbering system has been recommended to describe these forms. In a

Fischer projection of a natural L-glycerol derivative, the secondary hydroxyl group is

shown to the left of C-2; the carbon atom above this then becomes C-1 and that below isC-3. The prefix "sn" is placed before the stem name of the compound, when the

stereochemistry is defined. Their primary biological function is to serve as a store of

energy. As an example, the single molecular species 1,2-dihexadecanoyl-3-(9Z-octadecenoyl)-sn-glycerol is illustrated. (More...).

Diacylglycerols (less accurately termed "diglycerides") and

monoacylglycerols (monoglycerides) contain two moles and one mole of fatty acids per

mole of glycerol, respectively, and exist in various isomeric forms. They are sometimestermed collectively "partial glycerides". Although they are rarely present at greater than

trace levels in fresh animal and plant tissues, 1,2-diacyl-sn-glycerols are keyintermediates in the biosynthesis of triacylglycerols and other lipids, and they are vitalcellular messengers, generated on hydrolysis of phosphatidylinositol and related lipids by

a specific phospholipase C. Synthetic materials have importance in commerce. (More..).

2-Monoacyl-sn-glycerols are formed as intermediates or end-products

of the enzymatic hydrolysis of triacylglycerols; these and other positional isomers are

powerful surfactants. 2-Arachidonoylglycerol has important biological properties (as anendocannabinoid).

Acyl migration occurs rapidly in partial glycerides at room temperature, but especially on

heating, in alcoholic solvents or in the presence of acid or base, so special procedures arerequired for their isolation or analysis if the stereochemistry is to be retained. Synthetic

1/3-monoacylglycerols are important in commerce as surfactants. (More..).

Sterols and sterol esters: Cholesterolis by far themost common member of a group of steroids in animal tissues; it has a tetracyclic ring

system with a double bond in one of the rings and one free hydroxyl group. It is found

both in the free state, where it has an essential role in maintaining membrane fluidity, andin esterified form, i.e. as cholesterol esters. Other sterols are present in free and esterified
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form in animal tissues, but at trace levels only. Cholesterol is the precursor of thebile

acids and steroidal hormones.

In plants, cholesterol is rarely present in other than small amounts, but such phytosterolsas sitosterol, stigmasterol, avenasterol, campesterol and brassicasterol, and their fatty acid

esters are usually found, and they perform a similar function. (More..). Hopanoids arerelated lipids produced by some bacterial species.

Waxes: In their most common form, wax esters consist of fatty acids esterified to long-chain alcohols with similar chain-lengths. The latter tend to be saturated or have one

double bond only. Such compounds are found in animal, plant and microbial tissues and

they have a variety of functions, such as acting as energy stores, waterproofing and

lubrication.

In some tissues, such as skin, avian preen glands or plant leaf surfaces, the wax

components can be much more complicated in their structures and compositions. Theycan contain aliphatic diols, free alcohols, hydrocarbons (e.g. squalene), aldehydes and

ketones. (More..).

Tocopherols (collectively termed

vitamin E) are substituted benzopyranols (methyl tocols) that occur in vegetable oils.

Different forms (-, -, - and -) are recognized according to the number or position ofmethyl groups on the aromatic ring. -Tocopherol (with the greatest Vitamin E activity)

illustrated is an important natural antioxidant. Tocotrienols have similar ring structures

but with three double bonds in the aliphatic chain. (More...).

Free (unesterified) fatty acids are minor constituents of living tissues but are of biologicalimportance as precursors of lipids as an energy source and as cellular messengers.

(More...).

Glycerophospholipids

Phosphatidic acid or 1,2-diacyl-sn-glycerol-3-phosphate is found in trace amounts only in

tissues under normal circumstances, but it has great metabolic importance as a
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biosynthetic precursor of all other glycerolipids. It is strongly acidic and is usually

isolated as a mixed salt. One specific isomer is illustrated as an example.

Lysophosphatidic acid with one mole of fatty acid per mole of lipid (in positionsn-1) is amarker for ovarian cancer, and is a key cellular messenger. (More...).

Phosphatidylglycerol or 1,2-diacyl-sn-glycerol-3-

phosphoryl-1'-sn-glycerol tends to be a trace constituent of most tissues, but it is often themain component of some bacterial membranes. It has important functions in lung

surfactant, where its physical properties are significant, and in plant chloroplasts, where itappears to have an essential role in photosynthesis. Also, it is the biosynthetic precursorof cardiolipin. In some bacterial species, the 3'-hydroxyl of the phosphatidylglycerol

moiety is linked to an amino acid (lysine, ornithine or alanine) to form an O-

aminoacylphosphatidylglycerol or complex 'lipoamino acid'. (More..).

Cardiolipin (diphosphatidylglycerol or more precisely 1,3-bis(sn-3'-phosphatidyl)-sn-glycerol) is a unique phospholipid with in essence a dimeric structure, having four acyl

groups and potentially carrying two negative charges (and is thus an acidic lipid). It is an

important constituent of mitochondrial lipids especially, so heart muscle is a rich source.

Amongst other functions, it plays a key role in modifying the activities of the enzymes

concerned with oxidative phosphorylation. (More...).
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Lysobisphosphatidic acid or

bis(monoacylglycerol)phosphate is an interesting lipid as its stereochemical configuration

differs from that of all other animal glycero-phospholipids in that the phosphodiester

moiety is linked to positionssn-1 andsn-1' of glycerol, rather than to positionsn-3 towhich the fatty acids are esterified (some experts think that positionsn-2 is more likely

for the latter). It is usually a rather minor component of animal tissues, but is enriched inthe lysosomal membranes of liver and appears to be a marker for this organelle. The

glycerophosphate backbone is particularly stable, presumably because of the unusual

stereochemistry. (More..).

Phosphatidylcholine or 1,2-diacyl-sn-glycerol-

3-phosphorylcholine (or "lecithin", although the term is now used more often for themixed phospholipid by-products of seed oil refining) is usually the most abundant lipid in

the membranes of animal tissues, and it is often a major lipid component of plant

membranes, but only rarely of bacteria. With the other choline-containing phospholipid,sphingomyelin, it is a key structural component and constitutes much of the lipid in the

external monolayer of the plasma membrane of animal cells especially. (More...).

Lysophosphatidylcholine, which contains only one

fatty acid moiety in each molecule, generally inpositionsn-1, is sometimes present as a minor

component of tissues. It is a powerful surfactant

and is more soluble in water than most otherlipids.
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Phosphatidylethanolamine (once given the trivial

name "cephalin") is usually the second most abundant phospholipid class in animal and

plant tissues, and can be the major lipid class in microorganisms. As part of an important

cellular process, the amine group can be methylated enzymically to yield firstphosphatidyl-N-monomethylethanolamine and then phosphatidyl-N,N-

dimethylethanolamine, but these never accumulate in significant amounts; the eventual

product is phosphatidylcholine.

N-Acylphosphatidylethanolamine is a minor component of some plant tissues, especiallycereals, and it is occasionally found in animal tissues, where it is the precursor of some

biologically active amides. Lysophosphatidylethanolamine contains only one mole of

fatty acid per mole of lipid. (More...).

Phosphatidylserine is a weakly acidic lipid that is

present in most tissues of animals and plants and is also found in microorganisms. It islocated entirely on the inner monolayer surface of the plasma membrane and other

cellular membranes. Phosphatidylserine is an essential cofactor for the activation of

protein kinase C, and it is involved in many other biological processes, including bloodcoagulation and apoptosis (programmed cell death). (More...).

N-Acylphosphatidylserine has been detected in some animal tissues.

Phosphatidylinositol, containing the opticallyinactive form of inositol, myo-inositol, is a common constituent of animal, plant andmicrobial lipids. In animal tissues especially, it may be accompanied by small amounts of

phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate (and other

'poly-phosphoinositides'). These compounds have a rapid rate of metabolism in animalcells, and are converted to metabolites such as diacylglycerols and inositol phosphates,

which are important in regulating vital processes. For example, diacylglycerols regulate

the activity of a group of enzymes known as protein kinase C, which in turn control many
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key cellular functions, including differentiation, proliferation, metabolism and apoptosis

(see our web pages onphosphatidylinositol). In addition, phosphatidylinositol is the

primary source of the arachidonate used for eicosanoid synthesis in animals, and it isknown to be the anchor that can link a variety of proteins to the external leaflet of the

plasma membrane via a glycosyl bridge (glycosyl-phosphatidylinositol(GPI)-anchored

proteins) .

Phosphonolipids are lipids with a phosphonic acid

moiety esterified to glycerol, i.e. with a carbon-

phosphorus bond that is not easily hydrolysed by

chemical reagents. Phosphonylethanolamine, forexample, is found mainly in marine invertebrates and

in protozoa. A ceramide analogue is often found in

the same organisms (see below). (More...).

Ether lipids: Many glycerolipids, but mainlyphospholipids, and those of animal and microbial origin especially, contain aliphatic

residues linked either by an ether bond or a vinyl ether bond to position 1 ofL-glycerol.When a lipid contains a vinyl ether bond, the generic term "plasmalogen" is often used.They can be abundant in the phospholipids of animals and microorganisms, and

especially in the phosphatidylethanolamine fraction. In this instance, it has been

recommended that they should be termed "plasmanylethanolamine" and"plasmenylethanolamine", respectively.

On hydrolysis of glycerolipids containing an alkyl ether bond, 1-alkylglycerols are

released that can be isolated for analysis. Similarly, when plasmalogens are hydrolysed

under basic conditions, 1-alkenylglycerols are released. Aldehydes are formed on acidichydrolysis. With both groups of compound, the aliphatic residues generally have a chain-

length of 16 or 18, and they are saturated or may contain one additional double bond, thatis remote from the ether linkage (More...).
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'Platelet-activating factor' or 1-alkyl-2-acetyl-sn-glycerophosphorylcholine is an ether-containing phospholipid, which has been studied

intensively because it can exert profound biological effects at minute concentrations. For

example, it effects aggregation of platelets at concentrations as low as 10-11 M, and itinduces a hypertensive response at very low levels. Also, it is a mediator of inflammation

and has messenger functions. (More...).

1-Alkyl-2,3-diacyl-sn-glycerols, analogues of triacylglycerols, tend to be present in trace

amounts only in animal tissues, but can be major constituents of certain fish oils. Relatedcompounds containing a 1-alk-1'-enyl moiety ('neutral plasmalogens') are occasionally

present also. (More...).

Glycoglycerolipids

In plants, especially the photosynthetic tissues, a substantial proportion of the lipids

consists of 1,2-diacyl-sn-glycerols joined by a glycosidic linkage at positionsn-3 to acarbohydrate moiety. The main components are the mono- and

digalactosyldiacylglycerols, but related compounds have been found with up to four

galactose units, or in which one or more of these is replaced by glucose moieties. It isclear that these have an important role in photosynthesis, but many of the details have

still to be worked out.

In addition, a 6-O-acyl-monogalactosyldiacylglycerol is occasionally a component of

plant tissues. See our web pages dealing withplant galactolipids.
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A related unique plant glycolipid is

sulfoquinovosyldiacylglycerol or the "plant sulfolipid". It contains a sulfonic acid residue

linked by a carbon-sulfur bond to the 6-deoxyglucose moiety of amonoglycosyldiacylglycerol and is found exclusively in the chloroplasts.

Monogalactosyldiacylglycerols are not solely plant lipids as they have been found in

small amounts in brain and nervous tissue in some animal species. A range of complex

glyceroglycolipids have also been characterized from intestinal tract and lung tissue.They exist in both diacyl and alkyl acyl forms. Such compounds are destroyed by some

of the methods used in the isolation of glycosphingolipids, so they may be more

widespread than has been thought.

A complex glyco-glycero-sulfolipid, termed seminolipid, of which the main component is

1-O-hexadecyl-2-O-hexadecanoyl-3-O-(3'-sulfo--D-galactopyranosyl)-sn-glycerol, is the

principal glycolipid in testis and sperm. See our web pages onanimal

glycosyldiacylglycerols.

A further range of highly complex glycolipids occur in bacteria and other micro-

organisms, often with mannose as a carbohydrate moiety. These include acylated sugars

that do not contain glycerol.

Sphingomyelin and Glycosphingolipids

Sphingolipids consist of long-chain bases, linked by an amide bond to a fatty acid and via

the terminal hydroxyl group to complex carbohydrate or phosphorus-containing moieties.

Long-chain bases (sphingoids or sphingoid

bases) are the characteristic structural unit of sphingolipids. They are long-chain (12 to 22

carbon atoms) aliphatic amines, containing two or three hydroxyl groups, and often adistinctive trans-double bond in position 4. The commonest or most abundant of these in

animal tissues is sphingosine, ((2S,3R,4E)-2-amino-4-octadecen-1,3-diol) (illustrated).

More than a hundred long-chain bases have been found in animals, plants andmicroorganisms, and many of these may occur in a single tissue, but almost always as

part of a complex lipid as opposed to in the free form. The aliphatic chains can be

saturated, monounsaturated and diunsaturated, with double bonds of either the cis ortrans configuration, and they may sometimes have methyl substituents.
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In addition, saturated and monoenoic straight- and

branched-chain trihydroxy bases are found. For example, phytosphingosine ((2S,3S,4R)-2-amino-octadecanetriol) is the most common long-chain base of plant origin.

For shorthand purposes, a nomenclature similar to that for fatty acids can be used, i.e. the

chain-length and number of double bonds are denoted in the same manner with the prefix

"d" or "t" to designate di- and trihydroxy bases respectively. Thus, sphingosine is d18:1and phytosphingosine is t18:0. (More...).

Ceramides contain fatty acids linked by an amide bond to the amine

group of a long-chain base. In general, they are present at low levels only in tissues, but

they are key intermediates in the biosynthesis of the complex sphingolipids. In addition,

they have important functions in cellular signalling, and especially in the regulation ofapoptosis, and cell differentiation, transformation and proliferation.

Unusual ceramides have been located in the epidermis of the pig and humans; the fatty

acids linked to the sphingoid base consist of C30 and C32 (-hydroxylated components,with predominantly the essential fatty acid, linoleic acid, esterified to the terminal

hydroxyl group. They are believed to have a special role in preventing the loss of

moisture through the skin. (More...).

Sphingomyelin is a sphingophospholipid and consists of a ceramide unit linked atposition 1 to phosphorylcholine; it is found as a major component of the complex lipids

of all animal tissues but not of plants or micro-organisms.

It resembles phosphatidylcholine in many of its physical properties, and can apparently

substitute in part for this in membranes although it also has its own unique role. For

example, it is a major constituent of the plasma membrane of cells, where it isconcentrated together with sphingoglycolipids and cholesterol in tightly organized sub-

domains termed 'rafts'. Sphingosine tends to be the most abundant long-chain base

constituent, and it is usually accompanied by sphinganine and C20 homologues.
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Sphingomyelin is a precursor for a number of sphingolipid metabolites that have

important functions in cellular signalling, including sphingosine-1-phosphate (see below),

as part of the 'sphingomyelin cycle'. A correct balance between the various metabolites isvital for good health. Niemann-Pick disease is a rare lipid storage disorder that results

from of a deficiency in the enzyme responsible for the degradation of sphingomyelin.

(More...).

Ceramide phosphorylethanolamine is found in

the lipids of insects and some fresh water invertebrates; the phosphonolipid analogue,

ceramide 2-aminoethylphosphonic acid, has been detected in sea anemones and protozoa.Ceramide phosphorylinositol is also found in some organisms, and like

phosphatidylinositol, it can be an anchor unit for oligosaccharide-linked proteins in

membranes. (More...).

Neutral glycosylceramides: The most widespread glycosphingolipids are themonoglycosylceramides (or cerebrosides), and they consist of a basic ceramide unit

linked by a glycosidic bond at carbon 1 of the long-chain base to glucose or galactose.

They were first found in brain lipids, where the principal form is galactosylceramide, but

they are now known to be ubiquitous constituents of animal tissues. Glucosylceramide isalso found in animal tissues, and especially in skin, where it functions as part of the water

permeability barrier. It is the biosynthetic precursor of lactosylceramide, and thence of

the complex oligoglycolipids and gangliosides. In addition, glucosylceramide is found inplants, where the main long-chain base is phytosphingosine.

O-Acyl-glycosylceramides have been detected in small amounts in some tissues, as have

cerebrosides with monosaccharides such as xylose, mannose and fucose. (More...).

Di-, tri- and tetraglycosylceramides (oligoglycosylceramides) are present in most animaltissues at low levels. The most common diglycosyl form is lactosylceramide, and it canbe accompanied by related compounds containing further galactose or galactosamine

residues. Tri- and tetraglycosylceramides with a terminal galactosamine residue are

sometimes termed "globosides", while glycolipids containing fucose are known as"fucolipids". Lactosylceramide is the biosynthetic precursor of most of these with further

monosaccharide residues being added to the end of the carbohydrate chain (up to as many

as twenty). They are an important element of the immune response system. For example
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some glycolipids are involved in the antigenicity of blood group determinants, while

others bind to specific toxins or bacteria. As the complex glycosyl moiety is considered

to be of primary importance in this respect, it has received most attention frominvestigators. However, certain of these lipids have been found on occasion to have

distinctive long-chain base and fatty acid compositions, which enhance their biological

activity. Some glycolipids accumulate in persons suffering from rare disease syndromes,characterized by deficiencies in specific enzyme systems related to glycolipid

metabolism.

Sulfate esters of galactosylceramide and lactosylceramide (sulfoglycosphingolipids -

often referred to as "sulfatides" or "lipid sulfates"), with the sulfate group linked toposition 3 of the galactosyl moiety, are major components of brain lipids and they are

found in trace amounts in other tissues. (More...).

Complex plant sphingolipids, phytoglycosphingolipids, containing glucosamine,

glucuronic acid and mannose linked to the ceramide via phosphorylinositol, were isolated

and characterized from seeds initially, but related compounds are also known to bepresent in other plant tissues and in fungi.

Gangliosides are highly complex oligoglycosylceramides, which contain one or more

sialic acid groups (N-acyl, especially acetyl, derivatives of neuraminic acid, abbreviatedto "NANA") in addition to glucose, galactose and galactosamine.

The polar and ionic nature of these lipids renders them soluble in water (contrary to some

definitions of a lipid). They were first found in the ganglion cells of the central nervoussystem, hence the name, but are now known to be present in most animal tissues. The

long-chain base and fatty acid components of gangliosides can vary markedly between

tissues and species, and they are presumably related in some way to function.

Gangliosides have been shown to control growth and differentiation of cells, and theyhave important roles in the immune defence systems. They act as receptors for a number

of tissue metabolites and in this way may regulate cell signalling. Also, they bind

specifically to various bacterial toxins, such as those from botulinum, tetanus and

cholera. A number of unpleasant lipidoses have been identified involving storage ofexcessive amounts of gangliosides in tissues, the most important of which is Tay-Sachs

disease. (More...).

Sphingosine-1-phosphate is one of the simplest sphingolipids structurally. It is present atlow levels only in animal tissues, but it is a pivotal lipid in many cellular signalling

pathways (together with ceramide and ceramide-1-phosphate). For example, within cells,
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sphingosine-1-phosphate promotes cellular division (mitosis), while in the blood it may

play a critical role in platelet aggregation and thrombosis. (More...).

The fatty acids of sphingolipids: Although structures of fatty acids are discussed in

greater depth below, it is worth noting that the acyl groups of ceramides are very differentfrom those in the glycerolipids. They tend to consist of long-chain (C16 up to C26 but

occasionally longer) odd- and even-numbered saturated or monoenoic fatty acids and

related 2-D-hydroxy fatty acids, both in plant and animal tissues. Linoleic acid may bepresent at low levels in sphingolipids from animal tissues, but polyunsaturated

compounds are rarely found (although their presence is often reported in error).

Fatty Acids

The common fatty acids of plant tissues are C16 and C18 straight-chain compounds with

zero to three double bonds of a cis (orZ) configuration. Such fatty acids are also

abundant in animal tissues, together with other even numbered components with asomewhat wider range of chain-lengths and up to six cis double bonds separated by

methylene groups (methylene-interrupted). The systematic and trivial names of those

fatty acids encountered most often, together with their shorthand designations, are listedin the table.

The common fatty acids of animal and plant origin

Systematic name Trivial name Shorthand

Saturated fatty acids

ethanoic acetic 2:0

butanoic butyric 4:0

hexanoic caproic 6:0

octanoic caprylic 8:0

decanoic capric 10:0

dodecanoic lauric 12:0

tetradecanoic myristic 14:0

hexadecanoic palmitic 16:0

octadecanoic stearic 18:0

eicosanoic arachidic 20:0

docosanoic behenic 22:0

Monoenoic fatty acids

cis-9-hexadecenoic palmitoleic 16:1(n-7)
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cis-6-octadecenoic petroselinic 18:1(n-12)

cis-9-octadecenoic oleic 18:1(n-9)

cis-11-octadecenoic cis-vaccenic 18:1(n-7)

cis-13-docosenoic erucic 22:1(n-9)

cis-15-tetracosenoic nervonic 24:1(n-9)

Polyunsaturated fatty acids*

9,12-octadecadienoic linoleic 18:2(n-6)

6,9,12-octadecatrienoic -linolenic 18:3(n-6)

9,12,15-octadecatrienoic -linolenic 18:3(n-3)

5,8,11,14-eicosatetraenoic arachidonic 20:4(n-6)

5,8,11,14,17-eicosapentaenoic EPA 20:5(n-3)

4,7,10,13,16,19-docosahexaenoic DHA 22:6(n-3)

* all the double bonds are of the cis configuration

The most abundant saturated fatty acid in nature is hexadecanoic orpalmitic acid. It can also be designated a "16:0" fatty acid, the first numerals denoting the

number of carbon atoms in the aliphatic chain and the second, after the colon, denoting

the number of double bonds. All the even-numbered saturated fatty acids from C2 to C30have been found in nature, but only the C14 to C18 homologues are likely to be

encountered in appreciable concentrations in glycerolipids, other than in a restricted

range of commercial fats and oils.

Oleic orcis-9-octadecenoic acid, the most abundant monoenoicfatty acid in nature, is

designated as "18:1", or more precisely as 9c-18:1 or as 18:1(n-9) (to indicate that the lastdouble bond is 9 carbon atoms from the terminal methyl group).

The latter form of the nomenclature is of special value to biochemists. Similarly, the mostabundant cis monoenoic acids fall into the same range of chain-lengths, i.e. 16:1(n-7) and

18:1(n-9), though 20:1 and 22:1 are abundant in fish. Fatty acids with double bonds of

the trans (orE) configuration are found occasionally in natural lipids, or are formed

during food processing (hydrogenation) and so enter the food chain, but they tend to beminor components only of animal tissue lipids, other than of ruminants, where they are

formed naturally by biohydrogenation. Their suitability for human nutrition is currently acontroversial subject.

The C18polyunsaturatedfatty acids, linoleic orcis-9,cis-12-octadecadienoic acid (18:2(n-

6)) and -linolenic orcis-9,cis-12,cis-15-octadecatrienoic acid (18:3(n-3)), are major
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components of most plant lipids, including many of the commercially important

vegetable oils.

They are essential fatty acids in that they cannot be synthesised in animal tissues. On theother hand, as linoleic acid is almost always present in foods, it tends to be relatively

abundant in animal tissues. In turn, these fatty acids are the biosynthetic precursors in

animal systems of C20 and C22 polyunsaturated fatty acids, with three to six double bonds,via sequential desaturation and chain-elongation steps (desaturases in animal tissues can

only insert a double bond on the carboxyl side of an existing double bond). Those fatty

acids derived from linoleic acid, especially arachidonic acid (20:4(n-6)), are important

constituents of the membrane phospholipids in mammalian tissues, and are also theprecursors of the prostaglandins and other eicosanoids. In fish, linolenic acid is the more

important essential fatty acid, and polyunsaturated fatty acids of the (n-3) series,

especially eicosapentaenoic acid (20:5(n-3) or EPA) and docosahexaenoic acid (22:6(n-3)or DHA), are found in greater abundance.

Many other fatty acids that are important for nutrition and health do of course exist in

nature, and at present there is great interest in -linolenic acid (18:3(n-6)), available fromevening primrose oil -

- and in conjugated linoleic acid (mainly 9-cis,11-trans-octadecadienoate) or 'CLA', a

natural constituent of dairy products, that is claimed to have remarkable health-givingproperties.


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Branched-chain fatty acids are synthesised by many microorganisms (most often with aniso- or an anteiso-methyl branch) and they are synthesised to a limited extent in higher

organisms. They enter animal tissues via the diet, especially with ruminants.

Phytanic acid, 3,7,11,15-tetramethylhexadecanoic acid, is a metabolite of phytol and is

found in animal tissues, but generally at low levels only.

Fatty acids with many other substituent groups are found in certain plants and

microorganisms, and they may be encountered in animal tissues, which they enter via thefood chain. These substituents include acetylenic and conjugated double bonds, allenic

groups, cyclopropane, cyclopropene, cyclopentene and furan rings, and hydroxy-, epoxy-and keto-groups. For example, 2-hydroxy fatty acids are synthesised in animal and plant

tissues, and are often major constituents of the sphingolipids. 12-Hydroxy-octadec-9-

enoic or 'ricinoleic' acid is the main constituent of castor oil.

Eicosanoids and Related Lipids

The term eicosanoid is used to embrace biologically active lipid mediators (C20 fatty acids

and their metabolites), including prostaglandins, thromboxanes, leukotrienes and otheroxygenated derivatives, which exert their effects at very low concentrations. They are

produced primarily by three classes of enzymes, cyclooxygenases (COX-1 and COX-2),

lipoxygenases (LOX) and cytochrome P450 epoxygenase. The key precursor fatty acidsare 8c,11c,14c-eicosatrienoic (dihomo--linolenic or 20:3(n-6)), 5c,8c,11c,14c-

eicosatetraenoic (arachidonic or 20:4(n-6)) and 5c,8c,11c,14c,17c-eicosapentaenoic(20:5(n-3) or EPA) acids (see our web page on 'polyunsaturated fatty acids'). Morerecently docosanoids (resolvins and protectins) derived from 4c,7c,10c,13c,16c,19c-

docosahexaenoic acid (22:6(n-3) or DHA) have been described. Other eicosanoids are

produced by non-enzymic means (isoprostanes).
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Those derived from arachidonic acid appear to be of special importance and have been

most studied. The prostaglandins and thromboxanes have cyclic structures, generated by

cyclo-oxygenase enzymes, and are involved in the processes of inflammation. Thehydroxy-eicosatetraenoic acids are generated by lipoxygenases, and of these the 5-

lipoxygenase is especially important as it produces the first intermediate in the

biosynthesis of leukotrienes. The resolvins and protectins have anti-inflammatoryproperties.

Plant products, such as thejasmonatesand other oxylipins

derived from 9c,12c,15c-octadecatrienoic (-linolenic or 18:3(n-3)) acid are also

generated by the action of lipoxygenases. They are involved in responses to physicaldamage by animals or insects, stress and attack by pathogens. There are obvious

structural similarities between the jasmonates and prostanoids. Ourintroductory page on

eicosanoids will lead you to further information.

Of course, many more lipids occur in nature than can be described in this document. Ihave not touched on proteolipids, lipoproteins and lipopolysaccharides here, for example,

but there is information on these and other lipids elsewhere on this website. New lipids

continue to be found, and no doubt many remain to be discovered. Other than the LipidLibrary, the book "Les Lipides dans le Monde Vivant. Introduction la Lipidomique" by

Claude Leray (Lavoisier, France, 2010) and the websitehttp://www.cyberlipid.org are the

best sources of information on lipid structures.
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WHAT LIPIDS DO

Their Biological Functions

For many years, lipids were considered to be intractable and uninteresting oily materials

with two main functions to serve as a source of energy and as the building blocks of

membranes. They were certainly not considered to be appropriate candidates for suchimportant molecular tasks as intracellular signalling or local hormonal regulation. In

1929, George and Mildred Burr demonstrated that linoleic acid was an essential dietary

constituent, but it was many years before the importance of this finding was recognized

by biochemists in general. With the discovery by Bergstrm, Samuelsson and others in1964 that the essential fatty acid arachidonate was the biosynthetic precursor of the

prostaglandins with their effects on inflammation and other disease states, the scientific

world in general began to realize that lipids were much more interesting than they hadpreviously thought.

A major milestone was achieved in 1979 with the discovery of thefirst biologically active phospholipid, platelet-activating factor. At about the same time,

there arose an awareness of the distinctive functions of phosphatidylinositol and itsmetabolites. Since then, virtually every individual lipid class has been found to have

some unique biological role that is distinct from its function as a source of energy or as a

simple construction unit of a membrane. Indeed it is now recognised that lipids in

membranes function also in the trafficking of cellular constituents, the regulation of theactivities of membrane proteins and signalling.

All multi-cellular organisms, use chemical messengers to send information between

organelles and to other cells and as relatively small hydrophobic molecules, lipids are

excellent candidates for signalling purposes. The fatty acid constituents have well-defined structural features, such as cis-double bonds in particular positions, which can

carry information by binding selectively to specific receptors. In esterified form, they can

infiltrate membranes or be translocated across them to carry signals to other cells. Duringtransport, they are usually bound to proteins so their effective solution concentrations are

very low, and they are can be considered to be inactive until they reach the site of action

and encounter the appropriate receptor.


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Storage lipids, such as triacylglycerols, in their cellular context are inert, and indeed

esterification with fatty acids may be a method of de-activating steroidal hormones, for

example, until they are actually required. In contrast, polar phospholipids have bothhydrophobic and hydrophilic sites that can bind via various mechanisms to membrane

proteins and influence their activities. Glycosphingolipids carry complex carbohydrate

moieties that have a part to play in the immune system, for example. Lipids have beenimplicated in a number of human disease states, including cancer and cardiovascular

disease, sometimes in a detrimental and sometimes in a beneficial manner. In short, every

scientist should now be aware that lipids are just as fascinating as all the other groups oforganic compound that make up living systems.

In this web document, the main biological functions of some key lipids are briefly

summarized to give a general overview, but much more information is available here on

those pages dealing with specific lipid classes.

Fatty Acids

Fatty acids are one of the defining constituents of lipids and are in large part responsiblefor the distinctive physical and metabolic properties of the latter. However, they are also

important in non-esterified form, i.e. asfree (unesterified) fatty acids. They are released

from triacylglycerols during fasting to provide a source of energy and of structuralcomponents for cells (see below), where they are of course of vital importance. However,

it has become evident that there are a number of more dynamic functions of fatty acids,

which are attracting great interest. It has long been known that linoleic and linolenic acidsare essential fatty acids, in that they cannot be synthesised by animals and must come

from plants via the diet. They are precursors of arachidonic, eicosapentaenoic anddocosahexaenoic acids, which are vital components of all membrane lipids. However, we

cannot even now claim to fully understand the reasons for the unique requirements forthese fatty acids.

Dietary fatty acids of short and medium chain-length are not usually esterified but are

oxidized rapidly in tissues as a source of 'fuel' to support all the events necessary to keep

organisms functioning. Longer-chain fatty acids are usually esterified first totriacylglycerols or structural lipids in tissues. Although all lipids are in a state of dynamic

flux, membrane lipids are conserved in content and composition in essence, except under

conditions of extreme stress. Triacylglycerols are the primary storage form of long-chain

fatty acids for energy and structural purposes, and free acids can be mobilized quicklywhen required for transport in an appropriate form to the heart, liver and other tissues

where they can be oxidized.
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Polyunsaturated fatty acids are important as constituents

of the phospholipids, where they appear to confer distinctive properties to themembranes, in particular by decreasing their rigidity. The presence of saturated and

monoenoic acids ensure that there is a correct balance between rigidity and flexibility.

Indeed, saturated and 2-hydroxy fatty acids in sphingolipids appear to give additionalrigidity and hydrogen-bonding stability to the sub-domains of membranes termed 'rafts'.

The essential fatty acids, linoleic and linolenic acidsand their longer-chain polyunsaturated metabolites, such as arachidonic acid, can befound in most lipid classes, but they are also the precursors of many different types of

eicosanoids, including the hydroxyeicosatetraenes,prostanoids (prostaglandins,

thromboxanes and prostacyclins),leukotrienes(and lipoxins) andresolvins, not to forgetthe isoprostanes, which are formed by non-enzymic means. It is surely no coincidence

that plant hormones, such as thejasmonates, are also derived from the essential fatty

acids and have structural similarities. While, they are usually treated separately inbiochemical textbooks, it should not be forgotten that these compounds are in fact fatty

acids. Some of them are occasionally found esterified to phospholipids (and

glycosyldiacylglycerols in plants), although their short half-lives may preclude long-term

storage in this form. For example, the isoprostanes are all formed in situ in lipids withinmembranes. The eicosanoids are highly potent at nanomolar concentrations in the

regulation of innumerable biological activities, especially in relation to inflammatory

responses, pain and fever.

Fatty acids are also the biosynthetic precursors of many insect pheromones and of

secondary metabolites in plants.

Unesterified fatty acids can act as second messengers required for the translation of

external cellular signals, as they are produced rapidly as a consequence of the binding ofspecific agonists to plasma membrane receptors. Within cells, fatty acids can act to

amplify or otherwise modify signals to influence the activities of such enzymes as proteinkinases, phospholipases, and many more. They are involved in regulating geneexpression, mainly targeting genes that encode proteins with roles in fatty acid transport

or metabolism via effects on transcription factors, i.e. peroxisome proliferator-activated

receptors (PPARs) in the nuclei of cells. Such effects can be highly specific to particular

fatty acids. Thus, unesterified arachidonic acid may have some biological importanceper

se as part of the mechanism by which apoptosis (programmed cell death) is regulated.
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Tri-, Di- and Monoacylglycerols

Virtually all the natural fats and oils of commerce consist oftriacylglycerols, but here we

are concerned with their biological functions. As discussed briefly above triacylglycerolsare the storage lipid in animal and plant cells, where they occur as discrete dropletssurrounded by a protective monolayer of phospholipids and functional hydrophobic

proteins. Fatty acids are released when required by hydrolysis reactions catalysed by

lipases under the influence of hormones, though a high proportion is usually re-esterifiedto biochemically inert triacylglycerols for extracellular transport. One specialized form of

adipose tissue, brown fat, is highly vascularized and rich in mitochondria, which oxidize

fat so rapidly that heat is generated. This appears to be especially important in younganimals and in those recovering from hibernation. Triacylglycerols are the main lipid

component in the only material designed by nature entirely as a food, i.e. milk, though

triacylglycerols in seeds could perhaps be considered similarly as 'food' for the

developing plant embryo until it is capable of photosynthesis.

However, triacylglycerol depots have other functions. Subcutaneous depots serve as

insulation against cold in many terrestrial animals, as is obvious in the pig, which issurrounded by a layer of fat, and it is especially true for marine mammals. In the latter

and in fish, the lipid depots are less dense than water and so aid buoyancy with the result

that less energy is expended in swimming. More surprisingly, perhaps, triacylglycerolstogether with the structurally related glyceryl ether diesters and wax esters are the main

components of the sonar lens used in echolocation by dolphins and some whales.

sn-1,2-Diacylglycerols are formed as intermediates in the biosynthesis of triacyl-sn-

glycerols and via the action of a diacylglycerol kinase of phospholipids. In addition, theyfunction as second messengers in many cellular processes, modulating vital biochemical

mechanisms by activating members of the protein kinase C family of enzymes. They are

formed together with the important inositol phosphates by the action of the enzymephospholipase C onphosphatidylinositol and polyphosphoinositides mainly. Their

influence is believed to extend to the pathophysiology of cancer and other disease states.

2-Monoacylglycerols are produced when triacylglycerols are digested in the intestines of

animals, but they are re-esterified before they are transported elsewhere in the body. In
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general, monoacylglycerols are minor components of tissues, which are never permitted

to accumulate because their strong detergent properties would have a disruptive effect on

membranes.

2-Arachidonoylglycerol, a further product of phosphatidylinositol catabolism, isimportant in animal tissues as an endogenous ligand for cannabinoid receptors and as a

mediator of the inflammatory response.

Waxes

Waxes form a thin layer over all the green tissue of plants that is both a chemical and a

physical barrier. This layer serves many purposes, for example to limit the diffusion ofwater and solutes, while permitting a controlled release of volatiles that may deter pests

or attract pollinating insects. It provides protection from disease and insects, and helps

the plants resist drought. Waxes also have a waterproofing and protective role for insects.

Waxes can have a storage function, as in marine organisms and for example in the seedsof the jojoba plant. Bees use wax to produce the rigid structures of honeycombs. The

uropygial glands of birds secrete waxes, which they use to provide water-proofing forfeathers.

Some Other Simple Lipids

Before a fatty acid can be metabolized in tissues, it must usually be activated by

conversion to a Coenzyme A esteror acyl-CoA, with the fatty acid group linked to the

terminal thiol moiety. The thiol ester is a highly energetic bond that permits a faciletransfer of the acyl group to receptor molecules.Acyl-carnitines assist the transport andmetabolism of fatty acids in and out of mitochondria, where they are oxidized for energy

production. In so doing, carnitine maintains a balance between free and esterified

coenzyme A in cells.
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Long-chainN-acylethanolamines are ubiquitous

trace constituents of animal and human cells with important pharmacological properties.The nature of the fatty acid controls the biological functions. Anandamide orN-arachidonoylethanolamine has attracted special interest, because of its marked biological

activities, exerting its effects through binding to and activating specific cannabinoid

receptors. Like 2-arachidonoylglycerol, discussed above, it is an endogenous cannabinoid

or endocannabinoid. In contrast, oleoylethanolamide is an endogenous regulator of foodintake with potential as an anti-obesity drug, while palmitoylethanolamide is an anti-

inflammatory agent, and stearoylethanolamide is a pro-apoptotic agent.

Similarly, changing the nature of the amide moiety also changes the function. Thus, thesimple oleamide molecule orcis-9,10-octadecenamide, isolated from the cerebrospinal

fluid of sleep-deprived cats, has been identified as the signalling molecule responsible forcausing sleep. Many simple fatty acid derivatives of amino acids are now known and

their biological functions are slowly being revealed.

Sterols

Cholesterol is a ubiquitous component of all animal

tissues, where much of it is located in the membranes. It occurs in the free form and

esterified to long-chain fatty acids (cholesterol esters) in animal tissues, including theplasma lipoproteins. It is generally believed that the main function of cholesterol is to

modulate the fluidity of membranes by interacting with their complex lipid components,

specifically the phospholipids such as phosphatidylcholine and sphingomyelin, increasingthe degree of order by promoting a 'liquid-ordered phase'. There is also considerable

evidence for more intimate protein-cholesterol interactions that may regulate the

activities of certain membrane proteins. It is of course the precursor of bile acids, vitaminD and steroidal hormones..

In plants, cholesterol tends to be a minor component only of a complex phytosterol

fraction that includes campesterol, -sitosterol, stigmasterol, 5-avenasterol and

brassicasterols, while yeasts and fungi tend to contain ergosterol as their main sterolcomponent. Plant sterols are also able to regulate membrane fluidity and permeability,
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and they modulate the activity of membrane-bound enzymes. Some bacterial species

produce structurally and functionally related lipids, thehopanoids.

Complex Lipids in Membranes

Cellular membranes are semi-permeable barriers that enclose and define the cell and itsorganelles. They control the transport of materials, including signalling molecules, and

indeed many reactions occur within membranes, including energy production and

biosynthesis of cellular components. In addition, they can deform to enable budding,

fission and fusion. It is evident that the specific lipid compositions of membranes haveevolved to provide a barrier to the diffusion of ionic solutes and other molecules into

cellular compartments where they may not be required. At the same time, the membrane

environment provides a stable molecular platform for essential metabolic events and forintense signalling activity. Cellular membranes are the first site for receipt of

extracellular signals, they recruit and activate effector molecules, and they are the launchpad for activated effector molecules throughout the cell.

The characteristic feature of membrane lipids, which are essential for all of thesefunctions, is that they contain both hydrophobic and hydrophilic constituents, i.e. they are

amphiphilic. As such, they are weak surfactants and they tend to form aggregates in

bilayer or hexagonal-II arrangements in aqueous media in the normal temperature rangesthat prevail in living cells. However, in natural membranes, there is a mixture of lipid

types, which determine that bilayer structures predominate.

Glycerophospholipids, such as phosphatidylcholine,

phosphatidylethanolamine and so forth, together with the sphingolipids, such as

sphingomyelin and the glycosphingolipids, and cholesterol are essential structuralelements of all biological membranes. In the conventional model, which is illustrated

here in two dimensions, polar lipids form a bilayer with the polar head groups oriented

towards the aqueous phase while the hydrophobic fatty acyl moieties are arrangedinternally.
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Proteins, such as enzymes, transport systems or signalling receptors, can span the bilayer

and take up a considerable proportion of the membrane surface. They interact via their

basic amino acid residues with the ionic groups of polar lipids via electrostaticinteractions, generating a net charge that is mainly negative or zwitterionic. In the

process, their biological properties may be modified.

These membrane structures are not static, and free movement is possible within each

leaflet (lateral diffusion) and between leaflets (vertical or flip-flop diffusion). In addition,lipid molecules can rotate around their principal axis (rotational diffusion). The lateral

and rotational diffusions are responsible for the liquid characteristics of membranes, with

the constraint that the hydrophobic chains remain parallel to each other and perpendicularto the surface of the bilayer.

The distribution of lipids in each of the membrane leaflets is asymmetric with

phosphatidylcholine and sphingolipids located in the outer leaflet of the plasma

membrane, for example, while phosphatidylethanolamine and anionic phospholipids such

as phosphatidylinositol (and polyphosphoinositides) and phosphatidylserine occurprimarily in the inner leaflet. Cholesterol occurs in roughly equal proportions in both

faces, where it modulates the fluidity of membranes by its interaction with phospholipids.A membrane translocation machinery, which consumes large amounts of energy, is

required to maintain this asymmetry.

Each glycerophospholipid with its distinctive polar head group and characteristic fatty

acid composition modifies the properties of a membrane in a unique manner andcontributes to its overall properties. Phosphatidylcholine is often the most abundant lipid

in membranes, and it has a cylindrical shape, which does not induce curvature. On the

other hand, an increased concentration of cone-shaped lipids on one side and inverted

cones on the other side of a membrane will bring about curvature, which may be requiredfor membrane transport and fusion processes.

The balance between saturated, monoenoic and polyunsaturated fatty acids is

important in maintaining the optimum degree of fluidity of a given membrane.Docosahexaenoic acid, for example, adopts a more flexible and compact conformation

than more saturated chains with an average length only 60% of that for oleic chains, andthis in turn increases the conformational disorder of saturated chains in mixed-chain

phospholipids. In bacterial membranes, branched-chain and cyclopropane fatty acids

modify the fluidity in an analogous manner. When ether and plasmalogen forms of lipidsare also taken into account, membranes can contain a thousand distinct molecular species

of phospholipid. It is obviously impossible to quantify the relative importance of each of


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these to the physical and biological properties of membranes, and some general

assessments only are possible.

While the need to form stable bilayers is a primary prerequisite for all membranes, thereis also a requirement for a potential ability of the lipids to form non-bilayer structures for

some membrane-associated cell processes. For example, short-lived non-bilayerstructures with specific lipid components are probably formed in the processes of fusion

and fission of lipid bilayers and for cell division, while the activities of certainmembrane-associated proteins can be modulated by lipids that do not form lamellar

layers.

Further complexity is imposed by specific associations of phospholipids with membrane

proteins. Defined lipid species are required to stabilize protein structures, to control theinsertion and folding of proteins and membranes, and even for the assembly or

polymerization of enzyme complexes with direct effects on their functions. In addition,

many proteins are directed to membranes by covalent linkages to lipids, such as the

glycosyl phosphatidylinositol anchors, or by modification with myristoyl, palmitoyl,prenyl or sterol moieties (see below). The sphingolipids together with cholesterol arrange

themselves into distinct sub-domains or 'rafts' (see below) with certain membraneenzymes, and they act to compartmentalize these and their different biochemical

functions of course.

Glycerophospholipids

Phospholipids play multiple roles in cells other than by establishing permeability barriers.

For example, they provide a matrix for the assembly and function of a wide variety ofenzymes, they participate in the synthesis of macromolecules, and they act as molecular

signals to influence metabolic events. Anionic lipids like phosphatidylinositol and its

phosphorylated derivatives, which are concentrated on the cytoplasmic leaflet ofmembranes, exert a control on the properties of the membranecytosol interface and

consequently on many aspects of membrane trafficking, including vacuole formation

transport and fusion. Specific lipids of this kind are associated with particular organelles,

where in combination with other signalling molecules they can recruit effector proteinswith appropriate functions for each cellular compartment.

Phosphatidylcholine is a zwitterionic lipid and usually the most abundant phospholipid in

membranes of animal and plants, constituting a high proportion of the outer leaflet of theplasma membrane. It is also an integral component of the lipoproteins in plasma.

However, it may serve as a source of diacylglycerols with a signalling function, while the

plasmalogen form especially may provide arachidonate for eicosanoid production. In

addition, phosphatidylcholine is the biosynthetic precursor of sphingomyelin (see below)and many other signalling molecules and thus has an influence on innumerable metabolic

pathways.
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Platelet-activating factoror 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine, a closelyrelated lipid, was the first biologically active phospholipid to be discovered. Amongst the

innumerable activities that have been documented, it effects the aggregation of platelets

at concentrations as low as 10-11 M, it is a mediator of inflammation and it is involved inthe mechanism of the immune response.

Phosphatidylethanolamineis also a major component of membranes, especially in

bacteria, with distinctive physical properties because of its small head group and

hydrogen bonding capacity. In the bacteriumE. coli, it supports active transport by the

lactose permease, and other transport systems may require or be stimulated by it. Inanimal and plants, it acts as a 'chaperone' during the assembly of membrane proteins to

guide the folding path for the proteins and to aid in the transition from the cytoplasmic tothe membrane environment.

Phosphatidylinositol is an acidic or anionic phospholipid, a high proportion of which in

animal membranes consists of the 1-stearoyl,2-arachidonoyl molecular species, which is

of considerable biological importance. It is the primary donor of 1,2-diacylglycerols withtheir specific signalling functions (see above), and of inositol phosphates with many

different biological activities. In addition, it is the main source of arachidonic acid for the

production of eicosanoids and of endogenous cannabinoids. In all eukaryotes,

phosphatidylinositol serves as an anchor that links a variety of proteins to the externalleaflet of the plasma membrane via complex glycosyl bridges, i.e. glycosyl-

phosphatidylinositol(GPI)-anchored proteins.

A further acidic lipid,phosphatidylserine, contributes substantially to non-specificelectrostatic interactions in the inner leaflet of membranes. This normal distribution is

disturbed during platelet activation and in the process of cellular apoptosis when the lipid

is transferred from the inner to the outer leaflet of the plasma membrane and acts as asignal to scavenger cells. Phosphatidylserine chelates with calcium to act as the

foundation for bone growth. It is also an essential cofactor for the activation of many

enzymes, including protein kinase C, which is a key enzyme in signal transduction.
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Cardiolipin or diphosphatidylglycerol is a unique acidic phospholipid with fouracyl groups. In the mitochondria of cells, its primary location, many biological functions

of this lipid have been identified, but the main ones involve activation of those enzymes

concerned with oxidative phosphorylation. Indeed, it is integrated into their quaternary

structure, where it is an essential component of the interface between the enzymes andtheir environment and may stabilize the active sites. In higher plants, cardiolipin is an

integral constituent of the photosystem II complexes, which are also involved in

oxidative processes, and where it may be required for the maintenance of structural andfunctional properties.

Phosphatidic acid is generally a minor component of cells, but it is a key intermediate in

the biosynthesis of all other phospholipids. It is known to have signalling functions in

animal cells, by specific binding to particular proteins, and it may be even moreimportant in higher plants where it is formed rapidly in response to stresses of all kinds.

Lysophospholipids, i.e. with only one mole of fatty acid per mole of lipid, were long

thought to be merely intermediates in the biosynthesis of phospholipids that were

potentially disruptive to cells if allowed to accumulate, because of their powerfuldetergent properties. However,lysophosphatidic acid has been shown to have signalling

and other biological effects that are dependent on receptor mechanisms. It is produced by

a wide variety of cell types and most mammalian cells express receptors for it. Forexample, it is involved in the activation of protein kinases, adenyl cyclase and

phospholipase C, in the release of arachidonic acid, and much more. Interest was

stimulated especially by a finding that lysophosphatidic acid is significantly elevated inthe plasma of ovarian cancer patients compared to healthy controls, so that it may

represent a useful marker for the early detection of the disease.

Other lysophospholipids including the sphingolipid analogue, sphingosine-1-phosphate

(see below), exhibit a related range of activities. Lysobisphosphatidic acidhas a uniquestereochemistry and distinctive biological functions.

Glycosyldiacylglycerols

Mono- and digalactosyldiacylglycerols, andsulfoquinovosyldiacylglycerol are important

components of membranes of chloroplasts and related organelles, and indeed these are

the most abundant lipids in all photosynthetic tissues, including those of higher plants,algae and certain bacteria. They may substitute in part for phospholipids, especially when
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phosphorus is limiting, although the distinctive ability of monogalactosyldiacylglycerols

to form inverted micelles may be important for membrane structure and for interactions

with specific proteins. The thylakoid membrane where photosynthesis occurs in plantshas an asymmetric distribution of glycolipids, with much of the

digalactosyldiacylglycerol on the luminal leaflet, where it may assist the movement of

protons along the membrane surface to the ATPase. While many different functions havebeen ascribed to these lipids, it is clear that their primary importance is in their

interactions with the photosynthetic apparatus.

Glycosyldiacylglycerols have also been found in animal tissues, though usually in rathersmall amounts, and their role in mammalian membranes is poorly understood. However,

seminolipid or 1-O-hexadecyl-2-O-hexadecanoyl-3-O--D-(3'-sulfo)-galactopyranosyl-sn-glycerol, which was first found in mammalian spermatozoa and testes, is known to beessential for spermatogenesis and may have a role in myelination.

Sphingolipids

Sphingolipids are distinguished by the presence of a long-chain or sphingoid base, suchas sphingosine, to which a fatty acid is linked by an amide bond, and usually with the

primary hydroxyl group attached to complex phosphoryl or carbohydrate moieties. They

have an immense range of functions in tissues that are quite distinct from those of thecomplex glycerolipids. For example, sphingomyelin has structural similarities to

phosphatidylcholine, but has very different physical and biological properties, while the

complex oligoglycosylceramides and gangliosides have no true parallels among theglycerolipids.

Free sphingoid bases are found trace levels only in tissues, but they are mediators of anumber of cellular events. For example, they inhibit the enzyme protein kinase C, and

they are inhibitors of cell growth, although they stimulate cell proliferation and DNA

synthesis. Some of the structural features of the long-chain bases are only introduced
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after they are esterified with long-chain fatty acids to form ceramides, which are also the

primary precursors of the complex sphingolipids. In addition, ceramides have an

important role in cellular signalling, and especially in the regulation of apoptosis, and celldifferentiation, transformation and proliferation. In contrast,sphingosine-1-phosphate is

an especially important sphingosine metabolite and promotes cellular division (mitosis)

as opposed to apoptosis, so that the balance between the former and ceramide, ceramide-1-phosphate and sphingosine levels in cells is critical.

In fact, the biosynthesis and catabolism of sphingolipids involves a large number of

metabolites, many of which have distinctive biological activities. In animals the

relationships between these metabolites have been rationalized in terms of a'sphingomyelin cycle', in which each of the various compounds has characteristic

metabolic properties. Similar pathways occur in plants although sphingomyelin is not

involved.

Sphingomyelin is by far the most abundant sphingolipid in animal tissues. In addition to

serving as a source of key cellular metabolites, sphingomyelin is an important buildingblock of membranes and like its glycerolipid analogue phosphatidylcholine tends to be

most abundant in the plasma membrane of cells and especially in the outer leaflet. Thesphingolipids in general contain high proportions of longer-chain saturated and

monoenoic fatty acids, often accompanied by high proportions of 2-hydroxy but not

polyunsaturated fatty acids.

Sphingomyelin and other sphingolipids together with cholesterol are located in an

intimate association in specific sub-domains or 'rafts' (or related structures termed

'caveolae') of membranes. These are laterally segregated regions that form as a result ofselective affinities between sphingolipids and membrane proteins. As sphingolipids

containing long saturated acyl chains, they pack more tightly together, thus giving

sphingolipids much higher melting temperatures than glycerophospholipids. This tightacyl chain packing is essential for raft lipid organization, since the differential packing

facility of sphingolipids and cholesterol in comparison with glycerophospholipids leads

to phase separation in the membrane, giving rise to the sphingolipid-rich regions ('liquid-

ordered' phase) surrounded by glycerophospholipid-rich domains ('liquid-disordered'phase). The ordered phases are relatively resistant to attack by detergents, a property that

is sometimes used to define them.

An important result of this process is that rafts contain a variety of different proteins,

including glycosyl-phosphatidylinositol(GPI)-anchored proteins and tyrosine receptorkinases. These provide much of the important biological properties of rafts, and are also

essential to maintain their stability. Micro-domains or rafts that are enriched in
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sphingolipids (other than sphingomyelin), sterols and specific proteins have also been

detected in the plasma membrane of plant cells.

Monoglycosylceramides or cerebrosides are common constituents of membranes ofanimals and plants. Galactosylceramide is the principal glycosphingolipid in brain tissue

and myelin, while glucosylceramide is a major constituent of skin lipids, and is the sourceof the unusual complex ceramides that are found in the stratum corneum. It is also the

biosynthetic precursor of the oligoglycosphingolipids. Harmful quantities ofglucosylceramide accumulate in tissues of patients with Gaucher's disease, an inherited

metabolic disorder. In plants, specific glucosylceramides elicit defence responses against

fungal attack, and they appear to assist plants to withstand stresses brought about by coldand drought.

The membranes of animals and plants contain a wide range of complex

oligoglycosylceramides (several hundred different head groups). Most of these occur on

the external leaflet of the plasma membrane in rafts, where they are importantcomponents of the body's immune defence system, both as cellular immunogens and as

antigens. For example, certain glycolipids are involved in the antigenicity of blood group

determinants, while others bind to specific toxins or bacteria. Some function as receptorsfor cellular recognition, and they can be specific for particular tissues or tumours.

Glycosphingolipid sulfates are highly polar acidic molecules that are important in the

transport of sodium and potassium ions and osmoregulation in animal tissues; they may

also have a role in the protection of the intestinal mucosa against digestive enzymes.Gangliosides are complex oligoglycosylceramides containing sialic acid residues so they

are also highly polar and acidic. They are cell-type specific antigens that control the

growth and differentiation of cells and have an important role in the interactions between

cells, especially in the immune defence systems. They are especially important formyelination in brain and other nervous tissues. In addition, gangliosides act as receptors

of interferon, epidermal growth factor, nerve growth factor, insulin and many other

metabolites, and in this way they regulate cell signalling. Certain gangliosides bind tospecific bacterial toxins and they mediate interactions between microbes and host cells

during infections.

Proteolipids and Lipoproteins
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although lipid analyst tend to have a firm understanding of what is meant by the term

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