introduction to the study of lipids -...
TRANSCRIPT
Introduction to the Study of Lipids
Factors to Consider in the Study of Biomolecules
What are the features of the basic building blocks ? (ex: monosaccharides, alcohols, fatty acids, amino acids)
1) General structure and functional groups 2) Polar and non-polar portions
3) Hydrogen bonding capabilities
4) Acid-Base properties
5) Optical activity
6) Other chemical properties
Factors to Consider in the Study of Biomolecules
Do the small building blocks function by themselves ?
Are the building blocks put together into larger structures ?
What are the physical properties of these larger structures ?
How are the various types of biomolecules suited for their individual functions ?
Lipids
Classification of a compound as a lipid is based on its solubility behavior rather than the presence of a common functional group.
Lipids are compounds which dissolve in nonpolar or low polarity solvents such as toluene, dioxane, or
carbon tetrachloride.
Lipids
Note:
Other than triacylglycerols, most lipids are actually amphipathic:
One part of the molecule is hydrophobic. One part of the molecule is hydrophilic.
However, the hydrophobic part of a lipid molecule is usually much larger than the hydrophilic part.
Therefore, most lipids are insoluble (or very slightly soluble) in water.
Varieties of Lipids
• Triacylglycerols—animal fats and plant oils— sources of energy and a storage form of energy not required for immediate use.
• Glycerophospholipids, sphingolipids, and cholesterol are the primary structural components of the membranes that surround all cells and organelles.
• Steroid hormones and other hormone-like lipids act as chemical messengers, initiating or altering activity in specific target cells.
• The lipid-soluble (fat-soluble) vitamins - required for a variety of physiological functions.
• Bile salts - needed for the digestion of lipids in the intestinal tract.
Classification of Lipids
Hydrolyzable Lipids
Non-Hydrolyzable Lipids
Contain one or more straight chain fatty acids connected to
another structure through ester, amide, or glycoside linkages.
Do not contain hydrolyzable linkages.
Classification of Lipids
Fatty Acids
long hydrocarbon chain
carboxylic acid functional group
Fatty Acids
CO
ONa+
Biologically Important Fatty AcidsFatty acids may be saturated or unsaturated:
Saturated fatty acids: contain no C=C bonds. Palmitic (C-16) and stearic (C-18) acids are the most common.
Unsaturated fatty acids: contain one or more C=C bonds
(Almost all naturally occurring unsaturated fatty acids contain cis double bonds.)
(monounsaturated and polyunsaturated fatty acids).
Oleic (C-18, one C=C) - the most common.
Linoleic and linolenic acids - essential fatty acids for animals Synthesized only by plants. They must be supplied in the diet.
(The position of the double bonds in unsaturated fatty acids is sometimes indicated by an omega number (ω-). The ω-1 carbon is the methyl group farthest from the carbonyl carbon.) Linoleic and linolenic acids are ω-6 and ω-3 fatty acids.
COOH
COOH
COOH
COOH
COOH
COOH
COOH
COOH
COOH
COOH
COOH
Number of Carbons
Common Name Systematic Name Structure
Melting Point ºC
12 lauric dodecanoic 44
14 myristic tetradecanoic 58
16 palmitic hexadecanoic 63
18 stearic octadecanoic 69
20 arachidic eicosanoic 77
16 palmitoleic hexadecenoic 0
18 oleic octadecenoic 13
18 linoleic octadecadienoic -5
18 linolenic octadecatrienoic -11
20 arachidonic eicosadecaenoic -50
20 EPA eicosapentaenoic -50
Fatty Acids - Physical Properties
Fatty Acids - Structures
Waxes
Most waxes are fatty acid esters with long chain alcohols having only one –OH group.
The alcohols in waxes usually contain an even number of carbon atoms ranging from 14 to 36.
Beeswax Carnauba wax
Lanolins:
OCH3-(CH2)n-C-O
OCH3-(CH2)24-C-O-(CH2)29-CH3
OCH3-(CH2)14-C-O-(CH2)29-CH3
Triacylglycerols (Triglycerides)
Make up about 90% of our dietary lipid intake.
Animal Fats, Fish oils, Plant Oils
Triacylglycerols are triesters of glycerol:
Naturally occurring triacylglycerols contain a variety of fatty acid components. Any fat or oil is a mixture of many different triacylglycerols, few of which are “simple” (R1=R2=R3).
The percentage of unsaturated fatty acids varies with the source:
Animal sources other than fish: 40-60% Fish (high percentage of polyunsaturation): 75-80% Plant sources: 85-90%
Triacylglycerols Defined by Their “R” Groups
Triacylglycerols - Fatty Acid Composition
Triacylglycerols - Physical Properties
Triacylglycerols - Chemical Properties
Hydrolysis
Acidic Conditions
Basic Conditions
+
+
+-
-
-
Soaps
Soaps are produced by a process called “saponification” (technically, “basic hydrolysis of fats”)
H2C
C
H2C
OH
O
O
C
C
O
O
C
O
C
C
O
O
C
O
-O
-O
-O
Na+
Na+
Na+
Hot aq NaOH
H2C
C
H2C
OHH
OH
OH
C
O
CH2CH2
CH2CH2
CH2CH2
CH2CH2
CH2CH2
CH2CH2
CH2CH2
CH3-ONa+
Soaps
Soaps are the sodium or potassium salts of long-chain fatty acids.
hydrophilic region
hydrophobic region
How Do Soaps Work?
micelle micelle+grease
Synthetic Detergents
S
O
O-ONa+
O
S
O
-ONa+
O
S
O
-ONa+
O
Sodium Dodecyl Sulfate (SDS)
Sodium Alkyl Benzene Sulfonate (ABS)
Sodium Dodecyl Benzene Sulfonate (LAS)
N+CH2
N+
Synthetic Detergents
Na+
O
O
O
O
OS
O
O
-O
O
O
O
HO
Behavior of Soaps in Water
Triacylglycerols - Chemical PropertiesReduction
H2C
HC
H2C
O
O
O
O
O
O
H2C
HC
H2C
O
O
O
O
O
O
H2, Ni, heat, pressure
Corn oil—————>Solid Crisco
Triacylglycerols - Catalytic Hydrogenation
Catalytic hydrogenation - the addition of H2 to alkene double bonds in the presence of a metal catalyst (Ni or Pt).
This process is the basis for the conversion of plant oils into margarine and other products.
Some but not all of the double bonds in vegetable oils are hydrogenated in order to convert them into more solid, more
palatable forms.
Effectively, one can adjust the melting point of any lipid component of a food.
Saturated fatty acids (from animals and butter, for example) are implicated in the development of atherosclerosis.
Vegetable oils contain a much higher percentage of unsaturated fatty acids and almost no cholesterol.
Triacylglycerols in the DietWhat’s good about unsaturated triglycerides?
The catalysts used for hydrogenation reactions have a serious drawback. They also catalyze an unwanted
reaction: the isomerization of some of the remaining cis double bonds into trans double bonds.
Evidence suggests that trans double bonds raise blood cholesterol levels more than cis double bonds and perhaps
more than saturated fatty acids.
What’s bad about unsaturated triglycerides?
What’s bad about the hydrogenation process?
Saturated fats last longer because oxidation of the double bonds causes rancidity.
Triacylglycerol Digestion
Triacylglycerols in the diet are too large to diffuse through the intestinal membranes.
Prior to absorption, triacylglycerols are first digested in the small intestine (basic pH) with enzymes called lipases with the help of
bile salts.
The digestion in the intestines produces a mixture of primarily monoacylglycerols and fatty acids with some diacylglycerols and
glycerol.
The intestinal cells rebuild the triacylglycerols and combine them with proteins into particles called chylomicrons.
Chylomicrons are transported via the lymphatic system to the bloodstream where they are carried to various tissues.
In the tissues the triacylglycerols are separated from the proteins, hydrolyzed to fatty acids and glycerol, and metabolized to produce
energy.
Fatty acids not needed for energy are reconverted to triacylglycerols and stored in adipose cells (adipocytes or fat cells) as fat droplets.
Triacylglycerol Digestion
Triglycerides as an Energy Source
Triacylglycerols are the primary energy storage form in animals.
Triacylglycerols: 9.2 kcal/g dry weight Glycogen: 4.0 kcal/g dry weight
Glycogen is stored in a highly hydrated form whereas triacylglycerol is stored in an anhydrous form.
Taking this into account 1 g of triacylglycerol yields 7 times more
energy than 1 g of hydrated glycogen.
Normal liver and muscle glycogen levels can fill your energy needs for approximately 12 hours.
Adipose tissue triacylglycerols can fill your energy needs for several weeks to a couple of months.