Carbohydrates

Introduction:• Carbohydrates are the most abundant organic compounds in the plant world

• They are storehouses of chemical energy (glucose, starch, glycogen)

• They serve as components of supportive structures in plants (cellulose) and connective tissues in animals•They are essential components of nucleic acids (D-ribose (RNA) and 2-deoxy-D-ribose (DNA))

• serve as cell surface markers for cell-to-cell identification and communication

Formula for carbohydrates: Cn(H2O)m

example: Glucose (blood sugar) C6H12O6 or C6(H2O)6

Sucrose (table sugar) C12H22O11 or C12(H2O)11

Once thought to represent “hydrated carbon”. This formula isn’t always applicable, but has become “common” chemical terminology and persists.

•The majority of carbohydrates are polyhydroxy aldehydes and ketones.

•Therefore the chemistry of carbonyls and hydoxyl groups dominates here.

MonosaccharidesMonosaccharides = CnH2nOn

•One of the carbons of these compounds is either a ketone or an aldehyde.

•The suffix -ose is added to a molecule that is a carbohydrate, and prefixes tri-, tert-, and pent- are used to indicate the number of carbons.

•trioses to octoses are the most common monosaccharides, with the chemistry of pentoses and hexoses dominating.

•Monosaccharides containing an aldehyde functionality are called aldoses and those that contain a ketone functionality are known as ketoses.

For example, there are only two trioses:

CHO

CHOH

CH2OH

CH2OH

C

CH2OH

O

Glyceraldehyde(an aldotriose)

Dihydroxyacetone(a ketotriose)

•Often common names are used exclusively with carbohydrates and are firmly rooted in the literature.

•This is the case for glyceraldehyde which has the IUPAC name- 2,3-dihydroxypropanal. While okay for a small molecule like this, the IUPAC names become unwieldy for bigger systems and are never found in the biochemical or biological literature.

•As such, common names will be used throughout this discussion of carbohydrates.

Isomers

• Molecules with the same chemical formula but with a different arrangement of their atoms are called isomers

• One distinguishing feature of simple sugars is the spacial arrangement of their atoms

• Glucose and fructose are isomers

Glyceraldehyde exists as two different isomers (enantiomers).

(D)-Glyceraldehyde

(L)-Glyceraldehyde

CHO

C

CH2OH

HO H

CHO

C

CH2OH

OHH

Note: Common Amino Acids are “L”; common sugars “D”.

Amino Sugars

•Amino sugars contain an -NH2 group in the place of an -OH group.

•Only three amino sugars are common in nature: D-glucosamine, D-mannosamine, and D-galactosamine.

•N-Acetyl-D-glucosamine, is a component of many polysaccharides, including chitin, the hard shell-like exoskeleton of crustaceans and shellfish.CHO

NH2H

HHO

OHH

OHH

CH2OH

CHO

HNH

HHO

OHH

OHH

CH2OH

CHO

HH2N

HHO

OHH

OHH

CH2OH

CHO

NH2H

HHO

HHO

OHH

CH2OH

O

CH3

D-glucosamine D-Mannosamine D-Galactosamine N-Acetyl-D-glucosamine

Physical Properties of Carbohydrates

Monosaccharides are generally:

• Colorless, crystalline solids

•Soluble in water (good H-bonds between -OH groups and water)

•Most are sweet to the taste (see relative sweetness table)

The Cyclic Structure of Monosaccharides

•molecules containing a hydroxyl group and a carbonyl group can cyclize

•The ring form of glucose is the most common structure for glucose in the cell•The common way of representing cyclic structure of monosaccharides is to use the Haworth projection named after Sir Walter N. Haworth. •The monosaccharide is represented as a planar hexagon and pentagon, as the case may be, lying perpendicular to the plane of the paper.•Groups are attached either above or below the plane of the ring.

• When glucose forms a ring structure there is a 50% chance that the hydroxyl on carbon #1 will end up below the plane of the ring

• If this happens the molecule is called -glucose

• If the hydroxyl group ends up above the ring it is called -glucose

Glycosidic Linkages

• Glycosidic linkages are covalent bonds holding monosaccharides to one another

• they are formed by condensation reactions in which the H atom comes from a hydroxyl group of one sugar and the -OH comes from a hydroxyl group of another sugar

Disaccharides

• Consist of two monosaccharides linked together

• Three common disaccharides are:

1)Sucrose (common table sugar)– Glucose + fructose

2) Lactose (milk sugar)– Glucose +galactose

3) Maltose (product of starch digestion)– Glucose+glucose

Polysaccharides

Starches• Starches are polymers of glucose used for storage in plants• Two types are found:

1) Amylose consists of linear unbranched chains of glucose of several hundred units

2) Amylopectin differs from amylose in being highly branched– At approximately every 13 residues a side chain is attached to

the #6 carbon– Amylopectin consists of several thousand glucose residues

• The hydrolysis of starch is done by amylases

Polysaccharides

Glycogen

• Animal glucose storage molecule

• Similar to amylopectin except branches tend to be shorter and more frequent

• The liver and muscle are major depots of glycogen

Glycogen in liver cells

Cellulose

• Cellulose is probably the most abundant organic molecule on earth

• Constituent of plant cell walls• Major structural material from which

plants are made• Like starch cellulose is a long chain of

glucose residues• however, its structure is different

because of the way that glucose is bonded together

• Cellulose consists of beta 1-4 linkages which cause glucose monomers to ‘flip-flop’

• There are no side chains allowing these long linear molecules to lie close together

• This results in a series of stiff,

elongated fibrils

Oligosaccharides

• Sugars containing several simple sugars attached to one another (typically three to ten)

• Example is the blood groups

• often found as a component of glycoproteins or glycolipids and as such are often used as chemical markers, often for cell recognition

• example is ABO blood type specificity