CARBOHYDRATE CHEMISTRY DR AMINA TARIQ BIOCHEMISTRY

Introduction Carbohydrates are one of the three major classes of biological molecules. Carbohydrates are also the most abundant biological molecules. Carbohydrates derive their name from the general formula Cn (H2O).

functions Variety of important functions in living systems: Nutritional (energy storage, fuels, metabolic intermediates) Structural (components of nucleotides, plant and bacterial cell walls, arthropod exoskeletons, animal connective tissue)

Informational (cell surface of eukaryotes -- molecular recognition, cell-cell communication) osmotic pressure regulation (bacteria) Precursor for the synthesis of all other carbohydrates in the body.---- glycogen, ribose & deoxyribose, galactose.

Diseases associated with carbohydrate metabolism are: Diabetes mellitus Glycogen storage diseases Galactosemia Lactose intolerance

Carbohydrates & teeth All food nourishes the bacteria in your mouth, they thrive on sugar-based carbohydrates in sweet things like fruit, sugar, biscuits and raisins. The carbohydrates that result in the longest and most severe acid attacks are the sugars sucrose, fructose and glucose. The bacteria in the mouth can also form acid from the carbohydrates lactose and starch.

Some of the bacteria in undisturbed plaque in the mouth break down digestible carbohydrates such as sugar, fruit, crisps, bread, boiled potatoes and pasta in a process that creates acid that can result in tooth decay.

The formation of acid reduces acidity levels (the pH value) in the mouth, which, in turn, increases the solubility of calcium and phosphate from the teeth. This material is never replaced and is the first sign of tooth decay.

Different carbohydrates trigger different drops in acidity levels in the mouth, and the rates at which the original pH value is restored also vary. This is why different carbohydrates carry a different risk of caries.

Carbohydrates are carbon compounds that contain large quantities of hydroxyl groups.

Carbohydrates are chemically characterized as: Poly hydroxy aldehydes or Poly hydroxy ketones.

Sugars that contain an aldehyde group are called Aldoses. Sugars that contain a keto group are called Ketoses.

classification All carbohydrates can be classified as either: Monosaccharides Disaccharides Oligosaccharides Polysaccharides. Derived carbohydrates

Monosaccharides- one unit of carbohydrate Disaccharides- Two units of carbohydrates. Anywhere from two to ten monosaccharide units, make up an oligosaccharide. Polysaccharides are much larger, containing hundreds of monosaccharide units.

Carbohydrates also can combine with lipids to form glycolipids With proteins to form glycoproteins.

Derived Carbohydrates- derived from carbohydrates by various chemical reactions; e.g. oxidation products and reduction products.

Isomerism in monosaccharides Isomers are the molecules that have the same molecular formula.

Stereoisomers Stereoisomers are molecules that have the same molecular formula, but have a different arrangement of the atoms in space. (different structures). For example, a molecule with the formula AB2C2, has two ways it can be drawn:

Isomer 1

Isomer 2

Examples of isomers: Glucose Fructose Galactose Mannose Same chemical formula C6 H12 O6

EPIMERS EPIMERS are sugars that differ in configuration at ONLY 1 specific POSITION, other than the carbonyl carbon.

Examples of epimers : D-glucose & D-galactose (epimeric at C4) D-glucose & D-mannose (epimeric at C2) D-idose & L-glucose (epimeric at C5)

ENANTIOMERS Non-Superimposable COMPLETE mirror image (differ in configuration at EVERY CHIRAL CENTER.

The two members of the pair are designated as D and L forms. This designation is based on the presence of OH group on the penultimate asymmetric carbon. In D form the OH group on the asymmetric carbon is on the right. In L form the OH group is on the left side. D-glucose and L-glucose are enantiomers.

D and L monosaccharides are metabolized by specific enzymes. In mammals most are D monosaccharides.

Asymmetric carbon A carbon linked to four different atoms or groups farthest from the carbonyl carbon. Also called Chiral carbon.

Dihydoxyacetone is the only monosaccharide that does not possess any asymmetric carbon atom. All monosaccharides containing more than 3 carbon atoms have two or more asymmetric carbon atoms. D & L given- by seeing the configuration of H & OH around the penultimate carbon atom. (carbon atom situated farthest from the carbonyl carbon).

cyclization Less then 1%of CHO exist in an open chain form. Predominantly found in ring form. involving reaction of C-5 OH group with the C-1 aldehyde group or C-2 of keto group.

Six membered ring structures are called Pyranoses . five membered ring structures are called Furanoses .

Anomeric carbon The carbonyl carbon after cyclization becomes the anomeric carbon. This creates α and β configuration. In α configuration the OH is on the same of the ring in fischer projection. In Haworths it is on the trans side of CH2OH.

Such α and β configuration are called diastereomers and they are not mirror images. Enzymes can distinguished between these two forms: Glycogen is synthesized from α-D glucopyranose Cellulose is synthesized from β -D glucopyranose

MUTAROTATION Unlike the other stereoisomeric forms, α and β anomers spontaneously interconvert in solution. This is called mutarotation.

Optical Activity When a plane polarized light is passed through a solution containing monosaccharides the light will either be rotated towards right or left. This rotation is because of the presence of asymmetric carbon atom. If it is rotated towards left- levorotatory (-) If it is rotated towards right- dextrorotatory(+)

Vant Hoff's Law: Aldohexoses have 4 asymmetric carbon atoms i.e. from 2-4 . This law states that the number of possible optical isomers is equal to 2n where n is the number of asymmetric carbon atoms. 24 = 16 possible isomers

The reason that this is so important, however, is not because these particular compounds can rotate light. Instead, it has to do with the way that biological reactions take place. Biological reactions generally are catalyzed by enzymes and the enzymes are set up to specifically handle a particular orientation of atoms within a molecule.

An enzyme that would be able to take D-glucose, and hydrolyze it, or oxidize it, or do something else to it would not be able to carry out that same reaction on the mirror image of D-glucose

. That is because it wouldn't fit properly up against the enzyme as is necessary to carry out the reaction. As a consequence, different arrangements of the atoms in molecules, even just one or two of the OH's switched in position in the molecules shown above will make a biological difference. Therefore, it's very important to be very familiar with the orientations of the functional groups within a particular molecule.

Plane polarized light will neither be rotated to left or right when; No asymmetric carbon atom is present. Equal amounts of dextro and levorotatory isomers are present (racemic mixture). Meso compounds – they have asymmetric carbon atom but do not rotate light. Due to internal compensation. (Each molecule two halves, rotation of light is equal by both halves, so no rotation).

Reducing sugar Sugars in which the oxygen of the anomeric carbon is free and not attached to any other structure, such sugars can act as reducing agents and are called reducing sugars.

Sucrose is a non reducing sugar.