Introduction to chemistry of carbohydrates

1. Chemistry of Carbohydrates

Classification of Carbohydrates

There are three major classes of carbohydrates

• Monosaccharides (Greek, mono = one)

• Oligosaccharides (Greek, oligo= few) 2-10 monosaccharide units.

• Polysaccharides (Greek, Poly = many) >10 monosaccharide units.

1.1. Monosaccarides

Monosaccharides

Monosaccharides also called simple sugars. They consist of a single polyhydroxy aldehyde or ketone units. The most abundant monosaccharides in nature are the 6-carbon sugars like D-glucose and fructose.

Structure

Monosaccharide has a backbone, which is un-branched, single bonded carbon chain. One of the carbon atoms is double bonded to an oxygen atom to form carbonyl group. Each of the other carbon atoms has a hydroxyl group. 

For example, observing the structure of glucose below;

Classification of monosaccharides

Monosaccharides having aldehyde groups are called Aldoses and monosaccharides with Ketone group are Ketoses. 

Depending on the number of carbon atoms, the monosaccharides are named trioses (C₃), tetroses (C₄), pentoses (C₅), hexoses (C₆), heptoses (C₇).

Physical properties of Monosaccharides

1. They are colorless crystalline compounds
2. readily soluble in water.
3. Their solutions are optically active and exhibit the phenomenon of mutarotation.
4. Carbohydrates spontaneously change between the α and β configuration.
5. Exhibit asymmetric center and stereoisomerism

Asymmetric carbon is a carbon that has four different groups or atoms attached to it and having optically activity in solution.

All the monosaccharides except dihydroxyacetone contain one or more asymmetric or chiral carbon atoms and thus occur in optically active isomeric forms.

Optical Activity Monosaccarides

Epimers

When sugars are different from one another, only in configuration with regard to a single carbon atom (around one carbon atom) they are called epimers of each other. For example glucose and mannose are epimers. They differ only in configuration around C2. Mannose and Galactose are epimers of Glucose.

Oligosaccharides

Oligosaccharides contain 2 to 10 monosaccharide units. The most abundant oligosaccharides found in nature are the Disaccharides.

1.2. Disaccharides

Disaccharides are sugars produced when two monosaccharides are covalently bonded together by glycosidic linkages. 

Glycosidic bond is formed when the hydroxyl group on one of the sugars reacts with the anomeric carbon on the second sugar.  

Examples of monosaccharides include; sucrose, maltose, and Lactose

Biologically Important Disaccharides 

Maltose (glucose + glucose)

Maltose contains two D-glucose residues joined by a glycosidic linkage between OH at the first carbon atom of the first glucose residues and OH at the fourth carbon atom of the second glucose forming a α-(1,4) glycosidic linkage. 

Maltose is the major degradative product of Starch. Maltose is hydrolysed to two molecules of D-glucose by the intestinal enzyme maltase, which is specific for the α- (1, 4) glycosidic bond.

Lactose (galactose + glucose)

Lactose is a disaccharide of β-D galactose and β-D- glucose which are linked by β-(1,4) glycosidic linkage. Lactose acts as a reducing substance since it has a free carbonyl group on the glucose. It is found exclusively in milk of mammals (Milk sugar).

Sucrose (Cane sugar) (glucose + fructose)

Sucrose is a disaccharide of α-D-glucose and β-D-fructose. It is obtained from cane sugar and present in various fruits. 

In contrast to other disaccharides, sucrose contains no free anomeric carbon atom. Since the anomeric carbons of both its component monosaccharide units are linked to each other. For this reason, sucrose is non reducing sugar.

1.3. Polysaccharides

Polysaccharides, or polycarbohydrates, are the most abundant carbohydrates found in food. They are long-chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages.

Most of the carbohydrates found in nature occur in the form of high molecular polymers called polysaccharides.

There are two types of polysaccharides. These are:

• Homopolysaccharides that contain only one type of monosaccharide building blocks.

• Heteropolysaccharides, which contain two or more different kinds monosaccharide building blocks.

1.4. Homopolysaccharides

Homopolysaccharides

Example of Homopolysaccharides: Starch, glycogen, Cellulose and dextrins.

Starch

It is one of the most important storage polysaccharides in plant cells. It is especially abundant in  tubers, such as potatoes and in seeds such as cereals.

Starch consists of two polymeric units made of glucose called Amylose and Amylopectin but they differ in molecular architecture.

Amylose is unbranched with 250 to 300 D-Glucose units linked by α-(1, 4) linkages Amylopectin consists of long branched glucose residue (units) with higher molecular weight.

The inner part of glucose units in amylopectin are joined by α-(1,4) glycosidic linkage as in amylose, but the branch points of amylopectin are α- (1,6) linkages. The branch points repeat about every 20 to 30 (1-4) linkages. 

Glycogen

- Glycogen is the main storage polysaccharide of animal cells (Animal starch).

- It is present in liver and in skeletal muscle.

- Like amylopectin glycogen is a branched polysaccharide of D-glucose units in α - (1, 4) linkages, but it is highly branched.

- The branches are formed by α - (1,6) glycosidic linkage that occurs after every 8 -12 residues. Therefore, liver cell can store glycogen within a small space. Multiple terminals of branch points release many glucose units in short time.

Cellulose

Cellulose is the most abundant structural polysaccharide in plants. It is fibrous, tough, water insoluble. Cellulose is a linear unbranched homopolysaccharide of 10,000 or more D- glucose units connected by β-(1, 4) glycosidic bonds. Humans cannot use cellulose because they lack of enzyme (cellulase) to hydrolyze the β-( 1-4) linkages.

Dextrins

 These are highly branched homopolymers of glucose units with α-(1, 6), α-(1, 4) and α-(1, 3) linkages. Since they do not easily go out of vascular compartment they are used for intravenous infusion as plasma volume expander in the treatment of hypovolemic shock.

1.5. Hetero polysaccharides

Hetero polysaccharides

These are polysaccharides containing more than one type of sugar residues

1. Glycosaminoglycans, (GAGs or mucopolysaccharides)

They are long, usually unbranched, composed of a repeating disaccharide unit

 * They are negatively charged heteroplolysaccharide chains (polyanions)

• The amino sugar is either D-glucosamine or D-galactosamine in which the amino group is usually acetylated, thus eliminating its positive charges.

• The amino sugar may also be sulphated on carbon 4, 6, or on a monoacetylated nitrogen.

• The acidic sugar is either D-glucuronic acid or its carbon 6 epimer, L-uronic acid. For example Hyaluronic acid, Heparin and Chondroitin sulphate.

Function of Glycosaminoglycans (GAGS)

1. They have the special ability to bind large amounts of water, there by producing the gel-like matrix that forms the basis of the body’s ground substance.

2. Since they are negatively charged, for example, in bone, glycosaminoglycans attract and tightly bind cations like ca⁺⁺, they also take-up Na⁺ and K⁺

3. GAGs stabilize and support cellular and fibrous components of tissue while helping maintain

the water and salt balance of the body.

4. Its essential components of the extra cellular matrix, GAGs’ play an important role in mediating cell-cell interactions

• Ground substance is a part of connective tissue, which is a gel like substance containing water, salt, proteins and polysaccharides.

An example of specialized ground substance is the synovial fluid, which serves as a lubricant in joints, and tendon sheaths.

Heparin

• contains a repeating unit of D-glucuronic and D-glucosamine, with sulfate groups on some of the hydroxyl and amino-groups

• It is an important anticoagulant, prevents the clotting of blood by inhibiting the conversion of prothrombin to thrombin. Thrombin is an enzyme that acts on the conversion of plasma fibrinogen into the fibrin.

• It is found in mast cells in lung, liver skin and intestinal mucosa.

Glycoproteins (Mucoproteins)

Glycoproteins are proteins to which oligosaccharides are covalently attached. They differ from the glycosaminoglycans in that the length of the glycoprotein’s carbohydrate chain is relatively short (usually two to ten sugar residues in length, although they can be longer), whereas it can be very long in the glycosaminoglycans.

The glycoprotein carbohydrate chains are often branched.

be negatively charged.

For example:

- Glycophorin, a glycoprotein found in human red cell membranes.

- Human gastric glycoprotein (mucin).

- Many protein hormones, receptors are glycoproteins

Proteoglycans

When glycosaminoglycans are attached to a protein molecule the compound is called proteoglycan [proteoglycans = Glycosaminoglycans + proteins].