<h2>Carbohydrate Chemistry</h2>

As you go further metabolism of carbohydrates will be covered in great detail with various cycles such as glycolysis, glycogenolysis etc. More of that in another post. This post will inform briefly about the structure and

chemistry of carbohydrates

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• Carbohydrates (or sugars in layman terms) are organic molecules with a general formula Cn(H2O)n. The value of n may vary between 3 to 9. They can be polyhydroxy aldehydes CH=O or polyhydroxy ketones C=O which will form the functional group.
• Carbohydrate compounds are the most important source of energy for the activities of the cell and other body functions. Metabolism of the dietary carbohydrates in the body provides this energy.
• Dietary carbohydrates are subdivided into monosaccharides, the simplest sugars (glucose, fructose, galactose, ribose), disaccharides (lactose, maltose and sucrose), oligosaccharides (3-10 monosaccharides covalently linked to proteins or lipids, giving rise to glycoproteins and glycolipids respectively), and lastly polysaccharides consisting of more than 10 monosaccharides (starch, glycogen and cellulose).
• Polysaccharides can be homopolysaccharides if they are made up of the same repeating monosaccharide units or heteropolysaccharides if they contain different monosaccharide units eg heparin, dextran
• The carbon atoms from the sugar can also be used to synthesize new compounds
• All dietary carbohydrates will ultimately be broken down into monosaccharides, usually glucose before being abosrbed into the bloodstream
• The number of carbon atoms in the sugar can be inferred from the initial part of the name with suffix ose - thus triose (three carbon atoms), tetrose, pentose and hexose sugars

Types of carbohydrate isomers

Carbohydrate isomers have the same molecular formula but may differ in other properties. The various isomers are

1. Structural isomers - differing in structure eg glucose, fructose and galactose

2. Stereoisomers - having different spatial configuration. These include epimers (different orientation of OH and H around one carbon), enantiomers (sugars that are mirror images of each other eg D and L forms) and anomers (differ in orientation of H and OH around anomeric carbon eg alpha -glucose and beta - glucose)

3. Optical isomers - differ in optical rotation eg dextro and levo rotatory forms

Modified Sugars

Modified sugars are obtained when the hydroxyl group gets reduced or the alcohol group is oxidised or these are replaced by amino group

1.  OH group is reduced to H - eg Deoxyribonucleic acid (DNA)

2. Amino sugar - eg glucosamine, galactosamine - OH group is replaced by amino group

3. Alcohol group of first carbon is oxidised to COOH - eg gluconic acid

4. Acidic sugar - Last carbon of glucose is oxidised to COOH - eg glucuronic acid

5. Aldehyde group is reduced to alcohol - eg glycerol, mannitol, sorbitol

Read about mannitol and sorbitol 

Glycoproteins, glycolipids, glycosaminoglycans, and proteoglycans are composed of unmodified and modified sugars 

Some important glycoproteins and their functions

As mentioned previously, glyproteins are sugar molecules covalently linked to proteins. Some of the medically important glycoproteins are

1. Collagen - structural glycoprotein and an important component of connective tissue

2. Hormones - Thyroid stimulating hormone (TSH)

3. Prothrombin - clotting factor in blood

4. Transferrin and ceruloplasmin - transport proteins involved in transport of other molecules

5. Mucin - protective function and lubricant in lining of hollow organs

6. Immunoglobulins - important in immune function

7. Blood group antigens - important in blood grouping and blood banking

8. Glycophorin - membrane glycoprotein of red cells

Glycosaminoglycans (GAGS)

These are long, linear polysaccharides made up of repeating units of a specific disaccharide. The disaccharide units have one acetylated amino sugar and one uronic acid (see above)—an
acidic sugar such as D-glucuronic acid or D-iduronic acid. 

Glycosaminoglycans form important constituents of connective tissue (e.g., cartilage) and synovial fluid (i.e., joint lubricant).  

The six physiologically important glycosaminoglycans include heparin, heparan sulfate, dermatan sulfate, keratan sulfate (which does not contain an acid sugar), chondroitin sulfate, and hyaluronic acid.  

Functions of GAGS

Hyaluronic acid - polysaccharide consisting of alternating D-glucuronic acid and D-Nacetylglucosamine molecules - joint lubricant and important component of extracellular fluid

Heparin - strongly negative GAG and an important anticoagulant

Heparan sulphate - Occurs in glomerular basement membrane and thus involved in kidney filtration function

Dermatan sulphate - important component of skin, blood vessels and heart valves

Keratan sulphate - occurs in bone, cartilage and cornea

Chondroitin sulphate - occurs in cartilage (loss of chondroitin sulphate may result in joint degeneration and osteoarthritis).

Defective metabolism of GAGS results in a condition termed as mucopolysaccharidoses. Examples include Hunter's syndrome and Hurler's syndrome and both are characterized by significant accumulation of heparan sulphate and dermatan sulphate.

With this brief overview on chemistry of carbohydrates, I hope it will be easier to understand carbohydrate metabolism