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 VITAMIN C (Ascorbic acid)

Chemistry : Vitamin C is also known as an antiscorbutic vitamin. It is a water soluble vitamin. It has a structure similar to that of L-glucose and it is a derivative of glucose. Chemically, it is enediol lactone of gluconic acid. So it is a good reducing agent. Both oxidised and reduced forms of ascorbic acid are biologically active.

Sources : Citrous fruits (lemons and oranges), berries, melons, leafy vegetables, cabbage and tomatoes are good sources.

Physiological functions :

1. It is necessary for the functional activity of fibroblasts and osteoclasts

2. It is necessary for collagen synthesis

3. It is involved in oxidation -reduction reactions. It acts as a hydrogen transport agent

4. It helps in iron absorption by converting ferric iron to ferrous iron

5. It is involved in the conversion of folic acid to active tetrahydrofolic acid

Deficiency diseases : Deficiency of vitamin C produces scurvy. The symptoms are -

• Widespread haemorrhage

• Painful, swollen joints

• Defective teeth formation

• Defective bone formation

Requirement : 75 to 100 mg daily.

COENZYMES

 Coenzymes are non-protein organic compounds present in enzymes and associated with them. Coenzymes accelerate enzyme action. They differ from enzymes in the following aspects.

1. They are non-protein in nature

2. They have a low molecular weight

3. They are heat stable

4. They can be separated by dialysis

5. They are generally derived from vitamins

Classification of coenzymes : Coenzymes can be broadly classified as -

i) Groups transferring coenzymes -

TPP

Biotin

Coenzyme A

ii) Hydrogen transferring coenzymes -

NAD, NADP

FAD, FMN

Functions of Coenzymes : The important functions performed by coenzymes is to transfer hydrogen or group. The coenzymes accept the atom or a group from the substrate and transfer them to other molecules. The following is the list of coenzymes and the function performed -

NAD, NADP - Hydrogen transfer

FAD,FMN - Hydrogen transfer

TPP - Acetyl group transfer

Biotin - Carboxyl group transfer

Coenzyme A - Acyl group transfer

Co-ENZYME A (CoA) : It is the coenzyme form of pantothenic acid. It is composed of ATP, pantothenic acid and mercaptoethylamine. It is a group transferring coenzymes. The reaction of co-enzyme A is due to the presence of sulphydryl group. It accepts acetyl groups to form acetyl CoA, which takes part in a number of metabolic reactions.

CoA is required for -

• Conversion of Alpha-ketoglutarate to succinyl CoA in citric acid cycle

• Oxidation of fatty acids

ACETYL CoA : It is the coenzyme form of pantothenic acid. It is formed by the transfer of acetate to coenzyme A.

Acetyl CoA takes part in the following metabolic reactions -

• It is utilised in citric acid cycle

• It combines with choline to form acetylcholine

• It is the starting material for cholesterol synthesis

• It is the starting material for the synthesis of ketone bodies

• It is used in the synthesis and elongation of fatty acids

• It is used for the synthesis of steroid hormones