Pharma gyan Pandit

Biochemical Role of Vitamin B12 (Cyanocobalamin)

The different types of vitamin B12 described above are actually artifacts and the various groups attached to the Co atom are incorporated in the structure during isolation processes.  In the body all types of Vitamin B12 are converted to coenzymes B12 which are called cobamides. These coenzymes do not contain –CN or other groups mentioned above but contain one of the two groups attached to the Co atom.

Adenine nucleoside (5-deoxyadenosine). There are three coenzymes of this type which differ from each other in respect of benzimidazole portion of the vitamin B12 molecule. One of these has 5,6,dimethylbenzimidazole, the second has benzimidazole and the third has adenyl group; they are respectively called DBC, BC and AC. In human liver DBC is the most predominant coenzyme B12.

Methyle group: A coenzyme B12 containing a methyl group attached to the Co atom has also been discovered.

Reactions in which the coenzyme B12 are involved are given below

Transmethylation reaction in which thymine, methionine and choline are synthesized.

In this case the coenzyme B12 acts as a hydrogen transfer. Deoxyribonucleotides are utilized in the synthesis of DNA.

In vitamin B12 deficiency methylamalonic acid accumulates and is excreted in urine in increased amounts.

Other reactions in which this vitamin has been shown to take part are the conversion of folic acid to its active coenzyme forms, metabolism of several amino acids, choline and cholesterol, porphyrin biosynthesis and activation of amino acids for protein synthesis.