Biochemical Role of Vitamin B12

Abstract

Vitamin B12 func-tions most likely in the form of a coenzyme which differs from the vitamin in that in place of CN there is a 5[image]-deoxyadenosyl group attached to cobalt by the C5 of the 5[image]-deoxyribose moiety. The presence of a cobalt-to-carbon bond in B12 coenzyme accounts for many of the unique properties of these compounds. Visible light, cyanide, and acid cleave this bond. Four defined enzymatic reactions show a requirement for a cobamide compound. A cobamide coenzyme is essential for isomerization of glutamate to [beta]-methylaspartate, CoA to succinyl CoA, and the isomerization of methylmalonyl. It is also required for the enzymatic conversion of ethylene glycol to acetaldehyde, of propylene glycol to propion aldehyde. The synthesis of methionine requires a cobamide-containing protein. In addition, the vitamin or derivatives were implicated in lysine fermentation, deoxyribonucleic acid (DNA) formation, and sulfhydryl reduction.