All are true about Vitamin B12, except
Vitamin B12 is required in humans for two essential enzymatic reactions: the remethylation of homocysteine to methionine and the isomerization of methylmalonyl coenzyme A (CoA) that is produced during the degradation of some amino acids (isoleucine, valine, threonine, and methionine), and fatty acids with odd numbers of carbon atoms (Figure 28.5). When the vitamin is deficient, unusual fatty acids accumulate and become incorporated into cell membranes, including those of the nervous system. This may account for some of the neurologic manifestations of vitamin B12 deficiency. A. Structure of cobalamin and its coenzyme forms Cobalamin contains a corrin ring system that differs from the porphyrins in that two of the pyrrole rings are linked directly rather than through a methene bridge. Cobalt is held in the center of the corrin ring by four coordination bonds from the nitrogens of the pyrrole groups. The remaining coordination bonds of the cobalt are with the nitrogen of 5,6-dimethylbenzimidazole and with cyanide in commercial preparations of the vitamin in the form of cyanocobalamin (Figure 28.6). The coenzyme forms of cobalamin are 5&;-deoxy adenosylcobalamin, in which cyanide is replaced with 5&;-deoxyadenosine (forming an unusual carbon-cobalt bond), and methylcobalamin, in which cyanide is replaced by a methyl group Reference: Lippincott edition 5: page no: 375
Active form is methylcobalamine
Requires for conversion of homocysteine to methionine
Requires in metabolism of methylmalonyl CoA
Requires for conversion of pyruvate to lactate
vitamins