BIOMOLECULES AND ENZYMES

Detailed explanation-1: -In the cytoplasm, methionine synthase requires vitamin B-12 in the form of methylcobalamin and catalyzes the conversion of homocysteine to methionine by transfer of a methyl group from methyltetrahydrofolate.

Detailed explanation-2: -Vitamin B12 Methionine synthetase catalyzes the methylation of Hcy to regenerate methionine, serving as the methyl group carrier (via methylcobalamin) between the donor 5-methyltetrahydrofolate (5-methyl-FH4) and the acceptor Hcy (Fig 17.1). This reaction is a simple transfer of the single-C moiety.

Detailed explanation-3: -Methionine synthase is a B12-dependent enzyme, and deficiency of vitamin B12 can impair the remethylation of homocysteine, thus accumulating cellular folate as 5-methylTHF, a process termed the methyl trap hypothesis (Wagner et al., 1985).

Detailed explanation-4: -Vitamin B12: Physiology, Dietary Sources, and Requirements Vitamin B12, a cobalamin, is required for only two enzyme reactions in humans and animals: The conversion of methylmalonyl CoA to succinyl CoA and addition of a methyl group to homocysteine to form methionine.