Cobalamin-dependent methionine synthase ( EC:2.1.1.13 ) is a large modular protein that catalyses methyl transfer from methyltetrahydrofolate (CH3-H4folate) to homocysteine. During the catalytic cycle, it supports three distinct methyl transfer reactions, each involving the cobalamin (vitamin B12) cofactor and a substrate bound … expand

Cobalamin-dependent methionine synthase ( EC:2.1.1.13 ) is a large modular protein that catalyses methyl transfer from methyltetrahydrofolate (CH3-H4folate) to homocysteine. During the catalytic cycle, it supports three distinct methyl transfer reactions, each involving the cobalamin (vitamin B12) cofactor and a substrate bound to its own functional unit [ PUBMED:11731805 ]. The cobalamin cofactor plays an essential role in this reaction, accepting the methyl group from CH3-H4folate to form methylcob(III)alamin, and in turn donating the methyl group to homocysteine to generate methionine and cob(I)alamin.

Methionine synthase is a large enzyme composed of four structurally and functionally distinct modules: the first two modules bind homocysteine and CH3-H4folate, the third module binds the cobalamin cofactor and the C-terminal module binds S-adenosylmethionine. The cobalamin-binding module is composed of two structurally distinct domains: a 4-helical bundle cap domain (residues 651-740 in the Escherichia coli enzyme) and an α/β B12-binding domain (residues 741-896) ( IPR006158 ). The 4-helical bundle forms a cap over the α/β domain, which acts to shield the methyl ligand of cobalamin from solvent [ PUBMED:8939751 ]. Furthermore, in the conversion to the active conformation of this enzyme, the 4-helical cap rotates to allow the cobalamin cofactor to bind the activation domain ( IPR004223 ). The α/β domain is a common cobalamin-binding motif, whereas the 4-helical bundle domain with its methyl cap is a distinctive feature of methionine synthases.

This entry represents the 4-helical bundle cap domain. This domain is also present in other shorter proteins that bind to B12, and is always found N terminus to the α/β B12-binding domain.