| Type: | Domain | Name: | Cobalamin (vitamin B12)-binding module, cap domain |
| Description: | Cobalamin-dependent methionine synthase () 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 []. 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 colienzyme) and an alpha/beta B12-binding domain (residues 741-896) (). The 4-helical bundle forms a cap over the alpha/beta domain, which acts to shield the methyl ligand of cobalamin from solvent []. 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 (). The alpha/beta 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 alpha/beta B12-binding domain. | Short Name: | Cbl-bd_cap |
| DB identifier | Type | Name |
|---|---|---|
| IPR011822 | Family | Cobalamin-dependent methionine synthase |
| IPR015839 | Family | Bifunctional chloromethane dehalogenation methyltransferase corrinoid-binding protein CmuA |
| IPR017215 | Family | 5-methyltetrahydrofolate-homocysteine S-methyltransferase, bacterial |
| IPR012741 | Domain | Methyltransferase cognate corrinoid protein |
