is carried out by a single domain flavodoxin protein. The reductase protein is responsible for transfer of an electron from a reduced FMN cofactor to the inactive Cob(II), which enables regeneration of the active methylcobalamin enzyme via methyl transfer from S-adenosylmethionine to the reduced Cob(I) intermediate. This process is known as the reactivation cycle, and is thought to be gated from the normal catalytic cycle by large-scale conformational rearrangements within the enzyme. Because the oxidation of Cob(I) inevitably shuts down cob-dependent methionine synthase activity, defects or deficiencies in methionine synthase reductase have been implicated in some of the disease associations for methionine synthase deficiency. The mechanism of the cobalamin-independent (MetE) form, by contrast, proceeds through a direct methyl transfer from the activated N5-MeTHF to zinc thiolate homocysteine. Although the mechanism is considerably simpler, the direct transfer reaction is much less favorable than the cobalamin-mediated reactions and as a result the turnover rate for MetE is ~100x slower than that of MetH. As it does not contain the cobalamin cofactor, the cobalamin-independent enzyme is not prone to oxidative inactivation Structure High-resolution structures have been solved by X-ray crystallography for intact MetE both in the absence and presence of substrates and for fragments of MetH, although no structural description exists of a fully intact MetH enzyme. The available structures and accompanying bioinformatic analysis indicate minimal similarity in the overall structure, although there are similarities within the substrate-binding sites themselves. Cob-dependent MetH is divided into 4 separate domains. The domains, from N- to C-terminus, are denoted homocysteine binding (Hcy domain), N5-methylTHF binding (MTHF domain) Cobalamin-binding (Cob domain) and the S-adenosymethionine-binding or reactivation domain. The reactivation domain binds SAM and is the site of interaction with flavodoxin or Methionine Synthase Reductase during the reactivation cycle of the enzyme. The cobalamin-binding domain contains two subdomains, with the cofactor bound to the Rossman-fold B12-binding subdomain, which is in turn capped by the other subdomain, the four-helix bundle cap subdomain. The four-helix bundle serves to protect the cobalamin cofactor from unwanted reactivity, but can significantly change conformations to expose the cofactor allow it access to the other substrates during turnover. Both the Hcy and N5-MeTHF domains adopt a TIM barrel architecture; the Hcy domain contains the zinc-binding site, which in MetH consists of three cysteine residues coordinated to a zinc ion which in turn binds and activates Hcy. The N5-MeTHF binding domain binds and activates N5-MeTHF via