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. Author manuscript; available in PMC: 2011 Jun 11.
Published in final edited form as: Antioxid Redox Signal. 2007 Nov;9(11):1911–1921. doi: 10.1089/ars.2007.1795

Figure 4.

Figure 4

Reactions catalyzed by mammalian methionine synthase. During catalysis, the cobalamin cofactor of methionine synthase cycles between the methylcobalamin and cob(I)alamin forms, as the cofactor is alternately methylated by methyltetrahydrofolate (CH3-THF) and demethylated by homocysteine to form methionine. Cob(I)alamin is occasionally oxidized to form an inactive cob(II)alamin species; return of this species to the catalytic cycle requires a reductive remethylation of the cofactor in which an electron is provided by the auxiliary protein methionine synthase reductase (MSR) and a methyl group is provided by AdoMet, which is bound to the reactivation module of methionine synthase itself.