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Simplified schematic of the reaction of methionine synthase

This reaction serves as the interface of the folate cycle and the methionine remethylation cycle. The coenzyme, cobalamin (vitamin B12), utilizes cobalt to catalyze the transferring function in which the cobalt switches between having 1 or 3 valence electrons, dubbed cob(I)alamin, and cob(III)alamin. Over time, the cob(I)alamin cofactor of methionine synthase becomes oxidized to cob(II)alamin, rendering the enzyme inactive. Regeneration of methionine synthase requires reductive methylation via a reaction catalyzed by methionine synthase reductase in which S-adenosylmethionine is utilized as a methyl donor, reducing cob(II)alamin to cob(I)alamin (not shown). (Illustration created by "Boghog" on September 27, 2019. Modified by Dr. Douglas Lanska. Creative Commons Attribution-Share Alike 4.0 International License.)