(Methionine synthase) reductase

In enzymology, a [methionine synthase] reductase (EC 1.16.1.8) is an enzyme that catalyzes the chemical reaction

2 [methionine synthase]-methylcob(I)alamin + 2 S-adenosylhomocysteine + NADP⁺ ⇌ 2 [methionine synthase]-cob(II)alamin + NADPH + H⁺ + 2 S-adenosyl-L-methionine

The 3 products of this enzyme are methionine synthase-methylcob(I)alamin, S-adenosylhomocysteine, and NADP⁺, whereas its 4 substrates are methionine synthase-cob(II)alamin, NADPH, H⁺, and S-adenosyl-L-methionine.

Over time, the cob(I)alamin cofactor of methionine synthase becomes oxidized to cob(II)alamin rendering the enzyme inactive. Regeneration of functional enzyme requires reductive methylation via a reaction catalyzed by (methionine synthase) reductase in which S-adenosylmethionine is utilized as a methyl donor.

This enzyme belongs to the family of oxidoreductases, to be specific those oxidizing metal ion with NAD+ or NADP+ as acceptor. The systematic name of this enzyme class is [methionine synthase]-methylcob(I)alamin,S-adenosylhomocysteine:NADP+ oxidoreductase. Other names in common use include methionine synthase cob(II)alamin reductase (methylating), methionine synthase reductase, [methionine synthase]-cobalamin methyltransferase (cob(II)alamin, and reducing). It employs one cofactor, flavoprotein.