Methionine synthase (MTR)
Cofactors used
Clinical / pharmacological. B12-dependent; deficiency causes megaloblastic anaemia and the methyl-folate trap
What it does, reaction by reaction
Methionine Cycle / Transmethylation (SAM Cycle, homocysteine) Amino Acid & Nitrogen Metabolism · Cytosol
Homocysteine + 5-methyl-THF → methionine + THF
Converts Homocysteine 5-Methyl-THF into Methionine Tetrahydrofolate (THF)
› Notes
Methionine synthase (MTR) requires methylcobalamin (vitamin B12) and links the methionine cycle to the folate cycle. Reductive reactivation of oxidized cobalamin requires methionine synthase reductase, NADPH, FAD, FMN, and SAM; remethylation is the principal methionine-conserving route in most tissues.
One-Carbon Metabolism (Folate Cycle) Amino Acid & Nitrogen Metabolism · Cytosol + mitochondrion
5-Methyl-THF + homocysteine → THF + methionine
Converts 5-Methyl-THF Homocysteine into Tetrahydrofolate (THF) Methionine
› Notes
Methionine synthase (5-methyltetrahydrofolate–homocysteine methyltransferase) requires methylcobalamin (vitamin B12) and transfers the methyl group through a cobalamin intermediate. The reaction regenerates THF and prevents trapping of cellular folate as 5-methyl-THF.
Showing all 2 reactions.
What accelerates and inhibits it
Regulation is pathway-specific, so each context is listed separately
Methionine Cycle / Transmethylation (SAM Cycle, homocysteine)
Listed there as: Methionine synthase
5-methyl-THF, homocysteine, and functional B12 availability
B12 deficiency/inactivation; nitrous oxide oxidizes cobalamin and inhibits function
Mainly controlled by vitamin status and substrate flux
Recent literature
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