Methionine synthase (MS, MeSe, MTR) is primarily responsible for the regeneration of methionine from homocysteine in most individuals. In humans it is encoded by the MTRgene (5-methyltetrahydrofolate-homocysteine methyltransferase).[5][6] Methionine synthase forms part of the S-adenosylmethionine (SAMe) biosynthesis and regeneration cycle,[7] and is the enzyme responsible for linking the cycle to one-carbon metabolism via the folate cycle. There are two primary forms of this enzyme, the Vitamin B12 (cobalamin)-dependent (MetH) and independent (MetE) forms,[8] although minimal core methionine synthases that do not fit cleanly into either category have also been described in some anaerobic bacteria.[9] The two dominant forms of the enzymes appear to be evolutionary independent and rely on considerably different chemical mechanisms.[10]Mammals and other higher eukaryotes express only the cobalamin-dependent form. In contrast, the distribution of the two forms in Archaeplastida (plants and algae) is more complex. Plants exclusively possess the cobalamin-independent form,[11] while algae have either one of the two, depending on species.[12] Many different microorganisms express both the cobalamin-dependent and cobalamin-independent forms.[13]
^Zydowsky TM (1986). "Stereochemical analysis of the methyl transfer catalyzed by cobalamin-dependent methionine synthase from Escherichia coli B". Journal of the American Chemical Society. 108 (11): 3152–3153. doi:10.1021/ja00271a081.
