Glycine/sarcosine N-methyltransferase (, ApGSMT, glycine-sarcosine methyltransferase, GSMT, GMT, glycine sarcosine N-methyltransferase, S-adenosyl-L-methionine:sarcosine N-methyltransferase) is an enzyme with systematic name S-adenosyl-L-methionine:glycine(or sarcosine) N-methyltransferase (sarcosine(or N,N-dimethylglycine)-forming). It catalyses the following overall chemical reaction
The enzyme adds two methyl groups, with the intermediate, sarcosine, being methylated again to give the product N,N-dimethyl glycine. The methyl groups come from the cofactor, S-adenosyl methionine (SAM), which becomes S-adenosyl-L-homocysteine (SAH). The enzyme was characterised from the cyanobacterium, Aphanothece halophytica. N,N-dimethyl glycine is subsequently converted to betaine by the enzyme sarcosine/dimethylglycine N-methyltransferase.
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