Glycine/sarcosine/dimethylglycine N-methyltransferase (EC 2.1.1.162, GSDMT, glycine sarcosine dimethylglycine N-methyltransferase) is an enzyme with systematic name S-adenosyl-L-methionine:glycine(or sarcosine or N,N-dimethylglycine) N-methyltransferase (sarcosine(or N,N-dimethylglycine or betaine)-forming).^[1] This enzyme catalyses the following chemical reaction

3 S-adenosyl-L-methionine + glycine ${\displaystyle \rightleftharpoons }$ 3 S-adenosyl-L-homocysteine + betaine (overall reaction)

(1a) S-adenosyl-L-methionine + glycine ${\displaystyle \rightleftharpoons }$ S-adenosyl-L-homocysteine + sarcosine

(1b) S-adenosyl-L-methionine + sarcosine ${\displaystyle \rightleftharpoons }$ S-adenosyl-L-homocysteine + N,N-dimethylglycine

(1c) S-adenosyl-L-methionine + N,N-dimethylglycine ${\displaystyle \rightleftharpoons }$ S-adenosyl-L-homocysteine + betaine

This enzyme from the halophilic methanoarchaeon Methanohalophilus portucalensis can methylate glycine and all of its intermediates to form the compatible solute trimethylglycine.