Catalyzes the methylation of glycine by using S-adenosylmethionine (AdoMet) to form N-methylglycine (sarcosine) with the concomitant production of S-adenosylhomocysteine (AdoHcy), a reaction regulated by the binding of 5-methyltetrahydrofolate (PubMed:8281755). Plays an important role in the regulation of methyl group metabolism by regulating the ratio between S-adenosyl-L-methionine and S-adenosyl-L-homocysteine (By similarity). glycine + S-adenosyl-L-methionine = H(+) + S-adenosyl-L-homocysteine + sarcosine Inhibited by 5-methyltetrahydrofolate monoglutamate and by 5-methyltetrahydrofolate pentaglutamate, inhibition is much more effective by the pentaglutamate form than by the monoglutamate form. Two molecules of 5-methyltetrahydrofolate are bound per tetramer. The binding sites are localized between subunits. Inhibitor binding may preclude movements of the polypeptide chain that are necessary for enzyme activity. Homotetramer. Abundant in liver. Belongs to the class I-like SAM-binding methyltransferase superfamily. Glycine N-methyltransferase family. Extended N-terminus.