Characterization of Glycine SarcosineN-Methyltransferase and Sarcosine DimethylglycineN-Methyltransferase

ABSTRACT Glycine betaine is accumulated in cells living in high salt concentrations to balance the osmotic pressure. Glycine sarcosineN-methyltransferase (GSMT) and sarcosine dimethylglycineN-methyltransferase (SDMT) of Ectothiorhodospira halochloris catalyze the threefold methylation of glycine to betaine, with S-adenosylmethionine acting as the methyl group donor. These methyltransferases were expressed inEscherichia coli and purified, and some of their enzymatic properties were characterized. Both enzymes had high substrate specificities and pH optima near the physiological pH. No evidence of cofactors was found. The enzymes showed Michaelis-Menten kinetics for their substrates. The apparent Km andV max values were determined for all substrates when the other substrate was present in saturating concentrations. Both enzymes were strongly inhibited by the reaction productS-adenosylhomocysteine. Betaine inhibited the methylation reactions only at high concentrations.