Characterization of osmolyte betaine synthesizing sarcosine dimethylglycine <Emphasis Type="Italic">N</Emphasis>-methyltransferase from <Emphasis Type="Italic">Methanohalophilus portucalensis</Emphasis>

To overcome the extracellular salt stress, Methanohalophilus portucalensis FDF1^T synthesizes the compatible solute betaine through the methylation of glycine, sarcosine, and N,N-dimethylglycine. S-adenosylmethionine (AdoMet) is the methyl donor. The enzyme sarcosine dimethylglycine N-methyltransferase (SDMT) of M. portucalensis, that catalyzes the formation of N,N-dimethylglycine and glycine betaine, has been purified and characterized. SDMT, a monomer of 33 kDa with a pI at 5.03, has a narrow substrate specificity limited to using only sarcosine and dimethylglycine as substrates for the methyl transferase reaction. The K _m values for sarcosine and AdoMet were 2.29 and 0.21 mM, respectively, with a V _max of 0.83 μmol/mg-min (k _cat value of 0.44 s⁻¹). The K _m values for dimethylglycine and AdoMet were 3.76 and 0.59 mM, respectively, with a V _max of 4.88 μmol/mg-min (k _cat of 2.68 s⁻¹). A high concentration of the end product betaine (2.0 M) did not affect the SMT activity, but it slightly inhibited the DMT activity. Both activities were also not affected by potassium or sodium ions in concentrations of 200–1,000 mM. We compared this novel archaeal SDMT enzyme to other similar bacterial transferases as well as to the glycine sarcosine dimethylglycine methyltransferase found also in M. portucalensis.