Nyyssölä A, Reinikainen T, Leisola M
Helsinki University of Technology, Laboratory of Bioprocess Engineering, FIN-02015 HUT Espoo, Finland.
Appl Environ Microbiol. 2001 May;67(5):2044-50. doi: 10.1128/AEM.67.5.2044-2050.2001.
Glycine betaine is accumulated in cells living in high salt concentrations to balance the osmotic pressure. Glycine sarcosine N-methyltransferase (GSMT) and sarcosine dimethylglycine N-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 in Escherichia 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 K(m) and V(max) values were determined for all substrates when the other substrate was present in saturating concentrations. Both enzymes were strongly inhibited by the reaction product S-adenosylhomocysteine. Betaine inhibited the methylation reactions only at high concentrations.
甘氨酸甜菜碱在生活于高盐浓度环境的细胞中积累，以平衡渗透压。嗜盐嗜盐红螺菌的甘氨酸肌氨酸N-甲基转移酶（GSMT）和肌氨酸二甲基甘氨酸N-甲基转移酶（SDMT）催化甘氨酸三步甲基化生成甜菜碱，其中S-腺苷甲硫氨酸作为甲基供体。这些甲基转移酶在大肠杆菌中表达并纯化，对其一些酶学性质进行了表征。两种酶都具有高底物特异性，且最适pH接近生理pH。未发现辅因子的证据。这些酶对其底物表现出米氏动力学。当另一种底物以饱和浓度存在时，测定了所有底物的表观K(m)和V(max)值。两种酶都受到反应产物S-腺苷同型半胱氨酸的强烈抑制。甜菜碱仅在高浓度时抑制甲基化反应。
