Serine hydroxymethyltransferase (SHMT) and threonine aldolase are classified as fold type I pyridoxal-5’-phosphate-dependent enzymes and engaged in glycine biosynthesis from serine and threonine, respectively. The acidothermophilic archaeon Thermoplasma acidophilum possesses two distinct SHMT genes, while there is no gene encoding threonine aldolase in its genome. In the present study, the two SHMT genes (Ta0811 and Ta1509) were heterologously expressed in Escherichia coli and Thermococcus kodakarensis, respectively, and biochemical properties of their products were investigated. Ta1509 protein exhibited dual activities to catalyze tetrahydrofolate (THF)-dependent serine cleavage and THF-independent threonine cleavage, similar to other SHMTs reported to date. In contrast, the Ta0811 protein lacks amino acid residues involved in the THF-binding motif and catalyzes only the THF-independent cleavage of threonine. Kinetic analysis revealed that the threonine-cleavage activity of the Ta0811 protein was 3.5 times higher than the serine-cleavage activity of Ta1509 protein. In addition, mRNA expression of Ta0811 gene in T. acidophilum was approximately 20 times more abundant than that of Ta1509. These observations suggest that retroaldol cleavage of threonine, mediated by the Ta0811 protein, has a major role in glycine biosynthesis in T. acidophilum.

丝氨酸羟甲基转移酶 (SHMT) 和苏氨酸醛缩酶被归类为折叠 I 型 pyridoxal-5'-磷酸依赖性酶，分别参与丝氨酸和苏氨酸的甘氨酸生物合成。嗜酸嗜热古菌嗜酸嗜热菌具有两个不同的 SHMT 基因，而其基因组中没有编码苏氨酸醛缩酶的基因。在本研究中，两个 SHMT 基因（Ta0811 和 Ta1509）在大肠杆菌和柯达卡热球菌中异源表达, 并对其产品的生化性质进行了研究。Ta1509 蛋白表现出催化四氢叶酸 (THF) 依赖性丝氨酸裂解和非 THF 依赖性苏氨酸裂解的双重活性，类似于迄今为止报道的其他 SHMT。相比之下，Ta0811 蛋白缺乏参与 THF 结合基序的氨基酸残基，仅催化不依赖 THF 的苏氨酸裂解。动力学分析表明，Ta0811 蛋白的苏氨酸切割活性比 Ta1509 蛋白的丝氨酸切割活性高 3.5 倍。此外，Ta0811 基因在嗜酸乳杆菌中的mRNA 表达比 Ta1509 的丰度高约 20 倍。这些观察结果表明，由 Ta0811 蛋白介导的苏氨酸后羟醛切割在嗜酸嗜酸杆菌中的甘氨酸生物合成中起主要作用。