Washington State University, Pullman, 99164, USA.
Biochem J. 2010 Aug 15;430(1):97-105. doi: 10.1042/BJ20100566.
SHMT (serine hydroxymethyltransferase; EC 2.1.2.1) catalyses reversible hydroxymethyl group transfer from serine to H4PteGlun (tetrahydrofolate), yielding glycine and 5,10-methylenetetrahydrofolate. In plastids, SHMTs are thought to catalytically direct the hydroxymethyl moiety of serine into the metabolic network of H4PteGlun-bound one-carbon units. Genes encoding putative plastid SHMTs were found in the genomes of various plant species. SHMT activity was detected in chloroplasts in pea (Pisum sativum) and barley (Hordeum vulgare), suggesting that plastid SHMTs exist in all flowering plants. The Arabidopsis thaliana genome encodes one putative plastid SHMT (AtSHMT3). Its cDNA was cloned by reverse transcription-PCR and the encoded recombinant protein was produced in Escherichia coli. Evidence that AtSHMT3 is targeted to plastids was found by confocal microscopy of A. thaliana protoplasts transformed with proteins fused to enhanced green fluorescent protein. Characterization of recombinant AtSHMT3 revealed that substrate affinity for and the catalytic efficiency of H4PteGlu1-8 increase with n, and that H4PteGlu1-8 inhibit AtSHMT3. 5-Methyltetrahydrofolate and 5-formyltetrahydrofolate with one and five glutamate residues inhibited AtSHMT3-catalysed hydroxymethyl group transfer from serine to H4PteGlu6, with the pentaglutamylated inhibitors being more effective. Calculations revealed inhibition with 5-methyltetrahydrofolate or 5-formyltetrahydrofolate resulting in little reduction in AtSHMT3 activity under folate concentrations estimated for plastids.
丝氨酸羟甲基转移酶（SHMT；EC 2.1.2.1）催化可逆的羟甲基基团从丝氨酸转移到 H4PteGlun（四氢叶酸），生成甘氨酸和 5,10-亚甲基四氢叶酸。在质体中，SHMT 被认为催化性地将丝氨酸的羟甲基部分定向到 H4PteGlun 结合的一碳单元代谢网络中。在各种植物物种的基因组中发现了编码假定质体 SHMT 的基因。豌豆（Pisum sativum）和大麦（Hordeum vulgare）的叶绿体中检测到 SHMT 活性，表明质体 SHMT 存在于所有开花植物中。拟南芥（Arabidopsis thaliana）基因组编码一个假定的质体 SHMT（AtSHMT3）。通过反转录 PCR 克隆其 cDNA，并在大肠杆菌中生产重组蛋白。通过用融合增强型绿色荧光蛋白的蛋白转化的拟南芥原生质体的共聚焦显微镜，发现 AtSHMT3 被靶向质体的证据。重组 AtSHMT3 的特性表明，H4PteGlu1-8 的底物亲和力和催化效率随 n 增加，并且 H4PteGlu1-8 抑制 AtSHMT3。具有一个和五个谷氨酸残基的 5-甲基四氢叶酸和 5-甲酰基四氢叶酸抑制 AtSHMT3 催化的丝氨酸到 H4PteGlu6 的羟甲基基团转移，其中五谷氨酸基抑制剂更有效。计算表明，在估计质体中存在的叶酸浓度下，5-甲基四氢叶酸或 5-甲酰基四氢叶酸的抑制作用导致 AtSHMT3 活性降低很小。
