Helgadóttir Sunna, Rosas-Sandoval Guillermina, Söll Dieter, Graham David E
Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, Connecticut 06520, USA.
J Bacteriol. 2007 Jan;189(2):575-82. doi: 10.1128/JB.01269-06. Epub 2006 Oct 27.
Methanococcus maripaludis and Methanocaldococcus jannaschii produce cysteine for protein synthesis using a tRNA-dependent pathway. These methanogens charge tRNA(Cys) with l-phosphoserine, which is also an intermediate in the predicted pathways for serine and cystathionine biosynthesis. To establish the mode of phosphoserine production in Methanococcales, cell extracts of M. maripaludis were shown to have phosphoglycerate dehydrogenase and phosphoserine aminotransferase activities. The heterologously expressed and purified phosphoglycerate dehydrogenase from M. maripaludis had enzymological properties similar to those of its bacterial homologs but was poorly inhibited by serine. While bacterial enzymes are inhibited by micromolar concentrations of serine bound to an allosteric site, the low sensitivity of the archaeal protein to serine is consistent with phosphoserine's position as a branch point in several pathways. A broad-specificity class V aspartate aminotransferase from M. jannaschii converted the phosphohydroxypyruvate product to phosphoserine. This enzyme catalyzed the transamination of aspartate, glutamate, phosphoserine, alanine, and cysteate. The M. maripaludis homolog complemented a serC mutation in the Escherichia coli phosphoserine aminotransferase. All methanogenic archaea apparently share this pathway, providing sufficient phosphoserine for the tRNA-dependent cysteine biosynthetic pathway.
沼泽甲烷球菌和詹氏甲烷球菌利用一种依赖tRNA的途径合成半胱氨酸用于蛋白质合成。这些产甲烷菌用L-磷酸丝氨酸使tRNA（Cys）负载，L-磷酸丝氨酸也是预测的丝氨酸和胱硫醚生物合成途径中的一个中间体。为了确定甲烷球菌目细菌中磷酸丝氨酸的产生模式，研究发现沼泽甲烷球菌的细胞提取物具有磷酸甘油酸脱氢酶和磷酸丝氨酸转氨酶活性。从沼泽甲烷球菌中异源表达并纯化的磷酸甘油酸脱氢酶，其酶学性质与其细菌同源物相似，但对丝氨酸的抑制作用较弱。虽然细菌酶会被与别构位点结合的微摩尔浓度丝氨酸所抑制，但古细菌蛋白质对丝氨酸的低敏感性与其作为几种途径分支点的磷酸丝氨酸的位置是一致的。来自詹氏甲烷球菌的一种广泛特异性的V类天冬氨酸转氨酶将磷酸羟基丙酮酸产物转化为磷酸丝氨酸。这种酶催化天冬氨酸、谷氨酸、磷酸丝氨酸、丙氨酸和半胱氨酸的转氨作用。沼泽甲烷球菌的同源物弥补了大肠杆菌磷酸丝氨酸转氨酶中的serC突变。所有产甲烷古菌显然都共享这条途径，为依赖tRNA的半胱氨酸生物合成途径提供了足够的磷酸丝氨酸。
