Robertson D E, Noll D, Roberts M F
Department of Chemistry, Boston College, Chestnut Hill, Massachusetts 02167.
J Biol Chem. 1992 Jul 25;267(21):14893-901.
Methanogenic archaebacteria respond to osmotic stress by accumulating a series of organic molecules which function as compatible solutes. In two strains of marine methanogenic archaebacteria, Methanogenium cariaci and Methanococcus thermolithotrophicus, four key organic solutes are observed: L-alpha-glutamate, beta-glutamate, N epsilon-acetyl-beta-lysine, and betaine. The first three of these are synthesized de novo; betaine is transported into the Mg. cariaci cells from the medium. Mesophilic Mg. cariaci will preferentially transport betaine from the extracellular medium if it is present to counterbalance the external NaCl. In its absence it synthesizes N epsilon-acetyl-beta-lysine as the dominant osmolyte. This zwitterionic compound occurs at levels in Mg. cariaci which are considerably greater (based on mumol/mg of protein) than in Mc. thermolithotrophicus grown in media of the same ionic strength. Intracellular potassium ion concentrations, determined by 39K NMR spectroscopy and atomic absorption, differ significantly in the two cells. In Mc. thermolithotrophicus, intracellular K+ is balanced by the total concentration of anionic amino acid species, glutamate, and beta-glutamate. Turnover of the organic solutes has been monitored using 13C-pulse/12C-chase, and 15N-pulse/14N-chase experiments. Both beta-amino acids exhibit slower turnover rates when compared to L-alpha-glutamate or aspartate, consistent with their roles as compatible solutes. Biosynthetic information for the beta-amino acids is also provided by 13C-labeling experiments. beta-Glutamate shows a lag in 13C uptake from 13CO2, indicative of its biosynthesis from a precursor (probably a macromolecule) not in equilibrium with the soluble L-alpha-glutamate pool. Confirmation of a novel route for beta-glutamate synthesis and the production of the beta-lysine moiety from the diaminopimelate pathway is deduced from [13C2]acetate labeling patterns.
产甲烷古细菌通过积累一系列作为相容性溶质发挥作用的有机分子来应对渗透胁迫。在两种海洋产甲烷古细菌菌株，即卡里亚奇产甲烷菌（Methanogenium cariaci）和嗜热石炭营养甲烷球菌（Methanococcus thermolithotrophicus）中，观察到四种关键的有机溶质：L-α-谷氨酸、β-谷氨酸、Nε-乙酰-β-赖氨酸和甜菜碱。其中前三种是从头合成的；甜菜碱是从培养基转运到卡里亚奇产甲烷菌细胞中的。嗜温的卡里亚奇产甲烷菌如果有甜菜碱存在，会优先从细胞外培养基中转运甜菜碱来平衡外部的氯化钠。如果没有甜菜碱，它会合成Nε-乙酰-β-赖氨酸作为主要的渗透溶质。这种两性离子化合物在卡里亚奇产甲烷菌中的含量（基于每毫克蛋白质的微摩尔数）比在相同离子强度培养基中生长的嗜热石炭营养甲烷球菌中的含量要高得多。通过39K核磁共振光谱和原子吸收测定，两种细胞中的细胞内钾离子浓度有显著差异。在嗜热石炭营养甲烷球菌中，细胞内的K+由阴离子氨基酸种类、谷氨酸和β-谷氨酸的总浓度平衡。使用13C脉冲/12C追踪和15N脉冲/14N追踪实验监测了有机溶质的周转情况。与L-α-谷氨酸或天冬氨酸相比，两种β-氨基酸的周转速度都较慢，这与其作为相容性溶质的作用一致。13C标记实验也提供了β-氨基酸的生物合成信息。β-谷氨酸从13CO2摄取13C存在滞后现象，这表明它是由一种与可溶性L-α-谷氨酸库不平衡的前体（可能是一种大分子）生物合成的。从[13C2]乙酸盐标记模式推断出β-谷氨酸合成的新途径以及二氨基庚二酸途径产生β-赖氨酸部分的证据。
