Xing R Y, Whitman W B
Department of Microbiology, University of Georgia, Athens 30602.
J Bacteriol. 1991 Mar;173(6):2086-92. doi: 10.1128/jb.173.6.2086-2092.1991.
Methanococcus aeolicus, Methanococcus maripaludis, and Methanococcus voltae contain similar levels of four enzymes of branched-chain amino acid biosynthesis: acetohydroxy acid synthase, acetohydroxy acid isomeroreductase, dihydroxy acid dehydratase, and transaminase B. Following growth at low partial pressures of H2-CO2, the levels of these enzymes in extracts of M. voltae are reduced three- to fivefold, which suggests that their synthesis is regulated. The enzymes from M. aeolicus were found to be similar to the eubacterial and eucaryotic enzymes with respect to molecular weights, pH optima, kinetic properties, and sensitivities to O2. The acetohydroxy acid isomeroreductase has a specific requirement for Mg2+, and other divalent cations were inhibitory. It was stimulated threefold by K+ and NH4+ ions and was able to utilize NADH as well as NADPH. The partially purified enzyme was not sensitive to O2. The dihydroxy acid dehydratase is extremely sensitive to O2, and it has a half-life under 5% O2 of 6 min at 25 degrees C. Divalent cations were required for activity, and Mg2+, Mn2+, Ni2+, Co2+, and Fe2+ were nearly equally effective. In conclusion, the archaebacterial enzymes are functionally homologous to the eubacterial and eucaryotic enzymes, which implies that this pathway is very ancient.
嗜热栖热甲烷球菌、沼泽甲烷球菌和沃氏甲烷球菌中支链氨基酸生物合成的四种酶水平相似：乙酰羟酸合酶、乙酰羟酸异构还原酶、二羟酸脱水酶和转氨酶B。在低H₂-CO₂分压下生长后，沃氏甲烷球菌提取物中这些酶的水平降低了三到五倍，这表明它们的合成受到调控。已发现嗜热栖热甲烷球菌的这些酶在分子量、最适pH值、动力学性质和对O₂的敏感性方面与真细菌和真核生物的酶相似。乙酰羟酸异构还原酶对Mg²⁺有特定需求，其他二价阳离子有抑制作用。它受到K⁺和NH₄⁺离子的三倍刺激，并且能够利用NADH以及NADPH。部分纯化的酶对O₂不敏感。二羟酸脱水酶对O₂极其敏感，在25℃下5%O₂环境中的半衰期为6分钟。二价阳离子是酶活性所必需的，Mg²⁺、Mn²⁺、Ni²⁺、Co²⁺和Fe²⁺的效果几乎相同。总之，古细菌的这些酶在功能上与真细菌和真核生物的酶同源，这意味着这条途径非常古老。
