Krulwich T A, Ito M, Gilmour R, Sturr M G, Guffanti A A, Hicks D B
Department of Biochemistry, Mount Sinai School of Medicine of the City University of New York, NY 10029, USA.
Biochim Biophys Acta. 1996 Jul 18;1275(1-2):21-6. doi: 10.1016/0005-2728(96)00044-8.
Over a decade of work on extremely alkaliphilic Bacillus species has clarified the extraordinary capacity that these bacteria have for regulating their cytoplasmic pH during growth at pH values well over 10. However, a variety of interesting energetic problems related to their Na(+)-dependent pH homeostatic mechanism are yet to be solved. They include: (1) the clarification of how cell surface layers play a role in a category of alkaliphiles for which this is the case; (2) identification of the putative, electrogenic Na+/H+ antiporter(s) that, in at least some alkaliphiles, may completely account for a cytoplasmic pH that is over 2 pH units lower than the external pH; (3) the determination of whether specific modules or accessory proteins are essential for the efficacy of such antiporters; (4) the mechanistic basis for the increase in the transmembrane electrical potential at the high external pH values at which the potential-consuming antiporter(s) must be most active; and (5) an explanation for the Na(+)-specificity of pH homeostasis in the extremely alkaliphilic bacilli as opposed to the almost equivalent efficacy of K+ for pH homeostasis in at least some non-alkaliphilic aerobes. The current status of such studies and future strategies will be outlined for this central area of alkaliphile energetics. Also considered, will be strategies to elucidate the basis for robust H(+)-coupled oxidative phosphorylation by alkaliphiles at pH values over 10. The maintenance of a cytoplasmic pH over 2 units below the high external pH results in a low bulk electrochemical proton gradient (delta p). To bypass this low delta p, Na(+)-coupling is used for solute uptake even by alkaliphiles that are mesophiles from environments that are not especially Na(+)-rich. This indicates that these bacteria indeed experience a low delta p, to which such coupling is an adaptation. Possible reasons and mechanisms for using a H(+)-coupled rather than a Na(+)-coupled ATP synthase under such circumstances will be discussed.
在对极端嗜碱芽孢杆菌属进行了十多年的研究工作后，已阐明这些细菌在pH值远高于10的环境中生长时，具有非凡的调节细胞质pH的能力。然而，与其依赖Na⁺的pH稳态机制相关的各种有趣的能量问题仍有待解决。这些问题包括：（1）阐明细胞表面层在这类嗜碱菌中所起的作用；（2）鉴定假定的电生Na⁺/H⁺反向转运蛋白，在至少一些嗜碱菌中，该蛋白可能完全解释了比外部pH低超过2个pH单位的细胞质pH；（3）确定特定模块或辅助蛋白对于此类反向转运蛋白的功效是否至关重要；（4）在高外部pH值下跨膜电势增加的机制基础，此时消耗电势的反向转运蛋白必定最为活跃；（5）解释极端嗜碱芽孢杆菌中pH稳态对Na⁺的特异性，与之形成对比的是，至少一些非嗜碱需氧菌中K⁺对pH稳态几乎具有同等功效。本文将概述此类研究的现状以及嗜碱菌能量学这一核心领域的未来策略。同时，还将考虑阐明嗜碱菌在pH值超过10时通过H⁺偶联进行稳健氧化磷酸化的基础的策略。细胞质pH维持在比高外部pH低2个单位以上会导致低的整体电化学质子梯度（Δp）。为了绕过这种低Δp，即使是来自并非特别富含Na⁺环境的嗜温嗜碱菌，也会利用Na⁺偶联进行溶质摄取。这表明这些细菌确实经历了低Δp，而这种偶联是对其的一种适应。将讨论在这种情况下使用H⁺偶联而非Na⁺偶联的ATP合酶的可能原因和机制。
