Krulwich T A, Guffanti A A
Department of Biochemistry, Mount Sinai School of Medicine, City University of New York, New York 10029.
J Bioenerg Biomembr. 1992 Dec;24(6):587-99. doi: 10.1007/BF00762351.
Oxidative phosphorylation, which involves an exclusively proton-coupled ATP synthase, and pH homeostasis, which depends upon electrogenic antiport of cytoplasmic Na+ in exchange for H+, are the two known bioenergetic processes that require inward proton translocation in extremely alkaliphilic bacteria. Energy coupling to oxidative phosphorylation is particularly difficult to fit to a strictly chemiosmotic model because of the low bulk electrochemical proton gradient that follows from the maintenance of a cytoplasmic pH just above 8 during growth at pH 10.5 and higher. A large quantitative and variable discrepancy between the putative chemiosmotic driving force and the phosphorylation potential results. This is compounded by a nonequivalence between respiration-dependent bulk gradients and artificially imposed ones in energizing ATP synthesis, and by an apparent requirement for specific respiratory chain complexes that do not relate solely to their role in generation of bulk gradients. Special features of the synthase may contribute to the mode of energization, just as novel features of the Na+ cycle may relate to the extraordinary capacity of the extreme alkaliphiles to achieve pH homeostasis during growth at, or sudden shifts to, an external pH of 10.5 and above.
氧化磷酸化（涉及一种仅质子偶联的ATP合酶）和pH稳态（依赖于细胞质中Na⁺与H⁺进行电致反向转运）是已知的两种生物能量过程，在极端嗜碱细菌中需要质子向内转运。由于在pH 10.5及更高的环境中生长时，维持细胞质pH略高于8会导致较低的整体电化学质子梯度，因此能量与氧化磷酸化的偶联特别难以符合严格的化学渗透模型。在假定的化学渗透驱动力和磷酸化电位之间产生了很大的定量和可变差异。呼吸依赖的整体梯度与用于驱动ATP合成的人工施加的梯度之间的不等价性，以及对特定呼吸链复合物的明显需求（这些需求不仅仅与其在产生整体梯度中的作用相关），使情况更加复杂。合酶的特殊特征可能有助于能量供应模式，就像Na⁺循环的新特征可能与极端嗜碱菌在外部pH为10.5及以上的环境中生长或突然转变时实现pH稳态的非凡能力有关一样。
