Krulwich T A
J Membr Biol. 1986;89(2):113-25. doi: 10.1007/BF01869707.
The central problem for organisms which grow optimally, and in some cases obligately, at pH values of 10 to 11, is the maintenance of a relatively acidified cytoplasm. A key component of the pH homeostatic mechanism is an electrogenic Na+/H+ antiporter which--by virtue of kinetic properties and/or its concentration in the membrane--catalyzes net proton uptake while the organisms extrude protons during respiration. The antiporter is also capable of maintaining a constant pHin during profound elevations in pHout as long as Na+ entry is facilitated by the presence of solutes which are taken up with Na+. Secondary to the problem of acidifying the interior is the adverse effect of the large pH gradient, acid in, on the total pmf of alkalophile cells. For the purposes of solute uptake and motility, the organisms appear to largely bypass the problem of a low pmf by utilizing a sodium motive force for energization. However, ATP synthesis appears not to resolve the energetics problem by using Na+ or by incorporating the proton-translocating ATPase into intracellular organelles. The current data suggest that effective proton pumping carried out by the alkalophile respiratory chain at high pH may deliver at least some portion of the protons to the proton-utilizing catalysts, i.e., the F1F0-ATPase and the Na+/H+ antiporter, by some localized pathway.
对于那些在pH值为10至11时能最佳生长，在某些情况下甚至是必须在此pH值下生长的生物体来说，核心问题是维持相对酸化的细胞质。pH稳态机制的一个关键组成部分是一种电致Na⁺/H⁺反向转运蛋白，它凭借动力学特性和/或其在膜中的浓度，在生物体呼吸过程中排出质子时催化净质子摄取。只要有与Na⁺一起被摄取的溶质存在从而促进Na⁺进入，该反向转运蛋白就能在胞外pH大幅升高期间维持胞内pH恒定。在使内部酸化这一问题之后，胞内酸性、胞外碱性的大pH梯度对嗜碱菌细胞总质子动力势产生不利影响。出于溶质摄取和运动的目的，生物体似乎通过利用钠动力势来供能，在很大程度上避开了低质子动力势的问题。然而，ATP合成似乎无法通过利用Na⁺或将质子转运ATP酶纳入细胞内细胞器来解决能量学问题。目前的数据表明，嗜碱菌呼吸链在高pH值下进行的有效质子泵浦可能通过某种局部途径将至少一部分质子传递给利用质子的催化剂，即F₁F₀ - ATP酶和Na⁺/H⁺反向转运蛋白。
