Krulwich T A, Guffanti A A, Seto-Young D
Department of Biochemistry, Mount Sinai School of Medicine, City University of New York, N.Y.
FEMS Microbiol Rev. 1990 Jun;6(2-3):271-8. doi: 10.1111/j.1574-6968.1990.tb04100.x.
A Na+/H+ antiporter catalyses coupled Na+ extrusion and H+ uptake across the membranes of extremely alkalophilic bacilli. This exchange is electrogenic, with H+ translocated inward greater than Na+ extruded. It is energized by the delta chi 2 component of the delta mu H+ that is established during primary proton pumping by the alkalophile respiratory chain complexes. These complexes abound in the membranes of extreme alkalophiles. Combined activity of the respiratory chain, the antiporter, and solute transport systems that are coupled to Na+ re-entry, allow the alkalophiles to maintain a cytoplasmic pH that is several pH units more acidic than optimal external pH values for growth. There is no compelling evidence for a specific and necessary role for any ion other than sodium in pH homeostasis, and although there is very high cytoplasmic buffering capacity in the alkaline range, active mechanisms for pH homeostasis are crucial. Energization of the antiporter as well as the proton translocating F1F0-ATPase that catalyses ATP synthesis in the extreme alkalophiles must accommodate the problem of the low net delta mu H+ and the very low concentrations of protons, per se, in the external medium. This problem is by-passed by other bioenergetic work functions, such as solute uptake or motility, that utilize sodium ions for energy-coupling in the place of protons.
一种Na⁺/H⁺逆向转运蛋白催化嗜碱芽孢杆菌跨膜进行耦合的Na⁺外排和H⁺内吞。这种交换是生电的，向内转运的H⁺多于外排的Na⁺。它由嗜碱菌呼吸链复合物在初级质子泵浦过程中建立的ΔμH⁺的Δχ₂成分提供能量。这些复合物在极端嗜碱菌的膜中大量存在。呼吸链、逆向转运蛋白以及与Na⁺重新进入相耦合的溶质转运系统的联合活动，使嗜碱菌能够维持一种胞质pH，该pH比生长的最佳外部pH值酸性高几个pH单位。除了钠之外，没有令人信服的证据表明任何离子在pH稳态中具有特定且必要的作用，并且尽管在碱性范围内存在非常高的胞质缓冲能力，但pH稳态的主动机制至关重要。在极端嗜碱菌中，逆向转运蛋白以及催化ATP合成的质子转运F₁F₀ - ATP酶的供能必须适应外部介质中净ΔμH⁺低和质子本身浓度极低的问题。这个问题通过其他生物能学工作功能得以规避，例如溶质摄取或运动性，这些功能利用钠离子代替质子进行能量耦合。
