From the Departments of Biophysical Chemistry and.
J Biol Chem. 2014 May 9;289(19):13168-76. doi: 10.1074/jbc.M113.542993. Epub 2014 Mar 18.
Na(+)/H(+) exchangers are essential for regulation of intracellular proton and sodium concentrations in all living organisms. We examined and experimentally verified a kinetic model for Na(+)/H(+) exchangers, where a single binding site is alternatively occupied by Na(+) or one or two H(+) ions. The proposed transport mechanism inherently down-regulates Na(+)/H(+) exchangers at extreme pH, preventing excessive cytoplasmic acidification or alkalinization. As an experimental test system we present the first electrophysiological investigation of an electroneutral Na(+)/H(+) exchanger, NhaP1 from Methanocaldococcus jannaschii (MjNhaP1), a close homologue of the medically important eukaryotic NHE Na(+)/H(+) exchangers. The kinetic model describes the experimentally observed substrate dependences of MjNhaP1, and the transport mechanism explains alkaline down-regulation of MjNhaP1. Because this model also accounts for acidic down-regulation of the electrogenic NhaA Na(+)/H(+) exchanger from Escherichia coli (EcNhaA, shown in a previous publication) we conclude that it applies generally to all Na(+)/H(+) exchangers, electrogenic as well as electroneutral, and elegantly explains their pH regulation. Furthermore, the electrophysiological analysis allows insight into the electrostatic structure of the translocation complex in electroneutral and electrogenic Na(+)/H(+) exchangers.
钠离子/氢离子交换器对于所有生物体细胞内质子和钠离子浓度的调节至关重要。我们研究并实验验证了钠离子/氢离子交换器的一种动力学模型，其中单个结合位点可被钠离子或一个或两个氢离子交替占据。所提出的转运机制从根本上调节了极端 pH 值下的钠离子/氢离子交换器，防止了细胞质过度酸化或碱化。作为一个实验测试系统，我们首次对产电中性钠离子/氢离子交换器 Methanocaldococcus jannaschii 的 NhaP1（MjNhaP1）进行了电生理学研究，它是医学上重要的真核 NHE 钠离子/氢离子交换器的密切同源物。该动力学模型描述了 MjNhaP1 的实验观察到的底物依赖性，转运机制解释了 MjNhaP1 的碱性下调。因为该模型还解释了大肠杆菌（EcNhaA，在之前的一篇论文中已显示）的电活性 NhaA 钠离子/氢离子交换器的酸性下调，所以我们得出结论，它普遍适用于所有钠离子/氢离子交换器，无论是电活性的还是产电中性的，并巧妙地解释了它们的 pH 调节。此外，电生理学分析使我们深入了解了电中性和电活性钠离子/氢离子交换器中转运复合物的静电结构。
