Váró G, Needleman R, Lanyi J K
Department of Physiology and Biophysics, University of California, Irvine 92717, USA.
Biochemistry. 1995 Nov 7;34(44):14500-7. doi: 10.1021/bi00044a028.
The photocycle of the light-driven chloride pump, N. pharaonis halorhodopsin, is described by the scheme HR-->K--><==>L<==>N<==>O<==>HR'-->HR. From the chloride dependencies of the rate constants in this model we identify the N-->O and O-->HR' reactions as the steps where chloride release and uptake occur, respectively, during the transport. The dependencies of the rate constants on temperature describe a thermodynamic cycle in which enthalpy-entropy conversion occurs in the O-->HR' reaction. The dependencies of the rate constants on hydrostatic pressure indicate that a substantial volume decrease occurs at the L-->N reaction, a result of a large-scale conformational change. This is the opposite of the volume increase in the photocycle of the proton pump, bacteriorhodopsin, that is implicated in the access change of the active site during the transport and the passage of a proton from the cytoplasmic surface to the active site. The results together suggest a chloride transport mechanism, in which the equivalents of all the ion transfer steps in bacteriorhodopsin occur but in the reverse sense, so as to cause the extracellular-to-cytoplasmic translocation of a chloride ion instead of the cytoplasmic-to-extracellular transport of a proton.
光驱动氯离子泵——法老视紫红质（N. pharaonis halorhodopsin）的光循环由以下反应式描述：HR→K↔L↔N↔O↔HR'→HR。根据该模型中速率常数对氯离子的依赖性，我们确定N→O和O→HR'反应分别是运输过程中氯离子释放和摄取的步骤。速率常数对温度的依赖性描述了一个热力学循环，其中在O→HR'反应中发生了焓-熵转换。速率常数对静水压力的依赖性表明，在L→N反应中发生了大量的体积减小，这是大规模构象变化的结果。这与质子泵细菌视紫红质光循环中的体积增加相反，后者与运输过程中活性位点的可及性变化以及质子从细胞质表面传递到活性位点有关。这些结果共同表明了一种氯离子运输机制，其中细菌视紫红质中所有离子转移步骤的等效过程都发生了，但方向相反，从而导致氯离子从细胞外转运到细胞质中，而不是质子从细胞质转运到细胞外。
