LIGHT-DRIVEN CHLORIDE-ION TRANSPORT BY HALORHODOPSIN FROM NATRONOBACTERIUM-PHARAONIS .2. CHLORIDE RELEASE AND UPTAKE, PROTEIN CONFORMATION CHANGE, AND THERMODYNAMICS

The photocycle of the light-driven chloride pump, N. pharaonis halorho dopsin, is described hv by the scheme HR -->/hv K --> double left righ t arrow L double left right arrow N double left right arrow O double l eft right arrow HR' --> HR. From the chloride dependencies of the rate constants in this model we identify the N --> O and O --> HR' reactio ns as the steps where chloride release and uptake occur, respectively, during the transport. The dependencies of the rate constants on tempe rature describe a thermodynamic cycle in which enthalpy-entropy conver sion occurs in the O --> HR' reaction. The dependencies of the rate co nstants on hydrostatic pressure indicate that a substantial volume dec rease occurs at the L --> N reaction, a result of a large-scale confor mational change. This is the opposite of the volume increase in the ph otocycle of the proton pump, bacteriorhodopsin, that is implicated in the access change of-the active site during the transport and the pass age of a proton from the cytoplasmic surface to the active site. The r esults together suggest a chloride transport mechanism, in which the e quivalents of all the ion transfer steps in bacteriorhodopsin occur bu t in the reverse sense, so as to cause the extracellular-to-cytoplasmi c translocation of a chloride ion instead of the cytoplasmic-to-extrac ellular transport of a proton.