Transient spectroscopy of bacterial rhodopsins with an optical multichannel analyzer. 2. Effects of anions on the halorhodopsin photocycle.
We find that the photocycle of halorhodopsin (HR) in the presence of nitrate (but not chloride) consists of two parallel series of reactions. The first is essentially the same as that which occurs in the presence of chloride: HRhv----HRK----HRKL----HRL----HRO----HR. The second photocycle, however, which we describe as HRhv----HR'K----HRKO----HRO----HR, seems characteristic of what one would observe in the absence of chloride. Absorption spectra are calculated for all species but HRK and HR'K, which occur at shorter times (less than 60 ns) than we can resolve. At nitrate concentrations between 0.1 and 1 M, the proportion of HR which enters the first kind of photocycle increases in such a way as to suggest that nitrate can substitute for chloride, but much less effectively. At lower anion concentrations, the two photocycles are independent of one another, but at higher concentrations, they interact; i.e., the reaction HRKO----HRO----HRL can be observed. Thus, HRO must be common to the two photocycles. Kinetic fitting of the time dependence of HRL and HRO at different chloride concentrations provides evidence for the participation of chloride in the interconversion of HRL and HRO. The results are consistent with a model in which the photoreaction is influenced by the binding of an anion (either chloride or nitrate) to site II in HR: when an anion is bound, the HRK-initiated HRL-type photocycle is observed, but when the site is not occupied, the HR'K-initiated HRO-type photocycle is seen.
Department of Physiology and Biophysics, University of California, Irvine 92717.
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D001436:Bacteriorhodopsins / Q000032:analysis*; D001665:Binding Sites; D002712:Chlorides / Q000494:pharmacology; D006217:Halobacterium / Q000032:analysis* / Q000187:drug effects; D025602:Halorhodopsins; D007700:Kinetics; D009566:Nitrates / Q000494:pharmacology*; D010777:Photochemistry; D013057:Spectrum Analysis / Q000295:instrumentation
D002712:Chlorides; D025602:Halorhodopsins; D009566:Nitrates; D001436:Bacteriorhodopsins