A correlation between proton pumping and the bacteriorhodopsin photocycle.

In an attempt to establish a relationship between proton pumping and the photocycle intermediates of bacteriorhodopsin, we have studied the effects of pH and temperature on flash-induced proton pumping and the photointermediates O640 and M412. The relative quantum yield of flash-induced proton pumping is both pH and temperature dependent. It is high in the acid pH range and at low temperatures but decreases in the basic pH range and at high temperatures. The decay of M412 is biphasic. The amplitude of the slowly decaying component (Ms) was found to be pH dependent with a pK similar to that of the delta pH. The pH dependence of the fast-decaying component (Mf) is opposite to that of Ms and delta pH. Like that of Ms, the amplitude of O640 is high in the acid pH range, but unlike the amplitude of Ms, it declines very rapidly at pHs greater than 6.5; the amplitude of O640 becomes zero around pH 8. The temperature dependence of the amplitude of Ms was found to be similar to that of delta pH, being high at low temperatures and low in the high-temperature range. The temperature dependence of Mf and O640 is opposite to that of delta pH and Ms. Whereas, O640 and Ms seem to be likely candidates (from the pH dependence) for coupling between proton pumping and the photocycle, Ms is more likely because (i) the relative quantum yield of both Ms and delta pH seem not to go to zero at higher pHs and (ii) Ms and delta pH have similar temperature dependence.