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 O640and 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 M412is biphasic. The amplitude of the slowly decaying component (Ms) was found to be pH dependent with a pK similar to that of the Δ pH. The pH dependence of the fast-decaying component (Mf) is opposite to that of Msand Δ pH. Like that of Ms, the amplitude of O640is 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 O640becomes zero around pH 8. The temperature dependence of the amplitude of Mswas found to be similar to that of Δ pH, being high at low temperatures and low in the high-temperature range. The temperature dependence of Mfand O640is opposite to that of Δ pH and Ms. Whereas, O640and Msseem to be likely candidates (from the pH dependence) for coupling between proton pumping and the photocycle, Msis more likely because (i) the relative quantum yield of both Msand Δ pH seem not to go to zero at higher pHs and (ii) Msand Δ pH have similar temperature dependence.