Evidence for parallel photocycles and implications for the proton pump in bacteriorhodopsin.

In order to account for the large variety of kinetic phenomena in the light-induced reactions of bacteriorhodopsin's retinal chromophore (BR), a scheme of parallel photocycles has been proposed [W. Eisfeld, C. Pusch, R. Diller, R. Lohrmann and M. Stockburger, Biochemistry, 32 (1993) 7196-7215]. In the present study an experimental test for the validity of this model is described which is based on the fact that in the alkaline region the longest-living intermediates M(f), M(S) or N in each of the proposed cycles have significantly different lifetimes. A condition for the existence of parallel cycles would be that the population of M(f), M(S) or N is accompanied by a respective depletion of BR in each individual cycle. Dual-beam laser experiments were performed which showed that this condition is fulfilled. It is concluded that those proton transfer steps which are important for the function as a proton pump are the same for all cycles.