Interrelations of M-intermediates in bacteriorhodopsin photocycle

Abstract

The photocycles of the wild-type bacteriorhodopsin and the D96N mutant were investigated by the flash-photolysis technique. The M-intermediate formation (400 nm) and the L-intermediate decay (520 nm) were found to be well described by a sum of two exponents (time constants, τ1 = 65 and τ2 = 250 μs) for the wild-type bR and three exponents (τ1 = 55 μs, τ2 = 220 μs and τ3 = 1 ms) for the D96N mutant of bR. A component with τ = 1 ms was found to be present in the photocycle of the wild-type bacteriorhodopsin as a lag-phase in the relaxation of photoresponses at 400 and 520 nm. In the presence of Lu3+ ions or 80% glycerol this component was clearly seen as an additional phase of M-formation. The azide effect on the D96N mutant of bR suggests that the 1-ms component is associated with an irreversible conformational change switching the Schiff base from the outward to the inward proton channel. The maximum of the difference spectrum of the 1-ms component of D96N bR is located at 404 nm as compared to 412 nm for the first two components. We suggest that this effect is a result of the alteration of the inward proton channel due to the Asp96→Asn substitution. Proton release measured with pyranine in the absence of pH buffers was identical for the wild-type bR and D96N mutant and matched the M→M′ conformational transition. A model for M rise in the bR photocycle is proposed