Effects of mutagenetic substitution of prolines on the rate of deprotonation and reprotonation of the Schiff base during the photocycle of bacteriorhodopsin.

Membrane-buried proline residues are found in many transport proteins. To study their roles in the structure and function of bacteriorhodopsin (bR), effects of the individual substitutions of Pro-50, Pro-91 and Pro-186 on the deprotonation and reprotonation kinetics of the Schiff base (SB) were determined by flash photolysis. The obtained rate constants and the amplitudes of the slow and fast components were compared with those of ebR (wild-type bR, the native protein that is expressed in Escherichia coli). The deprotonation rates of PSB were found to be 10 times faster than that of ebR for P50A, P91A and P91G mutants, and 4 times faster for the P50G mutant. These mutations also increased the initial reprotonation rate of the SB, although the overall change in the reprotonation rate is not as significant as that in the deprotonation rate. Our results indicate that Pro-50 and Pro-91, as well as Pro-186, are important for the proton-pumping function of bR.