The proton release group of bacteriorhodopsin controls the rate of the final step of its photocycle at low pH.
The factors determining the pH dependence of the formation and decay of the O photointermediate of the bacteriorhodopsin (bR) photocycle were investigated in the wild-type (WT) pigment and in the mutants of Glu-194 and Glu-204, key residues of the proton release group (PRG) in bR. We have found that in the WT the rate constant of O --> bR transition decreases 30-fold upon decreasing the pH from 6 to 3 with a pKa of about 4.3. D2O slows the rise and decay of the O intermediate in the WT at pH 3.5 by a factor of 5.5. We suggest that the rate of the O --> bR transition (which reflects the rate of deprotonation of the primary proton acceptor Asp-85) at low pH is controlled by the deprotonation of the PRG. To test this hypothesis, we studied the E194D mutant. We show that the pKa of the PRG in the ground state of the E194D mutant, when Asp-85 is protonated, is increased by 1.2 pK units compared to that of the WT. We found a similar increase in the pKa of the rate constant of the O --> bR transition in E194D. This provides further evidence that the rate of the O --> bR transition is controlled by the PRG. In a further test, the E194Q mutation, which disables the PRG and slows proton release, almost completely eliminates the pH dependence of O decay at pHs below 6. A second phenomenon we investigated was that in the WT at neutral and alkaline pH the fraction of the O intermediate decreases with pKa 7.5. A similar pH dependence is observed in the mutants in which the PRG is disabled, E194Q and E204Q, suggesting that the decrease in the fraction of the O intermediate with pKa ca. 7.5 is not controlled by the PRG. We propose that the group with pKa 7.5 is Asp-96. The slowing of the reprotonation of Asp-96 at high pH is the cause of the decrease in the rate of the N --> O transition, leading to the decrease in the fraction of O.