Arginine-82 regulates the pKa of the group responsible for the light-driven proton release in bacteriorhodopsin.

In wild-type bacteriorhodopsin light-induced proton release occurs before uptake at neutral pH. In contrast, in mutants in which R82 is replaced by a neutral residue (as in R82A and R82Q), only a small fraction of the protons is released before proton uptake at neutral pH; the major fraction is released after uptake. In R82Q the relative amounts of the two types of proton release, "early" (preceding proton uptake) and "late" (following proton uptake), are pH dependent. The main conclusions are that 1) R82 is not the normal light-driven proton release group; early proton release can be observed in the R82Q mutant at higher pH values, suggesting that the proton release group has not been eliminated. 2) R82 affects the pKa of the proton release group both in the unphotolyzed state of the pigment and during the photocycle. In the wild type (in 150 mM salt) the pKa of this group decreases from approximately 9.5 in the unphotolyzed pigment to approximately 5.8 in the M intermediate, leading to early proton release at neutral pH. In the R82 mutants the respective values of pKa of the proton release group in the unphotolyzed pigment and in M are approximately 8 and 7.5 in R82Q (in 1 M salt) and approximately 8 and 6.5 in R82K (in 150 mM KCl). Thus in R82Q the pKa of the proton release group does not decrease enough in the photocycle to allow early proton release from this group at neutral pH. 3) Early proton release in R82Q can be detected as a photocurrent signal that is kinetically distinct from those photocurrents that are due to proton movements from the Schiff base to D85 during M formation and from D96 to the Schiff base during the M-->N transition. 4) In R82Q, at neutral pH, proton uptake from the medium occurs during the formation of O. The proton is released during the O-->bacteriorhodopsin transition, probably from D85 because the normal proton release group cannot deprotonate at this pH. 5) The time constant of early proton release is increased from 85 microseconds in the wild type to 1 ms in R82Q (in 150 mM salt). This can be directly attributed to the increase in the pKa of the proton release group and also explains the uncoupling of proton release from M formation. 6) In the E204Q mutant only late proton release is observed at both neutral and alkaline pH, consistent with the idea that E204 is the proton release group. The proton release is concurrent with the O-->bacteriorhodopsin transition, as in R82Q at neutral pH.