RELATIONSHIP OF PROTON UPTAKE ON THE CYTOPLASMIC SURFACE AND REISOMERIZATION OF THE RETINAL IN THE BACTERIORHODOPSIN PHOTOCYCLE - AN ATTEMPT TO UNDERSTAND THE COMPLEX KINETICS OF THE PH CHANGES AND THE N AND OINTERMEDIATES
Y. Cao et al., RELATIONSHIP OF PROTON UPTAKE ON THE CYTOPLASMIC SURFACE AND REISOMERIZATION OF THE RETINAL IN THE BACTERIORHODOPSIN PHOTOCYCLE - AN ATTEMPT TO UNDERSTAND THE COMPLEX KINETICS OF THE PH CHANGES AND THE N AND OINTERMEDIATES, Biochemistry, 32(38), 1993, pp. 10239-10248
In the bacteriorhodopsin photocycle the recovery of the initial BR sta
te from the M intermediate occurs via the N and O intermediates. The m
olecular events in this process include reprotonation of the Schiff ba
se and the subsequent uptake of a proton from the cytoplasmic side, as
well as reisomerization of the retinal from 13-cis to all-trans. We h
ave studied the kinetics of the intermediates and the proton uptake. A
t moderately low pH little of the N state accumulates, and the O state
dominates in the reactions that lead from M to BR. The proton uptake
lags behind the formation of O, suggesting the sequence N(0) <-> O(0)
+ H+ (from the bulk) --> O(+1) --> BR + H+ (to the bulk), where the su
perscripts indicate the net protonation state of the protein relative
to BR. Together with a parallel study of ours at moderately high pH, t
hese results suggest that the sequence of proton uptake and retinal re
isomerization depends on pH: at low pH the isomerization occurs first
and O accumulates, but at high pH the isomerization is delayed and the
refore N accumulates. Although this model contains too many rate const
ants for rigorous testing, we find that it will generate most of the c
haracteristic pH-dependent kinetic features of the photocycle with few
assumptions other than pH dependency for protonation at the proton re
lease and uptake steps.