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 O intermediates.
In the bacteriorhodopsin photocycle the recovery of the initial BR state from the M intermediate occurs via the N and O intermediates. The molecular events in this process include reprotonation of the Schiff base 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 have studied the kinetics of the intermediates and the proton uptake. At 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 superscripts indicate the net protonation state of the protein relative to BR. Together with a parallel study of ours at moderately high pH, these results suggest that the sequence of proton uptake and retinal reisomerization depends on pH: at low pH the isomerization occurs first and O accumulates, but at high pH the isomerization is delayed and therefore N accumulates. Although this model contains too many rate constants for rigorous testing, we find that it will generate most of the characteristic pH-dependent kinetic features of the photocycle with few assumptions other than pH dependency for protonation at the proton release and uptake steps.
Cao
Y
Y
Department of Physiology and Biophysics, University of California, Irvine 92717.
Brown
L S
LS
Needleman
R
R
Lanyi
J K
JK
D000595:Amino Acid Sequence; D001436:Bacteriorhodopsins / Q000378:metabolism*; D003593:Cytoplasm / Q000378:metabolism; D005798:Genes, Bacterial; D019617:Halobacterium salinarum / Q000235:genetics / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007536:Isomerism; D007700:Kinetics; D008027:Light; D011522:Protons; D011994:Recombinant Proteins / Q000378:metabolism; D012172:Retinaldehyde / Q000378:metabolism*; D013997:Time Factors
D011522:Protons; D011994:Recombinant Proteins; D001436:Bacteriorhodopsins; D012172:Retinaldehyde