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.
Department of Physiology and Biophysics, University of California, Irvine 92717.
Y
Y
Cao
L S
LS
Brown
R
R
Needleman
J K
JK
Lanyi
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