pH dependence of light-induced proton release by bacteriorhodopsin.
We have measured the current generated by light-activated proton release from bacteriorhodopsin into solution as a function of both pH and ionic strength. We find that proton release into solution decreases with increasing pH with an intrinsic pKa of 8.2 +/- 0.2. This pH dependence indicates that the deprotonation of a certain group inhibits or abolishes proton release. Under physiological conditions, this group either releases a proton directly into solution or interacts with the site of proton release. The most immediate candidates for this protonatable species are tyrosine-57, tyrosine-185, arginine-82, and water; acting individually or cooperatively. The salt dependence of the apparent pKa of this group also allows us to calculate the surface charge density of about -5 charges per bacteriorhodopsin, compatible with previous estimates.
Kono
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M
Biophysics Program, University of Illinois, Urbana 61801.
Misra
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Ebrey
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D001436:Bacteriorhodopsins / Q000737:chemistry* / Q000528:radiation effects; D006863:Hydrogen-Ion Concentration; D008027:Light; D009994:Osmolar Concentration; D011522:Protons
D011522:Protons; D001436:Bacteriorhodopsins