pH dependence of light‐induced proton release by bacteriorhodopsin

Abstract
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 pK a 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 pK a of this group also allows us to calculate the surface charge density of about −5 charges per bacteriorhodopsin, compatible with previous estimates.