The mechanism of proton transport in the light-driven pump bacteriorho
dopsin is beginning to be understood. Light causes the all-trans to 13
-cis isomerization of the retinal chromophore. This sets off a sequent
ial and directed series of transient decreases in the pK(a)'s of a) th
e retinal Schiff base, b) an extracellular proton release complex whic
h includes asp-85, and c) a cytoplasmic proton uptake complex which in
cludes asp-96. The timing of these pK(a) changes during the photoreact
ion cycle causes sequential proton transfers which result in the net m
ovement of a proton across the protein, from the cytoplasmic to the ex
tracellular surface.