PATHWAYS OF PROTON-TRANSFER IN THE LIGHT-DRIVEN PUMP BACTERIORHODOPSIN

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.