Chapter 10: A Structural View of Proton Transport by Bacteriorhodopsin

1 Introduction

Bacteriorhodopsin, ^{[1] [2] [3] [4]} and the more recently discovered growing number of other retinal-based light-driven proton pumps, ^{[5] [6] [7]} are transporters structurally closely related to G-protein coupled receptors. They are small ( ca. 24 kDa) heptahelical transmembrane proteins that contain a buried retinal much like in visual rhodopsin but in the all -trans isomeric state. Photoisomerization to 13 -cis, 15 -anti sets off a sequence of spectroscopically, crystallographically, and electrically detectable reactions ^[8] with a turn-over of about 100 s ^?1, in which protons are transferred between the retinal Schiff base and protein residues and between the protein and the bulk, protein conformational changes of increasing magnitude occur, and in the end the retinal regains its original isomeric state. The net result of this reaction cycle is the active translocation of a proton across the membrane, against an electrochemical potential well over what is required for physiological purposes like ATP synthesis. The events in this photocycle and the structural changes that accompany them have been studied by many groups and in great detail, with the hope that they will reveal how the proton transport is accomplished. Recent progress in this effort is impressive. FTIR spectra are providing more and more specific mechanistic insights. ^{[9] [10] [11] [12] [13]} As described below, X-ray diffraction is yielding extraordinarily high-resolution maps for the protein and...