Bacteriorhodopsin: structural insights revealed using X-ray lasers and synchrotron radiation

Abstract

2 Abstract: Directional transport of protons across an energy transducing membrane, or “proton pumping”, is ubiquitous in biology. Bacteriorhodopsin is a light-driven proton pump that is activated by a buried all-trans retinal chromophore being photo-isomerized to a 13-cis conformation. The mechanism by which photo-isomerization initiates directional proton transport against a proton concentration gradient has been studied by a myriad of biochemical, biophysical and structural techniques. X-ray free electron lasers (XFELs) have created new opportunities to probe the structural dynamics of bacteriorhodopsin at room temperature on time-scales from femtoseconds to milliseconds using time-resolved serial femtosecond crystallography (TRSFX). We review these recent developments and highlight where XFEL studies reveal new details concerning the structural mechanism of retinal photo-isomerization and proton pumping. We also discuss the extent to which these insights were anticipated by earlier intermediate trapping studies using synchrotron radiation. We believe that TR-SFX will open up the field for dynamical studies of other proteins that are not naturally light-sensitive.