Light-Triggered Opening and Closing of an Hydrophobic Gate Controls Vectorial Proton Transfer Across Bacteriorhodopsin

Abstract

Bacteriorhodopsin (BR) in the purple membrane (PM) of Halobacterium halobium is considered as a prototypic membrane protein. This integral membrane protein is the only protein species of the purple membrane and functions as a light-energized proton pump¹. The protein consists of a single polypeptide chain of 248 amino acids (M_r 26486 Da) traversing the lipid bilayer (about 8 lipids per BR) in seven α-helical segments (Fig. 1; see also Fig. 1 of ref. 2, this book). Photons are absorbed by the chromophore retinal, which is covalently linked via a protonated Schiff’s base to lysine-216 of the protein moiety. The antenna retinal is responsible for the characteristic purple colour of BR with its absorption maximum at 568 nm in the ground state. Upon photon absorption, BR undergoes a cyclic photoreaction with an overall half-time of about 10 ms via a series of at least five intermediates of different colour with rise times of femto-to milliseconds^3,4. During the photochemical cycle one^5,6,7 proton is vectorially translocated across BR, causing the formation of an electrochemical proton gradient. (Further information is reviewed elsewhere^2,4,8). This light-generated proton electrochemical potential across the energy-transducing membranes is utilized by the halobacteria as a driving force for ATP synthesis, active transport processes, and rotation of flagella. It is quite obvious that bacteriorhodopsin is a very attractive system for the investigation of molecular steps in light-driven vectoral H⁺-translocation. Bacteriorhodopsin is one of the most promising candidates to elucidate the link between the structure of a membrane transport protein, its dynamics, and the function.