BACTERIORHODOPSIN
Proton-pumping mechanism
Bacteriorhodopsin is the smallest autonomous light-harvesting protein that transforms the energy of light into that of transmembrane H+ gradient. It is arguably the simplest and best understood of all known active transport systems, and has been the focus of intensive biophysical and structural studies aimed at understanding its mechanism in atomic detail. Despite almost 30 years of intense research, there is still no unifying quantitative model of proton transfer in bacteriorhodopsin describing the entire photocycle. The ability of this molecular machine to pump protons effeciently in one direction, from the cytoplasmic to the extracellular side of the cell membrane, depends on concerted changes of charge states of only a small group of key residues, which makes it an attractive problem for a physicist.
Strong electrostatic interactions (up to 25 kT ) between the key titratable groups make the traditional description in terms of single-site pKs inappropriate; a full many-body description in terms of protonation states is necessary.
Left: A diagram of the bacteriorhodopsin photo-cycle showing transitions between conformational states. Absorption of a photon in the ground (BR) state leads to isomerization of the retinal Schiff base (S.B.) form all-trans to 13-cis in the K intermediate. After further relaxation to L, the proton transfer begins with the onset of the M state.
Right: A structure of bacteriorhodopsin showing key residues directly invoved in sequential proton transfer events (1 through 5) coupled to conformational changes: 1 -- protonation of Asp85 from the Schiff Base (S.B.); 2 -- proton release to the extracellular side; 3 -- reprotonation of the Schiff base from Asp96; 4 -- uptake of a proton form the cytoplasmic side (left) by Asp96; 5 -- proton transfer from Asp85 to the release group (R.G.), which is believed to involve Glu194, Glu204 and a H5O2+ cluster.
E-mail: onufriev@cs.vt.edu