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Bacteriorhodopsin is the smallest autonomous light-harvesting protein
that transforms the energy of light into that of transmembrane H+
It is arguably the simplest and best understood of all known active
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.
3D structural model of bacteriorhodopsin imbedded inside lipid membrane
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.
Energy diagram of the BR state
Transitions between protonation states in the photocycle