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Bacteriorhodopsin is the smallest autonomous light-harvesting protein,
transforming the energy of light into that of transmembrane H+ gradient. Here a
semi-quantitative model of its proton--pumping mechanism is developed, based on
calculated energy levels of the protonation states of the groups along
the proton transfer pathway in
the key conformational states of the photocycle -- BR, M, N, and O.
The model explains
the directionality of proton pumping -- from the cytoplasmic to the
extracellular side --
solely on the basis of a pattern of changing relative energies
of protonation states during the photocycle. The model operates with collective
states of protonation involving all key groups, rather than
individual group's proton affinities
pK traditionally used, which proves crucial for consistent
description of non-equilibrium charge transfer.
Predictions are made about several important structural aspects of
the states along the cycle whose high
resolution structures are not available.