Protonation reactions and their coupling in bacteriorhodopsin

Light-induced changes of the proton affinities of amino acid side groups ar e the driving force for proton translocation in bacteriorhodopsin. Recent p rogress in obtaining structures of bacteriorhodopsin and its intermediates with an increasingly higher resolution, together with functional studies ut ilizing mutant pigments and spectroscopic methods, have provided important information on the molecular architecture of the proton transfer pathways a nd the key groups involved in proton transport. III the present paper I con sider mechanisms of light-induced proton release and uptake and intramolecu lar proton transport and mechanisms of modulation of proton affinities of k ey groups in the framework of these data. Special attention is given to som e important aspects that have surfaced recently. These are the coupling of protonation states of groups involved in proton transport, the complex titr ation of the counterion to the Schiff base and its origin, the role of the transient protonation of buried groups in catalysis of the chromophore's th ermal isomerization, and the relationship between proton affinities of the groups and the pH dependencies of the rate constants of the photocycle and proton transfer reactions. (C) 2000 Elsevier Science B.V. All rights reserv ed.