PROTON RELEASE PATHWAY IN BACTERIORHODOPSIN - MOLECULAR-DYNAMICS AND ELECTROSTATIC CALCULATIONS

We use molecular dynamics, electrostatic, and quantumchemical calculat ions to discuss chromophore and protein structural changes as well as proton transfer pathways in the first half of the bacteriorhodopsin ph otocycle. A model for the molecular mechanism is presented, which acco unts for the complex pH dependence of the proton release and uptake pa ttern found for the M intermediates. The results suggest that transien t transfer of the Schiff base proton to a nearby tightly bound water m olecule is the primary step, which is accompanyied by dissipation of f ree energy to the protein. From there, the energetically most favorabl e proton transfer is to aspartate D85. Arginine R82 is involved in the protein reorientation switch, which catalyzes the pK(a) reduction of glutamate E204. This residue is, therefore, identified as extracellula r proton release group whose acid base equilibrium regulates the pH-de pendent splitting of the photocycle. (C) 1994 John Wiley & Sons, Inc.