ELECTROSTATIC AND CONFORMATIONAL EFFECTS ON THE PROTON TRANSLOCATION STEPS IN BACTERIORHODOPSIN - ANALYSIS OF MULTIPLE M-STRUCTURES

Molecular dynamic, electrostatic, and quantum chemical calculations ar e applied in order to analyze in a model-independent approach the driv ing forces for the rise and decay of the M state in the bacteriorhodop sin photocycle. We find that a protein conformational change involving the reorientation of arginine R82 away from the chromophore binding s ite toward the extracellular region after the protonation of the prima ry acceptor aspartate D85 induces the development of several M subpopu lations. They differ in the overall protein conformation and the total number and the distribution of protons and control the recovery of th e ground state in different ways. This protein conformational change c atalyzes extracellular proton release in the alkaline pH region and pr ovides favorable electrostatic and structural features for speeding up the reprotonation of the retinal Schiff base, simultaneously slowing down its reisomerization. The de- and reprotonation steps are decompos ed in single steps involving bound water molecules as intermediate pro ton binding sites. We show that, for each of the two overall transloca tions, the initial steps proceed near equilibrium, while further steps are unidirectional and fast.