Role of Arg82 in the early steps of the bacteriorhodopsin proton-pumping cycle.

Proton-transfer reactions in the bacteriorhodopsin light-driven proton pump are coupled with structural rearrangements of protein amino acids and internal water molecules. It is generally thought that the first proton-transfer step from retinal Schiff base to the nearby Asp85 is coupled with movement of the Arg82 side chain away from Asp85 and toward the extracellular proton release group. This movement of Arg82 likely triggers the release of the proton from the proton release group to the extracellular bulk. The exact timing of the movement of Arg82 and how this movement is coupled with proton transfer are still not understood in molecular detail. Here, we address these questions by computing the free energy for the movement of the Arg82 side chain. The calculations indicate that protonation of Asp85 leads to a fast reorientation of the Arg82 side chain toward the extracellular proton release group.