Regulation of the Proton Pumping Activity of Bacteriorhodopsin - The influence of Membrane Potential and Proton Gradient

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The membrane protein bacteriorhodopsin (bR) is a light-driven proton pump. The proton transfer involves protonation and deprotonation reactions of amino acids within the protein. The isomerization of the cofactor retinal initiates these chemical reactions. To understand the proton transport, it is necessary to determine the intramolecular structural and protonation changes at atomic detail and how they are coupled. To achieve this understanding, the following main questions will be addressed: - What are the protonation probabilities of the titratable groups in the different states of bR? - Which residues participate in proton transfer and form the proton transfer paths? - How do the membrane environment, the membrane potential, and the proton gradient influence the protonation probabilities? - What are the electrostatic differences between bacteriorhodopsin, halorhodopsin, sensory rhodopsin II, and rhodopsin, how do these differences determine their functional differences? To address the questions related to proton transfer, we will combine electrostatic calculations and methods to search for proton transfer paths. The results of these calculations will be used in more elaborate combined quantum mechanical/molecular mechanical calculations by other teams in the Forschergruppe to determine the proton transfer mechanism in more detail. The different properties of the various retinal proteins will be analyzed by comparing the elecrostatic potentials close to the retinal.