Dynamics of proton transfer in bacteriorhodopsin

Proton transfer in bacteriorhodopsin from the cytoplasm to the extracellular side is initiated from protonated asp96 in the cytoplasmic region toward the deprotonated Schiff base. This occurs in the transition from the photocycle late M state to the N state. To investigate this proton-transfer process, a quantum mechanics/molecular mechanics (QM/MM) model is constructed from the bacteriorhodopsin E204Q mutant crystal structure. Three residues, asp96, asp85, and thr89, as well as most of the retinal chromophore and the Schiff base link of lys216 are treated quantum mechanically and connected to the remaining classical protein through linker atom hydrogens. Structural transformation in the M state results in the formation of a water channel between the Schiff base and asp96. Since a part of this channel is lined with hydrophobic residues, there has been a question on the mechanism of proton transfer in a hydrophobic channel. Ab initio dynamics using the CHARMM/GAMESS methodology is used to simulate the transfer of the proton through a partially hydrophobic channel. Once sufficient water molecules are added to the channel to allow the formation of a single chain of waters from asp96 to the Schiff base, the transfer occurs as a fast (less than a picosecond) concerted event irrespective of the protonation state of asp85. Dynamic transfer of the proton from asp96 to the nearest water initiates the organization of a strongly bonded water chain conducive to the transfer of the proton to the Schiff base nitrogen.