Long-distance proton transfer with a break in the bacteriorhodopsin active site

Bacteriorhodopsin is a proton-pumping membrane protein found in the plasma membrane of the archaeon Halobacterium salinarium. Light-induced isomerization of the retinal chromophore from all-trans to 13-cis leads to a sequence of five conformation-coupled proton transfer steps and the net transport of one proton from the cytoplasmic to the extracellular side of the membrane. The mechanism of the long-distance proton transfer from the primary acceptor Asp85 to the extracellular proton release group during the O --> bR is poorly understood. Experiments suggest that this long-distance transfer could involve a transient state [O] in which the proton resides on the intermediate carrier Asp212. To assess whether the transient protonation of Asp212 participates in the deprotonation of Asp85, we performed hybrid Quantum Mechanics/Molecular Mechanics proton transfer calculations using different protein structures and with different retinal geometries and active site water molecules. The structural models were assessed by computing UV-vis excitation energies and C=O vibrational frequencies. The results indicate that a transient [O] conformer with protonated Asp212 could indeed be sampled during the long-distance proton transfer to the proton release group. Our calculations suggest that, in the starting proton transfer state O, the retinal is strongly twisted and at least three water molecules are present in the active site.