A molecular piston mechanism of pumping protons by bacteriorhodopsin.

In this review the proton-pumping mechanism proposed recently for bacteriorhodopsin [Chou, K. C. (1993) Journal of Protein Chemistry, 12: 337-350] is illustrated in terms of a phenomenological model. According to the model, theβ-ionone of the retinal chromophore in bacteriorhodopsin can be phenomenologically imagined as a molecular "piston". The photon capture by bacteriorhodopsin would "pull" it up while the spontaneous decrease in potential energy would "push" it down so that it would be up and down alternately during the photocycle process. When it is pulled up, the gate of pore is open and the water channel for the proton translocation is through; when it is pushed down, the gate of pore is closed and the water channel is shut up. Such a model not only is quite consistent with experimental observations, but also provides useful insights and a different view to elucidate the protonpumping mechanism of bacteriorhodopsin. The essence of the model might be useful in investigating the mechanism of ion-channels of other membrane proteins.