Mechanism of primary proton transfer in bacteriorhodopsin

Recent structures of putative intermediates in the bacteriorhodopsin photocycle have provided valuable snapshots of the mechanism by which protons are pumped across the membrane. However, key steps remain highly controversial, particularly the proton transfer occurring immediately after retinal trans-->cis photoisomerization. The gradual release of stored energy is inherently nonequilibrium: which photocycle intermediates are populated depends not only on their energy but also on their interconversion rates. To understand why the photocycle follows a productive (i.e., pumping), rather than some unproductive, relaxation pathway, it is necessary to know the relative energy barriers of individual steps. To discriminate between the many proposed scenarios of this process, we computed all its possible minimum-energy paths. This reveals that not one, but three very different pathways have energy barriers consistent with experiment. This result reconciles the conflicting views held on the mechanism and suggests a strategy by which the protein renders this essential step resilient.