Tuning of retinal twisting in bacteriorhodopsin controls the directionality of the early photocycle steps.
Productive proton pumping by bacteriorhodopsin requires that, after the all-trans to 13-cis photoisomerization of the retinal chromophore, the photocycle proceeds with proton transfer and not with thermal back-isomerization. The question of how the protein controls these events in the active site is addressed here using quantum mechanical/molecular mechanical reaction-path calculations. The results indicate that, while retinal twisting significantly contributes to lowering the barrier for the thermal cis-trans back-isomerization, the rate-limiting barrier for this isomerization is still 5-6 kcal/mol larger than that for the first proton-transfer step. In this way, the retinal twisting is finely tuned so as to store energy to drive the subsequent photocycle while preventing wasteful back-isomerization.
Bondar
Ana-Nicoleta
AN
Fischer
Stefan
S
Suhai
Sandor
S
Smith
Jeremy C
JC
D001436:Bacteriorhodopsins / Q000737:chemistry*; D007536:Isomerism; D008958:Models, Molecular; D015394:Molecular Structure; D010777:Photochemistry; D012172:Retinaldehyde / Q000737:chemistry*; D013997:Time Factors
D001436:Bacteriorhodopsins; D012172:Retinaldehyde