The Energetics and Molecular Dynamics of the Proton Pumping Photocycle in Bacteriorhodopsin

Robert R. Birge, Albert F. Lawrence, Thomas M. Cooper, Craig T. Martin, David F. Blair, Sunney I. Chan, Nonlinear Electrodynamics in Biological Systems, 107-120, 1984

The energy stored in the primary photochemical event of light adapted bacteriorhodopsin (~16 kcal mol^-1) is shown to be sufficient to pump two, but not three, protons per photocycle. A two channel model of the proton pumping photocycle is presented which accommodates the available experimental data. This model is energetically efficient because the primary event is used to destabilize both channels simultaneously. Two separate electrostatic gradients are generated, and the dark reactions that occur following the primary event restabilize both channels via proton translocation processes that ultimately pump two protons. The “primary” channel pumps the first proton in parallel. with the formation of the M₄₁₂ intermediate. The “secondary” channel pumps the second proton during the O₆₆₀→bR₅₆₈ dark reaction provided the electrochemical gradient is below a given threshold. Ground state 13-cis→all-transreisomerization is accomplished during the N₅₂₀→O₆₆₀ dark reaction due to formation of a protonated Schiff base chromophore with a single (secondary) counterion near the β-ionylidene ring. This latter counterion stabilizes a chromophore resonance structure which has a high degree of bond order reversal lowering significantly the barrier to ground state C₁₃=C₁₄isomerization.