Stability of a Nonequilibrium Biochemical Cycle Revealed by Single-Molecule Spectroscopy

Biological machinery relies on nonequilibrium dynamics to maintain stable directional fluxes through complex reaction cycles. In stabilizing the reaction cycle, the role of microscopic irreversibility of elementary transitions, and the accompanying entropy production, is of central interest. Here, we use multidimensional single-molecule spectroscopy to demonstrate that the reaction cycle of bacteriorhodopsin is coupled through both reversible and irreversible transitions, with directionality of trans-membrane H+ transport being ensured by the entropy production of irreversible transitions. We observe that thermal destabilization of the process is the result of diminishing thermodynamic driving force for irreversible transitions, leading to an exponentially increasing variance of flux through the transitions. We show that the thermal stability of the reaction cycle can be predicted from the Gibbs-Helmholtz relation.