ABSTRACT
Diffraction before destruction experiments with powerful X-ray pulses enabled time-resolved serial crystallography and the creation of movies by assembling the snapshots of protein movements after the protein quake was caused by photon absorption. Similar dynamics are orchestrated by archaeal bacteriorhodopsin and human rhodopsin to perform, respectively, the simplest known photosynthesis and photon detection. The free-energy transduction machinery of bacteriorhodopsin saves just enough of photon free-energy to power the ATP-synthase by a created electrochemical proton gradient. High quantum yield and energy conversion into biologically friendly form would be impossible in the absence of topologically closed membrane impermeable to protons. We used the bacteriorhodopsin photocycle to follow and optimize its light-activated proton pumping. The simulations were designed to answer the following questions: which transitions are associated with the highest entropy production, what are rate-limiting steps, and what is the best choice for the optimization of the catalytic conversion by using our maximal transitional entropy production theorem. Small increases in overall entropy production, net proton flux, and energy conversion efficiency can be achieved only after MTEP optimization for the last (recovery step) in the bacteriorhodopsin photocycle. The tentative conclusion is that biological evolution already used almost all available options to channel bacteriorhodopsin structure-function connection towards the most efficient dissipation of free energy gradients for physiological situations.
