Mechanism of the Primary Charge Transfer Reaction in the Cytochrome bc Complex.

The bc complex is a critical enzyme for the ATP production in photosynthesis and cellular respiration. Its biochemical function relies on the so-called Q-cycle, which is well established and operates via quinol substrates that bind inside the protein complex. Despite decades of research, the quinol-protein interaction, which initiates the Q-cycle, has not yet been completely described. Furthermore, the initial charge transfer reactions of the Q-cycle lack a physical description. The present investigation utilizes classical molecular dynamics simulations in tandem with quantum density functional theory calculations, to provide a complete and consistent quantitative description of the primary events that occur within the bc complex upon quinol binding. In particular, the electron and proton transfer reactions that trigger the Q-cycle in the bc complex from Rhodobacter capsulatus are studied. The coupled nature of these charge transfer reactions was revealed by obtaining the transition energy path connecting configurations of the Q-site prior and after the transfers. The analysis of orbitals and partial charge distribution of the different states of the Q-site has further supported the conclusion. Finally, key structural elements of the bc complex that trigger the charge transfer reactions were established, manifesting the importance of the environment in the process, which is furthermore evidenced by free energy calculations.