Electrostatic control of midpoint potentials in the cytochrome subunit of the Rhodopseudomonas viridis reaction center.

The photosynthetic reaction center of Rhodopseudomonas viridis has four hemes in a linear arrangement with alternating high- and low-potential sites. Their midpoints are -60, 20, 310, and 380 mV [Dracheva, S. M., Drachev, L. A., Konstantinov, A. A., Semenov, A. Y., Skulachev, V. P., Arutjunjan, A. M., Shuvalov, V. A. & Zaberezhnaya, S. M. (1988) Eur. J. Biochem. 171, 253-264]. Electrostatic calculations reproduce the 440-mV midpoint spread and assignments of high- and low-potential hemes. When calculations on model compounds to connect the theoretical midpoints to the standard hydrogen electrode are used, the absolute electrochemical midpoints for the reaction center hemes are also in good agreement with experiment. The free energy of oxidation is found to be dependent on pairwise interactions with charged amino acids, heme propionic acids, previously oxidized hemes, and axial ligands and on the reaction field induced by heme oxidation.