Modeling the Effects of Mutations on the Free Energy of the First Electron Transfer from Q_A^- to Q_B in Photosynthetic Reaction Centers

Abstract

Numerical calculations of the free energy of the first electron transfer in genetically modified reaction centers from Rhodobacter (Rb.) sphaeroides and Rb. capsulatus were carried out from pH 5 to 11. The multiconformation continuum electrostatics (MCCE) method allows side chain, ligand, and water reorientation to be embedded in the calculations of the Boltzmann distribution of cofactor and amino acid ionization states. The mutation sites whose effects have been modeled are L212 and L213 (the L polypeptide) and two in the M polypeptide, M43(44) and M231(233) in Rb. capsulatus (Rb. sphaeroides). The results of the calculations were compared to the experimental data, and very good agreement was found especially at neutral pH. Each mutation removes or introduces ionizable residues, but the protein maintains a net charge close to that in native RCs through ionization changes in nearby residues. This reduces the effect of mutation and makes the changes in state free energy smaller than would be found in a rigid protein. The state energy of QA-QB and QAQB- states have contributions from interactions among the residues as well as with the quinone which is ionized. For example, removing L213Asp, located in the QB pocket, predominantly changes the free energy of the QA-QB state, where the Asp is ionized in native RCs rather than the QAQB- state, where it is neutral. Side chain, hydroxyl, and water rearrangements due to each of the mutations have also been calculated showing water occupancy changes during the QA- to QB electron transfer.