Calculated Protein and Proton Motions Coupled to Electron Transfer: Electron Transfer from Q_A- to Q_B in Bacterial Photosynthetic Reaction Centers

Citations Over TimeTop 10% of 1999 papers

Abstract

Reaction centers from Rhodobacter sphaeroides were subjected to Monte Carlo sampling to determine the Boltzmann distribution of side-chain ionization states and positions and buried water orientation and site occupancy. Changing the oxidation states of the bacteriochlorophyll dimer electron donor (P) and primary (QA) and secondary (QB) quinone electron acceptors allows preparation of the ground (all neutral), P+QA-, P+QB-, P0QA-, and P0QB- states. The calculated proton binding going from ground to other oxidation states and the free energy of electron transfer from QA-QB to form QAQB- (DeltaGAB) compare well with experiment from pH 5 to pH 11. At pH 7 DeltaGAB is measured as -65 meV and calculated to be -80 meV. With fixed protein positions as in standard electrostatic calculations, DeltaGAB is +170 meV. At pH 7 approximately 0.2 H+/protein is bound on QA reduction. On electron transfer to QB there is little additional proton uptake, but shifts in side chain protonation and position occur throughout the protein. Waters in channels leading from QB to the surface change site occupancy and orientation. A cluster of acids (GluL212, AspL210, and L213) and SerL223 near QB play important roles. A simplified view shows this cluster with a single negative charge (on AspL213 with a hydrogen bond to SerL233) in the ground state. In the QB- state the cluster still has one negative charge, now on the more distant AspL210. AspL213 and SerL223 move so SerL223 can hydrogen bond to QB-. These rearrangements plus other changes throughout the protein make the reaction energetically favorable.

Related Papers

• → Structural and preliminary molecular dynamics studies of the Rhodobacter sphaeroides reaction center and its mutant form L(M196)H + H(M202)L(2014)3 cited
• The Photosynthetic Bacterial Reaction Center: Structure and Dynamics(2013)
• → On the role of the light‐harvesting B880 in the correct insertion of the reaction center of Rhodobacter capsulatus and Rhodobacter sphaeroides(1987)22 cited
• → Structure of the Reaction Center from Rhodobacter sphaeroides: Further Refinement of R-26 and 2.4.1 Structures and Interactions between the Reaction Center and Herbicides(1990)4 cited
• → Absorption Detected Magnetic Resonance (ADMR) and Holeburning Measurements on Reaction Center Triplet States of Rhodobacter sphaeroides R 26(1990)