Temperature dependence of nanosecond charge recombination in mutant Rhodobacter sphaeroides reaction centers: modelling of the protein dynamics

Abstract

We investigated the influence of a range of factors—temperature, redox midpoint potential of an electron carrier, and protein dynamics—on nanosecond electron transfer within a protein. The model reaction was back electron transfer from a bacteriopheophytin anion, HA−, to an oxidized primary electron donor, P+, in a wild type Rhodobacter sphaeroides reaction center (RC) with a permanently reduced secondary electron acceptor (quinone, QA−). Also used were two modified RCs with single amino acid mutations near the monomeric bacteriochlorophyll, BA, located between P and HA. Both mutant RCs showed significant slowing down of this back electron transfer reaction with decreasing temperature, similar to that observed with the wild type RC, but contrasting with a number of single point mutant RCs studied previously. The observed similarities and differences are explained in the framework of a (P+BA− ↔ P+HA−) equilibrium model with an important role played by protein relaxation. The major cause of the observed temperature dependence, both in the wild type RC and in the mutant proteins, is a limitation in access to the thermally activated pathway of charge recombination via the state P+BA− at low temperatures. The data indicate that in all RCs both charge recombination pathways, the thermally activated one and a direct one without involvement of the P+BA− state, are controlled by the protein dynamics. It is concluded that the modifications of the protein environment affect the overall back electron transfer kinetics primarily by changing the redox potential of BA and not by changing the protein relaxation dynamics.

Pushing the limits of flash photolysis to unravel the secrets of biological electron and proton transfer

“…We are happy to announce that today, this virtual volume unites 18 contributions by scientists from 17 countries (Canada, Czech republic, Finland, France, Germany, Hungary, India, Italy, Japan, Lebanon, Poland, Russia, Spain, Switzerland, Taiwan, UK, US) touching upon the wide variety of the subjects dealt with by Klaus at various times [25,[127][128][129][130][131][132][133][134][135][136][137][138][139][140][141][142][143].…”

“…Three papers are dedicated to photosynthetic systems [127][128][129]: Agostini et al used optically detected magnetic resonance (ODMR) to study the antenna triplet states in heliobacterial reaction centers. They propose that the triplet states are localised on the cytoplasmatic side of the complex, not far apart from each other but far away from both the electron transport chain and the carotenoid bound to the complex, thus suggesting a sacrificial role for ROS protection [127].…”

“…Gibasiewicz et al used nanosecond spectroscopy with 532 nm excitation to investigate the temperature dependence of charge recombination in mutant Rhodobacter sphaeroides reaction centers. They studied the role of protein dynamics and concluded that the modifications of the protein environment affect the overall back electron transfer kinetics primarily by changing the redox potential of the accessory bacteriochlorophyll B A and not by changing the protein relaxation dynamics [129].…”