P5
Andreas Labahn
Proton uptake and conformational changes associated with the first and second electron transfer to QB in bacterial reaction center from Rhodobacter sphaeroides
The primary steps of energy transduction in bacterial photosynthesis occur in the reaction centre (RC) that converts light into electrochemical energy. Light excites the primary electron donor which in bacterial reaction centers from the purple bacterium Rhodobacter sphaeroides is a bacterial chlorophyll dimer. Following the absorption of a photon an electron is transferred from the primary electron donor P via several intermediate acceptors to the primary electron acceptor QA and further to the secondary electron acceptor QB. In the presence of exogenous cytochrome c2 the photooxidized primary donor is rereduced. The absorption of a second photon induces the transfer of a second electron and subsequently the double reduction and protonation of the secondary quinone. This leads to the formation of a dihydroquinol, QBH2, which is released from the reaction centre.
A basic question is still unknown, namely, how the first electron transfer from QA- to QB is coupled to conformational changes and proton uptake. To unravel the sequential steps upon electron transfer we measure the QA-QB→QAQB- transition with time-resolved FTIR spectroscopy (step-scan technique). Decoupling of the protonation and electron transfer is accomplished by chemically modifying the cofactors in the reaction center while maintaining the native pathway for proton transfer to QB. In addition we construct mutants to selectively label amino acid residues in the RC. Kinetic FTIR measurements on mutant RCs reveal proton transfer rates between residues along the pathway from the aqueous phase to QB and provide insights into possible conformational changes. Subsequently, we plan to extent the measurements to study the mechanism of the formation of dihydroquinol.