Quinone (QB) Reduction by B-Branch Electron Transfer in Mutant Bacterial Reaction Centers from Rhodobacter sphaeroides : Quantum Efficiency and X-ray Structure †,‡

The photosynthetic reaction center (RC) from purple bacteria converts light into chemical energy. Although the RC shows two nearly structurally symmetric branches, A and B, light-induced electron transfer in the native RC occurs almost exclusively along the A-branch to a primary quinone electron acceptor QA. Subsequent electron and proton transfer to a mobile quinone molecule QB converts it to a quinol, QBH2. We report the construction and characterization of a series of mutants in Rhodobacter sphaeroides designed to reduce QB via the B-branch. The quantum efficiency to QB via the B-branch ΦB ranged from 0.4% in an RC containing the single mutation Ala-M260 → Trp to 5% in a quintuple mutant which includes in addition three mutations to inhibit transfer along the A-branch (Gly-M203 → Asp, Tyr-M210 → Phe, Leu-M214 → His) and one to promote transfer along the B-branch (Phe-L181 → Tyr). Comparing the value of 0.4% for ΦB obtained in the AW(M260) mutant, which lacks QA, to the 100% quantum efficiency for ΦA al...