Quinone (Q(B)) reduction by B-branch electron transfer in mutant bacterial reaction centers from Rhodobacter sphaeroides: Quantum efficiency and X-ray structure

摘要

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 Q(A). Subsequent electron and proton transfer to a mobile quinone molecule Q(B) converts it to a quinol, Q(B)H(2). We report the construction and characterization of a series of mutants in Rhodobacter sphaeroides designed to reduce Q(B) via the B-branch. The quantum efficiency to Q(B) via the B-branch &UPhi;(B) 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 &UPhi;(B) obtained in the AW(M260) mutant, which lacks Q(A), to the 100% quantum efficiency for &UPhi;(A) along the A-branch in the native RC, we obtain a ratio for A-branch to B-branch electron transfer of 250:1. We determined the structure of the most effective (quintuple) mutant RC at 2.25 &ANGS; (R-factor = 19.6%). The Q(A) site did not contain a quinone but was occupied by the side chain of Trp-M260 and a Cl-. In this structure a nonfunctional quinone was found to occupy a new site near M258 and M268. The implications of this work to trap intermediate states are discussed.