How Does the QB Site Influence Propagate to the QA Site in Photosystem II?

The redox potential of the primary quinone QA [Em(QA)] in photosystem II (PSII) is lowered by replacement of the native plastoquinone (PQ) with bromoxynil (BR) at the secondary quinone QB binding site. Using the BR-bound PSII structure presented in the previous Fourier transform infrared and docking calculation studies, we calculated Em(QA) considering both the protein environment in atomic detail and the protonation pattern of the titratable residues. The calculated Em(QA) shift in response to the replacement of PQ with deprotonated BR at the QB binding site [ΔEm(QA)PQ→BR] was −55 mV when the three regions, QA, the non-heme iron complex, and QB (QB = PQ or BR), were treated as a conjugated supramolecule (QA–Fe–QB). The negative charge of BR apparently contributes to the downshift in ΔEm(QA)PQ→BR. This downshift, however, is mostly offset by the influence of the residues near QB. The charge delocalization over the QA–Fe–QB complex and the resulting H-bond strength change between QA and D2-His214 are crucial factors that yield a ΔEm(QA)PQ→BR of −55 mV by (i) altering the electrostatic influence of the H-bond donor D2-His214 on Em(QA) and (ii) suppressing the proton uptake events of the titratable residues that could otherwise upshift ΔEm(QA)PQ→BR during replacement of PQ with BR at the QB site.