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

The redox potential of the primary quinone Q(A) [E-m(Q(A))] in photosystem II (PSII) is lowered by replacement of the native plastoquinone (PQ) with bromoxynil (BR) at the secondary quinone Q(B) binding site. Using the BR-bound PSII structure presented in the previous Fourier transform infrared and docking calculation studies, we calculated E-m(Q(A)) considering both the protein environment in atomic detail and the protonation pattern of the titratable residues. The calculated E-m(Q(A)) shift in response to the replacement of PQ with deprotonated BR at the Q(B) binding site [Delta E-m (Q(A))(PQ -> BR)] was -55 mV when the three regions, Q(A), the non-heme iron complex, and Q(B) (Q(B) = PQ or BR), were treated as a conjugated supramolecule (Q(A) - Fe - Q(B)). The negative charge of BR apparently contributes to the downshift in Delta E-m(Q(A))PQ -> BR. This downshift, however, is mostly offset by the influence of the residues near Q. The charge delocalization over the Q(A) -Fe - Q(B) complex and the resulting H-bond strength change between Q(A) and D2-H1s214 are crucial factors that yield a Delta E-m(Q(A))(PQ -> BR) of -55 mV by (i) altering the electrostatic influence of the H-bond donor D2-His214 on E-m(Q(A)) and (ii) suppressing the proton uptake events of the titratable residues that could otherwise upshift Delta E-m(Q(A))(PQ -> BR) during replacement of PQwith BR at the Q(B) site.