How Does the Q_B Site Influence Propagate to the Q_A Site in Photosystem II?

Abstract

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 [ΔE(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 ΔE(m)(Q(A))(PQ→BR). This downshift, however, is mostly offset by the influence of the residues near Q(B). The charge delocalization over the Q(A)-Fe-Q(B) complex and the resulting H-bond strength change between Q(A) and D2-His214 are crucial factors that yield a ΔE(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 ΔE(m)(Q(A))(PQ→BR) during replacement of PQ with BR at the Q(B) site.

Systematic High‐Accuracy Prediction of Electron Affinities for Biological Quinones

“…). There, the electron affinities of the primary and secondary acceptor plastoquinones Q A and Q B , the factors that control them, and the sequence of reduction and protonation of Q B are questions with critical implications for biological photosynthesis, and remain open challenges for both experiment and theory …”

Structure/Function/Dynamics of Photosystem II Plastoquinone Binding Sites

“…The authors observed more substantial contribution of the non-covalent interactions of Q A head group with D2-Phe261 and D2-Trp253 and Q B head group with D1-Phe255 and D1-Phe265 to the total PQ binding energies than the above mentioned H-bonding. Instead, it has been unambiguously demonstrated the importance of the H-bonding patterns of PSII PQs in determining their redox potential [40, 41]. The relative contribution of PSII integral lipids and cofactors to the total Q B interaction energy is less than 20% and is mainly limited to the PQ tail (Table 1 and Table 2 ).…”

Afterglow Thermoluminescence Measured in Isolated Chloroplasts

“…Within the PSII RC, the rapid release of an electron from the reaction center P680 to a pheophytin (Pheo) molecule produces the charge separated state, P680 + Pheo − (3). Two quinones, Q A and Q B , are located at the acceptor side of PSII with a H‐bounded nonheme iron in between (4). Charge stabilization takes place through electron transfer from Pheo − to the primary quinone acceptor, Q A , in less than 200 ps (5) and reduction of the cationic radical P680 + by Y Z (tyrosine 161 of D1 subunit), in 10–500 ns (6).…”

“…Charge recombination in PSII can be analyzed by thermoluminescence (TL) in pre‐illuminated photosynthetic materials such as intact leaves, isolated thylakoid membranes, or PSII submembranes fractions (14,15). Charge recombination of P680 + Q A − is known to occur probably via the intermediate state P680 + Pheo − (4). However, this type of recombination cannot be studied by themoluminescence.…”