Redox potential of the terminal quinone electron acceptor Q_B in photosystem II reveals the mechanism of electron transfer regulation

Photosystem II (PSII) extracts electrons from water at a Mn₄CaO₅ cluster using light energy and then transfers them to two plastoquinones, the primary quinone electron acceptor Q_A and the secondary quinone electron acceptor Q_B. This forward electron transfer is an essential process in light energy conversion. Meanwhile, backward electron transfer is also significant in photoprotection of PSII proteins. Modulation of the redox potential (E_m) gap of Q_A and Q_B mainly regulates the forward and backward electron transfers in PSII. However, the full scheme of electron transfer regulation remains unresolved due to the unknown E_m value of Q_B. Here, for the first time (to our knowledge), the E_m value of Q_B reduction was measured directly using spectroelectrochemistry in combination with light-induced Fourier transform infrared difference spectroscopy. The E_m(Q_B^-/Q_B) was determined to be approximately +90 mV and was virtually unaffected by depletion of the Mn₄CaO₅ cluster. This insensitivity of E_m(Q_B^-/Q_B), in combination with the known large upshift of E_m(QA^-/QA), explains the mechanism of PSII photoprotection with an impaired Mn₄CaO₅ cluster, in which a large decrease in the E_m gap between Q_A and Q_B promotes rapid charge recombination via QA-.