Redox potential of the terminal quinone electron acceptor QB 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 QB 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(Q(A)(-)/Q(A)), 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 Q(A)(-).