POSSIBLE ROLE OF CYTOCHROME b559 IN PHOTOSYSTEM II

Abstract

This chapter discusses the role of cytochrome b559 in photosystem II. The low potential form of cytochrome b559 binds a proton and that the binding by the reduced form is stronger than the binding by the oxidized form. In an experiment discussed in the chapter, this scheme for the high potential and low potential forms was integrated into the process of oxygen evolution by assuming that the proton bound to cytochrome b559 was the one that had been released in the splitting of water. The low potential, unprotonated form is reduced by photosystem II, possibly via plastoquinone in some type of Q cycle. This reduced form then binds a proton that was released from one of the S-state transitions that also supplied the electron to reduce PII+. The reduced and high potential form in turn reduces cytochrome f or plastocyanin with little loss of redox energy. The oxidized protonated form, which has a much lower affinity for the proton, releases its proton into the lumen of the thylakoid so that the cycle can begin again.