Mechanism of Electron Flow through the Q<sub>B</sub> Site in Photosystem II. 1. Kinetics of the Reduction of Electron Acceptors at the Q<sub>B</sub> and Plastoquinone Sites in Photosystem II Particles from the Cyanobacterium <italic>Synechococcus vulcanus</italic>

Abstract

Various derivatives of benzoquinone (BQ) were found to be reduced at two sites [the QB and plastoquinone (PQ) sites] in photosystem II particles from Synechococcus vulcanus, and the relationship between the structures of BQs and the kinetics of such reduction at each site were studied. Affinities of BQs for both the QB and the PQ site and the maximum turnover rates at the two sites were estimated by computer simulation of the dependence on the concentration of the BQ of the rate of oxygen evolution. Affinities of BQs for the QB sites determined by this method agreed well with those determined from competition between BQs and DCMU for the QB site. All methyl-substituted BQs had low affinities for the QB site, and tetramethyl-p-benzoquinone had almost no affinity. An increase in the number of chlorine atoms in the quinone ring increased the affinity, and the position of such substitutions had a great effect on the affinity when two positions on the ring were occupied by two chlorine atoms or methyl groups. The affinities of BQs for the PQ site were almost the same for all BQs tested in this experiment but the maximum turnover rates at this site varied greatly from one derivative to another. The results are consistent with the hypothesis that the binding of the PQ molecule to the QB site is attributable not to its quinone ring but to its isoprenoid chain and, moreover, that the electron transfer through the QB site occurs not by replacement of the PQ molecule but by donation of electrons to another PQ molecule.