Mechanisms of Electron Flow through the QB Site in Photosystem II. 3. Effects of the Presence of Membrane Structure on the Redox Reactions at the QB Site

Abstract

Reduction kinetics of chloro- or methyl-substituted benzoquinones (BQs) in spinach PSII membrane fragments or n-heptyl-β-D-thioglucoside-extracted PSII core complexes (HTG-PSII) was studied and compared to that in eyanobacterial PSII core complexes [Satoh et al. (1995) Plant Cell Physiol. 36: 597]. It was found that the BQs accept electrons at two sites (the QB and PQ sites) in the both spinach preparations as in the cyanobacterial preparation. Maximum turnover rates ( Vmax) and binding affinities (Km) of the two sites were estimated. Comparison of the values in PSII membrane fragments with those in HTG-PSII showed that removal of the membrane structure or light-harvesting chlorophyll a/b protein complexes from the PSII core complexes had little effect on the characteristics of the QB site, indicating that the HTG-PSII have the intact QB site and are good materials to study the site. The Km and Vmax values were comparable to those in the cyanobacterial preparation. Low affinity (high Km, values) and high Vmax values of methyl-substituted BQs to the QB site and almost the same rate of intrinsic electron flow through the QB site in the both spinach preparations further support the hypothesis that the plastoquinone (PQ) molecule at the QB site is not replaced by another PQ molecule but, after reduction, only its head group goes out of the QB pocket, and comes back after transferring the electrons and protons to a free PQ molecule [Satoh et al. (1993) Z. Naturforsch. 48c: 174].