Oxidation‐Reduction Physical Chemistry of the Acceptor Quinone Complex in Bacterial Photosynthetic Reaction Centers: Evidence for a New Model of Herbicide Activity

摘要: The oxidation-reduction midpoint potential (Em) of the primary quinone (QA) acceptor complex bacterial photosynthetic reaction centers has been measured as a function pH in presence and absence ubiquinone o-phenanthroline (o-Phen). Reaction centers, isolated from Rhodopseudomonas sphaeroides, were incorporated into egg phosphatidylcholine vesicles. Contrary to earlier reports, Em was found exhibit pH-dependence very similar that observed chromatophores, with slope - 60 mV/pH up p K for Q−A/Q−A(H+) at 9.5–10.0. In reconstitute secondary (QB), Em/pH curve QA shifted lower potentials, indicating binding Qn (actually QBH2) suppressed by reduction QA. o-Phen, an inhibitor electron transport QB, raised pK and, pH-values below but not above this pK, reversed effects QB. o-Phen lowered had no effect it. These results are discussed terms interactions between sites QB (A- B-sites). It is suggested compete B-site mutually exclusive fashion, their relative strengths modulated redox protonation state preparations low content, inhibits photochemistry suggesting it can also A-site. Competitive displacement other inhibitors mode action broad class herbicides active Photosystem II plants. affinities various states concluded order strength is: Q−B → QBH2. This accounts atypical stability semiquinone average quinol, compared free pool. may be significant functioning quinones communicating reducing equivalents center complexes intact membrane.