Herbicide/Quinone binding interactions in photosystem II

Many inhibitors prevent the oxidation of the primary electron-accepting quinone (Q_A) by the secondary quinone (Q_B) in photosystem II by displacem ent of Q_B from its binding site. On theother hand, plastoquinone-1 and 6-azido-5-decyl-2,3-dim ethoxy-/?-benzoquinone displace herbicides. Binding studies show the herbicide/quinone interaction to be (apparently) competitive. The herbicide binding is influenced differentially by various treatments. In this paper it is shown that the affinity of, for example, bromoxynil is decreased by thylakoid unstacking orby light- or reductant-induced reduction of certain thylakoid com ponents, whereas atrazine affinity remains unchanged. Furthermore, absence of HCO₃⁻ in the presence of formate leads to an affinity decrease of bromoxynil and atrazine, but to an increase in i-dinoseb affinity. Other differential photosystem II herbicide effects are known from the literature. Since different and unrelated groups of Q_A oxidation inhibitors have been found, and because of the above-mentioned dissimilarities in binding characteristics for different inhibitor groups, the hypothesisof non-identical, but “overlapping” binding sites for different herbicide groups and the native quinone must be more extensively defined. In this m anuscript we evaluate both the competitive herbicide/quinone binding m odel, and a m odel in which binding of one ligand alters the protein conformation resulting in a dram atic decrease in the binding affinity of ligands from other chemical groups; in this model ligands from the sam e or related chem ical groups bind competitively. Thus, the latter model proposes that only one herbicide or quinone m olecule can be bound with high affinityto the herbicide/quinone binding environm ent, but it depends on the chemical structure of the ligands whether the binding interaction between two ligands is trulycompetitive or more indirect (allosteric), mediated through the protein conformation.