Herbicide/Quinone Binding Interactions in Photosystem II

Many inhibitors prevent the oxidation o f the primary electron-accepting quinone (QA) by the secondary quinone (QB) in photosystem II by displacem ent o f Q B from its binding site. On the other 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 or by lightor reductant-induced reduction o f certain thylakoid com ponents, whereas atrazine affinity remains unchanged. Furthermore, absence o f H C O j in the presence o f 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 hypothesis o f non-identical, but “overlapping” binding sites for different herbicide groups and the native quinone must be more extensively defined. In this manuscript we evaluate both the competitive herbicide/quinone binding m odel, and a m odel in which binding o f one ligand alters the protein conformation resulting in a dramatic decrease in the binding affinity o f 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 molecule can be bound with high affinity to the herbicide/quinone binding environment, but it depends on the chemical structure of the ligands whether the binding interaction between two ligands is truly competitive or more indirect (allosteric), mediated through the protein conformation.