Interactions of herbicides and azidoquinones at a Photosystem II binding site in the thylakoid membrane

Abstract

6-Azido-5-decyl-2,3-dimethoxy- p-benzoquinone (6-azido-Q 0C 10) was found to replace the native plastoquinone at B (the second stable electron acceptor to Photosystem II (PS II)). The 6-azido-Q 10C 10 would accept electrons from the primary electron-accepting quinone, Q, thus allowing electron transport through PS II to the plastoquinone pool in thylakoids. The synthetic azidoquinone also competes with the PS II herbicides ioxynil and atrazine for binding. This observation strongly favors the hypothesis that PS II herbicides block electron transport by replacing the native quinone which acts as the second electron carrier on the reducing side of PS II (termed B). Covalent linkage of 6-azido-Q 0C 10 to its binding environment by ultraviolet irradiation greatly reduces herbicide-binding affinity but does not lead to a loss in herbicide-binding sites. We take this as evidence that covalent attachment of 6-azido-Q 0C 10 allows some freedom of quinone head-group movement such that the herbicides can enter the binding site. This indicates that the protein determinants which regulate quinone and herbicide binding are very closely related, but not identical. A compound somewhat related to 6-azido-Q 0C 10 is 2-azido-3-methoxy-5-geranyl-6-methyl- p-benzoquinone (2-azido-Q 2). This compound was found to be an ineffective competitor with respect to herbicide binding. Thus, interactions with protein-binding determinants are highly dependent on the molecular structure of quinones. The 2-azido-Q 2 was an inhibitor of electron flow in the intersystem portion of the chain.