Redirecting electron transfer in photosystem II from water to redox-active metal complexes

A negatively charged region on the surface of photosystem II (PSII) near Q _A has been identified as a docking site for cationic exogenous electron acceptors. Oxygen evolution activity, which is inhibited in the presence of the herbicide 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), is recovered by adding Co ^III complexes. Thus, a new electron-transfer pathway is created with Co ^III as the new terminal electron acceptor from Q _A ^-. This binding site is saturated at ∼2.5 mM [Co ^III], which is consistent with the existence of low-affinity interactions with a solvent-exposed surface. This is the first example of a higher plant PSII in which the electron-transfer pathway has been redirected from the normal membrane-associated quinone electron acceptors to water-soluble electron acceptors. The proposed Co ^III binding site may enable efficient collection of electrons generated from photochemical water oxidation by PSII immobilized on an electrode surface.