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.