In Photosystem II electrons from water splitting pass through a primary quinone electron acceptor (Q) to the secondary plastoquinone (Q). The D2 protein forms the Q-binding site and the D1 protein forms the Q-binding site. A non-heme iron sits between Q and Q resulting in a quinone-Fe-acceptor complex that must be activated before assembly of the oxygen-evolving complex can occur. An extended loop (residues 223-266) between the fourth (helix D) and fifth (helix E) helices of the D1 protein activates forward electron transfer via a conformational change that stabilizes a bidentate bicarbonate ligand to the non-heme iron while simultaneously stabilizing the binding of Q. We show that positioning of D1:Phe265 to provide a hydrogen bond to the distal oxygen of Q is required for forward electron transfer. In addition, mutations targeting D1:Phe265, resulted in a 50 mV decrease in the Q/Q midpoint potential.
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