Abstract
In Photosystem II electrons from water splitting pass through a primary quinone electron acceptor (QA) to the secondary plastoquinone (QB). The D2 protein forms the QA-binding site and the D1 protein forms the QB-binding site. A non-heme iron sits between QA and QB 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 QB. We show that positioning of D1:Phe265 to provide a hydrogen bond to the distal oxygen of QB is required for forward electron transfer. In addition, mutations targeting D1:Phe265, resulted in a 50 mV decrease in the QB/QB– midpoint potential.