Phe265 of the D1 protein is required to stabilize plastoquinone binding in the QB-binding site of photosystem II in Synechocystis sp. PCC 6803

• Published in: Biochemical and biophysical research communications 2024-11, Vol.733, p.150692, Article 150692
• Main Authors: Brown, Toby J., Vass, Imre, Summerfield, Tina C., Eaton-Rye, Julian J.
• Format: Article
• Language: English
• Subjects: Photosynthesis; Photosystem II; Plastoquinone; Variable fluorescence
• ISSN: 0006-291X; 1090-2104; 1090-2104
• DOI: 10.1016/j.bbrc.2024.150692

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. •Photosystem II (PS II) has two plastoquinone electron acceptors QA and QB.•A conformational change of the DE-Loop of the D1 protein enables QB to bind.•Mutations at Phe265 do not disrupt PS II assembly but slow electron transfer to QB.•Phe265 tunes the redox midpoint potential of the QB/QB– couple.•The aromaticity of Phe265 of the DE-loop is required for QB binding.