The D1:Ser268 residue of Photosystem II contributes to an alternative pathway for QB protonation in the absence of bound bicarbonate.

Photosystem II catalyses the splitting of water and the reduction in plastoquinone in thylakoid membranes of all oxygenic photosynthetic organisms. The final step in quinol formation is protonation of the reduced secondary quinone electron acceptor Q B 2 - ( H + ) to give QB H2 . The proton for this step is hypothesized to be provided by a hydrogen-bond network incorporating amino acids from the Photosystem II D1 and D2 reaction center proteins, together with several bound waters and a bicarbonate ion ligated to a non-heme iron found between the primary plastoquinone electron acceptor QA and QB . The aim of this study was to investigate the role of bicarbonate and the D1:Ser268 residue in the formation of QB H2 . Using targeted mutagenesis of the D1 protein in the cyanobacterium Synechocystis sp. PCC 6803, we have created two mutants, S268A and S268T. Our D1:Ser268 mutants exhibited increased sensitivity to formate-induced inhibition of electron transfer between QA and QB and indicate that D1:Ser268 and bicarbonate support the second protonation in the formation of QB H2 via two different pathways that both lead to the protonation of Q B 2 - ( H + ) by D1:His215.