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

Abstract

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 normalQnormalB2‐false(H+false)to give QBH2. 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 QBH2. 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 QBH2 via two different pathways that both lead to the protonation of normalQnormalB2‐false(H+false) by D1:His215.

The Interaction between PsbT and the DE Loop of D1 in Photosystem II Stabilizes the Quinone–Iron Electron Acceptor Complex

“…Bicarbonate dissociation is predicted to have a photoprotective effect, by shifting PS II into a low-productivity, low-damage state, to prevent photodamage. , This may benefit individual centers under high-light conditions, by slowing photodamage; however, if bicarbonate dissociation were to occur in all centers, then the result would likely be inhibitory to the organism due to low PS II activity, although turnover of the quinone–iron acceptor complex is still possible in the absence of bicarbonate via an alternative route for Q B 2– (H + ) protonation Figure d suggests that the bicarbonate is bound in the majority of ΔPsbT centers; however, it is currently unclear why the ΔPsbT strain is not affected in a manner similar to that of the F239A strain.…”

“…A non-heme iron is located between Q A and Q B , which is bound by four histidine ligands provided by D1 and D2 as well as a bidentate bicarbonate ligand . An extended loop of ∼40 residues connecting the D and E transmembrane helices of D1 lies directly above the quinone-iron acceptor complex (Figure ), with some of the DE loop residues interacting with the bound bicarbonate. , The removal of the bicarbonate ligand has been shown to affect Q A to Q B electron transfer, , and it has been hypothesized to be involved in the protonation of Q B . − Additionally, the removal of the bicarbonate ligand is hypothesized to provide a photoprotective effect. , …”

The hydrophobicity of mutations targeting D1:Val219 modifies formate and diuron binding in the quinone‐Fe‐acceptor complex of Photosystem II

“…1991b; Forsman et al . 2020; Forsman and Eaton‐Rye 2020). The reduced rate of Q A to Q B electron transfer following bicarbonate displacement by formate slows the rate of variable fluorescence decay following a saturating actinic flash (Eaton‐Rye and Govindjee 1984; Robinson et al .…”

“…The mutation of D1 residues surrounding the bicarbonate ligand to the NHI has frequently resulted in sensitivity to high‐light conditions (Forsman et al . 2019a; Forsman and Eaton‐Rye 2020; Forsman and Eaton‐Rye 2021). Consequently, the rate of oxygen evolution in the control and mutant strains was measured during and after a high‐light treatment to observe the sensitivity of the mutants to 1500 μmol photons m −2 s −1 .…”

“…Mutations around the bicarbonate ligand frequently result in an increased susceptibility to photodamage in the mutant strains (Forsman et al . 2019a; Forsman and Eaton‐Rye 2020). In this study, we have investigated the role of the Val219 residue in D1 by creating both the V219I and V219A mutants in the cyanobacterium Synechocystis sp.…”