In oxygenic photosynthesis, Photosystem II (PS II) catalyses the oxidation of water and transfers electrons to a plastoquinone electron acceptor (Q^A) and subsequently to the secondary acceptor (Q^B) ultimately forming plastoquinol (PQH²).

This entire process occurs in the presence of light and at ambient temperatures, highlighting the efficiency of PS II in splitting water and providing electrons for subsequent steps in the photosynthetic pathway. PS II operates as a 700 kDa dimeric membrane-protein complex with each monomer containing the D1, D2, CP43, and CP47 reaction centre subunits, 13 additional low-molecular-weight (LMW) proteins, the Mn⁴CaO⁵ oxygen-evolving complex (OEC), and various cofactors. The D1 and D2 reaction centre subunits form a heterodimer having five membrane-spanning helices (A–E) with an extended loop connecting the D and E helices that contribute to the binding sites of Q^A and Q^B, respectively.

In this study, conserved residues in the D2 DE loop were mutated in the cyanobacterium Synechocystis sp. PCC 6803 to investigate their roles in PS II assembly and function. In addition, a bicarbonate molecule, bound to a non-heme iron (NHI) between Q^A and Q^B, is also stabilized by the DE loops of D1 and D2.

The bicarbonate contributes to a hydrogen bond network incorporating several residues within the DE loops that facilitate electron and proton transfer leading to the formation of PQH² . Residues within the DE loop of the D2 polypeptide, including D2- Met246, D2- Val247, D2-Asn250, and D2-Ser254 were chosen for investigation due to their proximity to Q^A and PsbT, a LMW protein.

Additionally, D2-Arg265 , an arginine cap located on the E-helix of D2 , was targeted because of its proximity to the NHI . In the D2 mutant strains, a consistent observation was impaired PS II assembly and modified acceptor-side electron transfer kinetics. Substitutions of residues near Q^A also influenced donor-side electron transfer processes associated with the OEC .

Most mutants were susceptible to high-light-induced damage; however, in the presence of bicarbonate, oxygen evolution rates were generally rescued. Nevertheless, the D2-M246A, D2-S254H, and D2-V247D strains exhibited impaired oxygen evolution even when bicarbonate was added. Furthermore, the accumulation of pre-assembly modules containing the CP43 chlorophyll-binding protein was a recurrent theme in the D2 mutants. These findings highlight the D2 DE loop's contribution to stabilising the bicarbonate-NHI interactions and Q^A binding.

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Zoom link: https://otago.zoom.us/j/97756704741
Password: bioc