Post-Translationally Generated Cysteine-Sulfonate Functions as a Ligand in the Oxygen-Evolving Complex of Photosystem II

Photosynthetic oxygen evolution is catalyzed by the Mn4CaO5 cluster in the oxygen-evolving complex (OEC) of photosystem II. The Mn4CaO5 cluster is coordinated by six carboxylate ligands and one imidazole ligand derived from amino acid residues. However, the precise roles of these amino acid ligands remain unclear. In this study, we genetically replaced a key carboxylate ligand, D1-Asp170, with Cys and investigated the effects of this mutation on the structure and reactions of the OEC. Measurements of O2 evolution, together with thermoluminescence and delayed luminescence analyses, indicated that the Mn4CaO5 cluster was formed in the D1-D170C mutant but exhibited low O2-evolving activity and markedly impaired S-state transitions beyond the S3 state. LC–MS analysis revealed that D1-Cys170 in the mutant was completely oxidized to cysteine sulfonic acid (Cys-SO3H). Quantum mechanics/molecular mechanics (QM/MM) calculations further demonstrated that the resulting cysteine sulfonate (Cys-SO3–) functions as a bridging ligand between the Mn and Ca ions, replacing the carboxylate group of Asp. This represents a unique example of an amino acid-derived sulfonate group serving as a ligand to the Mn4CaO5 cluster. The pronounced functional effects of replacing the carboxylate with a sulfonate ligand highlight the critical role of D1-Asp170 in regulating OEC reactions.