Evolutionary origins of the photosynthetic water oxidation cluster: Bicarbonate permits mn²⁺ photo-oxidation by anoxygenic bacterial reaction centers

The enzyme that catalyzes water oxidation in oxygenic photosynthesis contains an inorganic cluster (Mn₄CaO₅) that is universally conserved in all photosystem II (PSII) protein complexes. Its hypothesized precursor is an anoxygenic photobacterium containing a type 2 reaction center as photo-oxidant (bRC2, iron-quinone type). Here we provide the first experimental evidence that a native bRC2 complex can catalyze the photo-oxidation of Mn²⁺ to Mn³⁺, but only in the presence of bicarbonate concentrations that allows the formation of (bRC2)Mn²⁺(bicarbonate)_1-2 complexes. Parallel-mode EPR spectroscopy was used to characterize the photoproduct, (bRC2)Mn³⁺(CO₃^2-), based on the g tensor and ⁵⁵Mn hyperfine splitting. (Bi)carbonate coordination extends the lifetime of the Mn³⁺ photoproduct by slowing charge recombination. Prior electrochemical measurements show that carbonate complexation thermodynamically stabilizes the Mn³⁺ product by 0.9-1 V relative to water ligands. A model for the origin of the water oxidation catalyst is presented that proposes chemically feasible steps in the evolution of oxygenic PSIIs, and is supported by literature results on the photoassembly of contemporary PSIIs.