Artificial Carbonic Anhydrase-Ruthenium Enzyme for Photocatalytic Water Oxidation

Bovine carbonic anhydrase (BCA) is an enzyme that regulates cellular pH by catalyzing CO₂ hydration. In this work, we used its well-defined zinc-containing active site to host a series of four sulfonamide-functionalized ruthenium-based water oxidation catalysts Ru1 to Ru4, thereby producing four BCA-Ru1 to BCA-Ru4 artificial metalloenzymes (ArMs). The four ruthenium complexes differed either by the nature of the spectator ligand (bda^2– or tda^2–) bound to the catalytic center or by the length of the linker between the axially ruthenium-bound pyridine moiety and the zinc-binding sulfonamide. The two ArMs BCA-Ru1 and BCA-Ru2 were catalytically active for photocatalytic water oxidation in aqueous solution in the presence of [Ru(bpy)₃](ClO₄)₂ as a photosensitizer, Na₂S₂O₈ as an electron acceptor, and blue light (450 nm). The most active artificial metalloenzyme, BCA-Ru1, could drive photocatalytic O₂ production at particularly low ArM concentrations (5 μM), yielding a turnover number (TON) of 348 and a turnover frequency (TOF) of 9 min^–1 that was 1 order of magnitude higher than for the enzyme-free catalyst. A molecular dynamics study was performed to model the interaction between the ruthenium catalyst and the BCA protein. Overall, the protein scaffold modified the second coordination sphere around the catalytic center, which enhanced the activity and stability of two out of the four water oxidation catalysts in aqueous solution, modifying their pH dependence and suppressing the need for adding any organic solvents in solution. Altogether, these results demonstrate how useful artificial metalloenzymes can be for the making of artificial photosynthetic systems.