A proposed role for inorganic carbon in water oxidation

Abstract

This is an article on the peroxydicarbonic acid (PODCA) hypothesis of photosynthetic water oxidation, which follows our first article in this general area (Castelfranco et al., Photosynth Res 94:235–246, 2007). In this article I have expanded on the idea of a protein-bound intermediate containing inorganic carbon in some chemically bound form. PODCA is conceived in this article as constituting a bridge between two proteins of the oxygen-evolving complex (OEC) that are essential for the evolution of O₂. Presumably, these are two proteins which have been shown to possess Mn-dependent carbonic anhydrase activity (Lu et al., Plant Cell Physiol 46:1944–1953, 2005; Shitov et al., Biochemistry (Moscow) 74:509–517, 2009). One of these proteins may be the D_I of the OEC core and the other may be the PsbO extrinsic protein. I attempt to relate briefly the PODCA hypothesis to the role of two cofactors for O₂ evolution: Ca²⁺ and inorganic carbon. In this scheme, inorganic carbon (HCO₃ ⁻) mediates the oxidation of peroxide to dioxygen, thus avoiding the homolytic cleavage of the peroxide into two free radicals. I visualize the role of Ca²⁺ in the binding of PODCA to two essential photosystem II proteins. I propose that PODCA alternates between two Phases. In Phase 1, PODCA is broken down with the production of O₂. In Phase 2, PODCA is regenerated.