title: 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: Ca(2+) 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 Ca(2+) 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.
authors.affiliation: Department of Plant Biology, University of California, Davis, CA, 95616, USA, ridieliz@yahoo.com.
authors.forename: Paul A
authors.initials: PA
authors.lastname: Castelfranco
mesh_terms: D002244:Carbon / Q000378:metabolism*; D002250:Carbon Radioisotopes; D002255:Carbonic Acid / Q000378:metabolism; D008954:Models, Biological; D010084:Oxidation-Reduction; D010100:Oxygen; D045332:Photosystem II Protein Complex / Q000378:metabolism; D014867:Water / Q000378:metabolism*
chemical_list: D002250:Carbon Radioisotopes; D045332:Photosystem II Protein Complex; D014867:Water; D002255:Carbonic Acid; D002244:Carbon; D010100:Oxygen