A proposed role for inorganic carbon in water oxidation.

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.

Hypothesis: the peroxydicarbonic acid cycle in photosynthetic oxygen evolution.

Peroxydicarbonic acid (Podca), a proposed intermediate in photosynthetic oxygen evolution, was synthesized electrochemically. Consistent with literature descriptions of this compound, it was shown to be a highly reactive molecule, spontaneously hydrolyzed to H2O2, as well as susceptible to oxidative and reductive decomposition. In the presence of Mn2+ or Co2+, Podca was quickly broken down with release of O2. The liberation of O2, however, was partially suppressed at high O2 concentrations. In the presence of Ca-washed photosystem II-enriched membranes lacking extrinsic proteins, Podca was decomposed with the release of O2, but only under conditions favoring photosynthetic electron flow (light plus a Hill oxidant). A model is proposed that details how peroxydicarbonic acid could act as an oxygen-evolving intermediate. The hypothesis is consistent with the well-established Kok model and with recent findings related to the chemistry of oxygen evolution.

Studies on chlorophyll biosynthesis and other things.

Our research on chlorophyll biosynthesis, over a period of approximately twenty years, has been described, emphasizing those areas in which our laboratory made significant and timely contributions. References to some of our most important articles are included. Portions of the chlorophyll biosynthetic pathway, in which our own laboratory was not involved, for example, the reduction of protochlorophyllide to chlorophyllide and the phytylation of the latter to yield chlorophyll a, have not been covered in this article. Those events which preceded my involvement with chlorophyll biosynthesis, but which contributed to the formation of my own scientific personality, are mentioned briefly in the Introduction. My non-scientific avocations have been included at the request of the reviewers and Govindjee.