Murburn precepts for the light reaction of oxygenic photosynthesis
Kelath Murali Manoj, N. M. Bazhin, Abhinav Parashar, Daniel Andrew Gideon, Vivian David Jacob, Deepak Haarith, Afsal Manekkathodi
Abstract
Robert Emerson’s original observation (1957) that “oxygenesis occurs even with far-red light excitation of Photosystem I” is incompatible with the extant Kok-Joliot cycle’s foundation that “photolysis occurs only at red-light stimulated Photosystem II harboring MnComplex”. Further, the Z-scheme of electron transfer cannot account for Emerson’s observations of enhanced oxygenesis by simultaneous excitation of the two photosystems with both red and far-red light because serially connected components would surely increase systemic resistance to flow of charges, impeding the overall electron transfer process from water to NADP+. To address such discrepancies, we propose that the photo-excitation of various pigments leads to the formation of aquated electrons (eaq) and diffusible reactive oxygen species (DROS) in milieu, which are stabilized by a pool of redox-active elements within chloroplasts. Subsequently, the ‘eaq+DROS’ pool is utilized and routed via disordered and parallel reactions by the ‘photosystem switches’ for NADP reduction, O2 liberation and ADP phosphorylation. The stochastic ‘murburn’ model is thermodynamically and kinetically favorable and evidenced by the identification of multiple ADP-binding sites on PS II/Cytochrome b6f, and structure/distribution of the concerned proteins, complexes and pigments. The new model also explains the observed synergy in functioning of photosystems and plants’ photosynthetic spectral range of 400-700 nm.