Oxidative and photosynthetic phosphorylation mechanisms

Models for protocellular photophosphorylation

Several photoreactions for transducing light energy have been analyzed for their relevance as models for protocellular photophosphorylation. Inorganic ions and compounds could have played a role in protocellular photophosphorylation. Organic catalysts may have been the next significant agents used by protocells for photophosphorylation. Membranous photophosphorylation probably became the most recent type of photoenergy transduction to be acquired by protocells; it is still used by modern cells although components of the other types of phosphorylation are found in present day cells. Recorded yields of energy-rich phosphates from the model reactions discussed are small. Arguments are advanced that such yields could have been sufficient to have fueled protocellular metabolism which was probably very slow compared to modern cellular metabolism. Future prospects for research in this area are discussed.

Primary photochemical reactions in chloroplast photosynthesis

Photosynthesis begins with the absorption of light energy and this absorbed energy is transferred to special sites, termed reaction centres. At these sites, the light energy is transformed into chemical products through an oxidation-reduction reaction that generates the primary reactants, an oxidized pigment molecule (P+) and a reduced electron acceptor (A–) (Clayton, 1972). The subsequent reactions of these species in the dark ultimately results in the formation of chemical products required for the fixation of CO2. In this essay we will discuss the nature of the primary reactants generated in the light reactions of chloroplast photosynthesis, stressing recent advances in the identification and characterization of such reactants.

A new photosynthetic pigment, "P430": its possible role as the priary electron acceptor of photosystem I

The technique of flash kinetic spectrophotometry was used to demonstrate a broad absorption band around 430 nm, which was kinetically different from P700, in several photosystem-I particles from spinach and blue-green algae. The component represented by this absorption band, designated as "P430", was bleached as fast as P700. Its recovery in the dark was accelerated by ferredoxin and by various artificial electron acceptors with redox potentials as low as -521 mV. The recovery kinetics have been demonstrated to be identical to those of a concomitant reduction of several of the artificial electron acceptors used. Tentatively, "P430" has been proposed as a possible primary electron acceptor of photosystem I.