The reaction mechanism of Photosystem I reduction by plastocyanin and cytochrome c6 follows two different kinetic models in the cyanobacterium Pseudanabaena sp. PCC 6903

Plastocyanin (Pc) and cytochrome c₆ (Cyt) have been purified to homogeneity from the cyanobacterium Pseudanabaena sp. PCC 6903, which occupies a unique divergent branch in the evolutionary tree of oxygen-evolving photosynthetic organisms. The two metalloproteins have similar molecular masses (9–10 kDa), as well as almost identical isoelectric points (ca. 8) and midpoint redox potentials (ca. 350 mV, at pH 7). Their reaction mechanism of electron transfer to Photosystem I (PS I) has been analyzed by laser-flash absorption spectroscopy. The kinetic traces with Pc correspond to monophasic kinetics, whereas those with Cyt are better fitted to biphasic curves. The observed pseudo first-order rate constant (k_obs) with Pc and that for the slower phase with Cyt exhibit saturation profiles at increasing donor protein concentrations, thereby suggesting that the two metalloproteins are able to form transient complexes with PS I. The ionic strength dependence of the rate constants for complex formation makes evident the electrostatic nature of intermediate complexes. The experimental findings indicate that the PS I reduction kinetics in Pseudanabaena follow a type II mechanism with Pc and a type III mechanism with Cyt, according to the different kinetic models proposed previously (Hervás M, Navarro JA, Díaz A, Bottin H and De la Rosa MA (1995) Biochemistry 34: 11321–11326)]. From an evolutionary point of view, this reinforces our previous observation that PS I was first adapted to operate efficiently with positively charged Cyt rather than with Pc.