[Role of ferredoxin in pseudo-cyclic electron transport in isolated pea chloroplasts]

The pseudo-cyclic electron transport in isolated pea chloroplasts in a medium without exogenous cofactors but containing methylviologen and ferredoxin was studied. In order to establish the steps limiting the electron transport, the rates of electron transport a different pH values and under phosphorylating and non-phosphorylating conditions in the presence of the uncoupler--gramicidin D--were determined. It was shown that in the absence of exogenous cofactors the total electron transport is limited by a stage of electron transfer from the acceptor site of photosystem I to oxygen. An addition of ferredoxin or methylviologen stimulates oxygen consumption. It was shown that the plot of the dependence of the rate of oxygen consumption on ferredoxin concentration is two-phase in contrast to that of methylviologen concentration, which is indicative of the existence of two competitive reactions of oxygen reduction involving ferredoxin (with different Km values for the protein).