Ferredoxin–NADP+ reductase is the site of oxygen reduction in pseudocyclic electron transport

Abstract

The effects of ferredoxin (Fd) and Fd–NADP+ reductase (FNR) on the oxygen photoreduction by photosystem I (PS I) in spinach (Spinacia oleracea L.) thylakoid membranes were investigated using a unique photoelectrochemical cell. This cell was previously shown to monitor the Mehler reaction products of photosynthetic oxygen reduction and represents an excellent tool for studying pseudocyclic electron transport. The magnitude of the photocurrent produced by the thylakoids was increased by as much as 40% in the presence of 60 μM Fd. If thylakoids were supplemented by both Fd and FNR, an additional increase of photocurrent was observed. All these reactions were inhibited by catalase, an enzyme that degrades H2O2, to demonstrate that O2 reduction was involved in all the photoreactions studied. The fact that more O2 was consumed in the presence of FNR was interpreted as evidence that the most effective site of oxygen reduction on the acceptor side of PS I is on FNR and not on Fd. The in vivo implication is that during pseudocyclic electron transport, NADP+ and oxygen directly compete for PS I electrons, with the former having significantly faster reaction kinetics. The advantageous physiological consequences of such a competition are (i) pseudocyclic electron transport would represent a true attenuating mechanism of the redox state of the NADP+–NADPH pool, (ii) oxygen would be a contingent acceptor under high illumination stress, helping to cope with the resultant elevated electron transport rates, and (iii) this mechanism is indisputably a faster response to stress than cyclic electron transport. Key words: Spinacia oleracea, photosystem I, thylakoid membranes, ferredoxin–NADP+ reductase, pseudocyclic electron transport, photoelectrochemistry.