Interaction between ferredoxin and ferredoxin nicotinamide adenine dinucleotide phosphate reductase in pyridine nucleotide photoreduction and some partial reactions. I. Inhibition of ferredoxin nicotinamide adenine dinucleotide phosphate reductase by ferredoxin.

Purified ferredoxin has been shown to inhibit reactions mediated by the flavoprotein ferredoxin-NADP reductase. Ferredoxin inhibits the transfer of electrons from NADPH to ferricyanide (diaphorase activity) to NAD (transhydrogenase) and the photoreduction of pyridine nucleotides during the Hill reaction. On the basis of the kinetics of inhibition, it is suggested that the flavoprotein has two binding sites for substrates. At one site the enzyme binds NADPH and at the other site NADP or NAD. Ferredoxin inhibits the flavoprotein by competing with the binding of substrates at both sites. At proper ferredoxin concentrations, NAD can be photoreduced by isolated chloroplasts at a rate of 50 pmoles per mg of chlorophyll per hour. Since chloroplasts isolated by conventional methods have lost part of the ferredoxin-NADP reductase it is suggested that NAD might be photoreduced at appreciable rates in uivo.