ABSTRACT
Ferredoxin-NADP+ reductase, the essential catalyst of NADP+ photoreduction, is a flavoprotein ubiquitous in photosynthetic eukaryotic cells and Cyano-bacteria. The cyanobacteria enzyme has a much lower content of methionine and lysine and more histidines. The diaphorase activity of ferredoxin-NADP reductase (FNR) is commonly used for measuring the enzyme in crude preparations. Iodonitrotetrazolium chloride (INT) is the acceptor of choice because of its specificity as substrate of FNR. The amino acid sequence obtained by sequencing the spinach FNR was then fully confirmed by cDNA analysis. It has been suggested that FNR binds flavin adenine dinucleotide (FAD) to form a preholoenzyme before transport into the chloroplast, where maturation to the holoenzyme takes place. Indirect evidence for the presence in the FAD-binding domain of cysteines whose integrity is required for proper functioning of the enzyme comes from several experiments. The enzyme can be reduced to both the semiquinone and the dihydroquinone.
