ferredoxinNADPredukáz

Ferredoxin-NADP+ reductase, often abbreviated as FNR, is a flavoprotein enzyme found in the stroma of chloroplasts and in photosynthetic bacteria. Its primary role is to catalyze the final step in the linear electron transport chain of photosynthesis. Specifically, it transfers electrons from reduced ferredoxin to NADP+, producing NADPH. This NADPH is a crucial reducing agent required for the Calvin cycle, the process by which carbon dioxide is fixed into sugars. FNR is a homodimeric protein, with each subunit containing a flavin adenine dinucleotide (FAD) prosthetic group. The mechanism involves a series of electron transfer steps, with ferredoxin donating an electron to FNR, which then reduces NADP+. The enzyme's activity is regulated by light intensity and the redox state of the photosynthetic electron transport chain. Under high light conditions, increased electron flow to ferredoxin leads to higher FNR activity and thus more NADPH production. Conversely, under low light or stress conditions, FNR activity can be down-regulated. Variations of FNR exist, with some also capable of catalyzing the reverse reaction, the reduction of ferredoxin by NADPH, which can be important under specific conditions or in non-photosynthetic tissues.