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Plant-type ferredoxin–NADP(H) reductases (FNRs) are flavoenzymes harboring one molecule of noncovalently bound flavin adenine dinucleo-doi:10.1016/j.jmb.2010.01.063 Available online at wwwIntroduction Plant-type ferredoxin–NADP(H) reductases (FNRs) constitute a family of monomeric enzymes harboring one molecule of noncovalently bound flavin adenine dinucleotide (FAD) as prosthetic group.1–4 FNRs were found in chloroplasts, non-photosynthetic plastids, apicoplasts, cyanobacteria, and phototrophic and heterotrophic bacteria.3,5 They catalyze reversible electron transfer between the iron–sulfur protein ferredoxin (Fd) or flavin-mono-nucleotide-containing flavodoxin and NADP(H)