Atomic structure of ferredoxin-NADP+ reductase: prototype for a structurally novel flavoenzyme family.

The three-dimensional structure of spinach ferredoxin-NADP+ reductase (NADP+, nicotinamide adenine dinucleotide phosphate) has been determined by x-ray diffraction at 2.6 angstroms (A) resolution and initially refined to an R factor of 0.226 at 2.2 A resolution. The model includes the flavin-adenine dinucleotide (FAD) prosthetic group and the protein chain from residue 19 through the carboxyl terminus at residue 314 and is composed of two domains. The FAD binding domain (residues 19 to 161) has an antiparallel beta barrel core and a single alpha helix for binding the pyrophosphate of FAD. The NADP binding domain (residues 162 to 314) has a central five-strand parallel beta sheet and six surrounding helices. Binding of the competitive inhibitor 2'-phospho-AMP (AMP, adenosine monophosphate) places the NADP binding site at the carboxyl-terminal edge of the sheet in a manner similar to the nucleotide binding of the dehydrogenase family. The structures reveal the key residues that function in cofactor binding and the catalytic center. With these key residues as a guide, conclusive evidence is presented that the ferredoxin reductase structure is a prototype for the nicotinamide dinucleotide and FAD binding domains of the enzymes NADPH-cytochrome P450 reductase, NADPH-sulfite reductase, NADH-cytochrome b5 reductase, and NADH-nitrate reductase. Thus this structure provides a structural framework for the NADH- or NADPH-dependent flavoenzyme parts of five distinct enzymes involved in photosynthesis, in the assimilation of inorganic nitrogen and sulfur, in fatty-acid oxidation, in the reduction of methemoglobin, and in the metabolism of many pesticides, drugs, and carcinogens.