Structural basis for the electron transfer from an open form of NADPH-cytochrome P450 oxidoreductase to heme oxygenase

Significance Heme oxygenase (HO) is a key enzyme for heme degradation that is deeply involved in iron homeostasis, defensive reaction against oxidative stress, and signal transduction mediated by carbon monoxide. To complete a single HO reaction, seven electrons supplied from NADPH-cytochrome P450 reductase (CPR) are required. Based on crystallography, X-ray scattering, and NMR analyses of CPR, it has been proposed that CPR has a dynamic equilibrium of the “closed-open transition.” The crystal structure of the transient complex of CPR with heme-bound HO clearly demonstrated that it is the open form of CPR that can interact with and transfer electrons to heme-bound HO. Moreover, the complex structure provides a scaffold to research the protein–protein interactions between CPR and other redox partners. NADPH-cytochrome P450 oxidoreductase (CPR) supplies electrons to various heme proteins including heme oxygenase (HO), which is a key enzyme for heme degradation. Electrons from NADPH flow first to flavin adenine dinucleotide, then to flavin mononucleotide (FMN), and finally to heme in the redox partner. For electron transfer from CPR to its redox partner, the ‘‘closed-open transition’’ of CPR is indispensable. Here, we demonstrate that a hinge-shortened CPR variant, which favors an open conformation, makes a stable complex with heme–HO-1 and can support the HO reaction, although its efficiency is extremely limited. Furthermore, we determined the crystal structure of the CPR variant in complex with heme–HO-1 at 4.3-Å resolution. The crystal structure of a complex of CPR and its redox partner was previously unidentified. The distance between heme and FMN in this complex (6 Å) implies direct electron transfer from FMN to heme.