Crystal structure of a NADPH‐cytochrome P450 oxidoreductase (CYPOR) and heme oxygenase 1 fusion protein implies a conformational change in CYPOR upon NADPH/NADP+ binding

: Heme oxygenase-1 (HMOX1) catalyzes heme degradation utilizing reducing equivalents supplied from NADPH-cytochrome P450 reductase (CYPOR). Recently, we determined the complex structure of NADP+ -bound open-conformation stabilized CYPOR and heme-HMOX1, but the resolution was limited to 4.3 A. Here, we determined the crystal structure of the fusion protein of open-conformation stabilized CYPOR and heme-HMOX1 at 3.25 A resolution. Unexpectedly, no NADP+ was bound to this fusion protein in the crystal. Structural comparison of the NADP+ -bound complex and the NADP+ -free fusion protein suggests that NADP+ binding regulates the conformational change in the FAD-binding domain of CYPOR. As a result of this change, the FMN-binding domain of CYPOR approaches heme-bound HMOX1 upon NADP+ binding to enhance the electron-transfer efficiency from FMN to heme.