♦ See referenced article, J. Biol. Chem. 2009, 284, 36628–36637
NADPH-cytochrome P450 reductase (CPR) catalyzes the transfer of electrons from NADPH to acceptors such as cytochrome P450s via its two flavin cofactors FMN and FAD. The crystal structure of CPR shows a conformation that is ideally suited for interflavin electron transfer but far less suitable for subsequent transfer to a large acceptor protein. A possible explanation is that CPR can also adapt a more flexible conformation allowing for greater domain movement. In this Paper of the Week, Jacqueline Ellis and colleagues use a combination of NMR and small-angle x-ray scattering (SAXS) to address this possibility directly. Comparing the NMR spectra of the full CPR protein versus the isolated FMN domain showed that several residues are in different environments in the two structures, whereas SAXS experiments revealed that oxidized and reduced CPR have different overall shapes. Together, this study provides evidence that two conformations of CPR are present in equilibrium in solution; a compact state (the crystal structure) and an extended state, which likely is required for CPR to transfer electrons to acceptors. In addition, the relative contributions of each state to the equilibrium are affected by the binding of substrates and products, providing evidence that these two conformations are physiologically relevant.
The two conformations of CPR; the compact form (right), which is appropriate for FMN-FAD electron transfer, and the extended form (left), with the FMN exposed, which is appropriate for electron transfer to cytochrome P450. The cofactors are highlighted as follows: FMN, yellow; FAD, orange; NADPH, blue.