Crystal structure of the FMN-binding domain of human cytochrome P450 reductase at 1.93 angstrom resolution

The crystal structure of the FMN-binding domain of human NADPH-cytochrome P 450 reductase (P450R-FMN), a key component in the cytochrome P450 monnoxyge nase system, has been determined to 1.93 Angstrom resolution and shown to b e very similar both to the global fold in solution (Barsukov I et al., 1997 , J Biomal NMR 10:63-75) and to the corresponding domain in the 2.6 Angstro m crystal structure of intact rat P450R (Wang hi et al., 1997, Proc Nat. Ac ad Sci USA 93:8411-8416). The crystal structure of P450R-FMN reported here confirms the overall similarity of its alpha-beta-alpha architecture to tha t of the bacterial flavodoxins, but reveals differences in the position, nu mber, and length of the helices relative to the central beta-sheet. The mar ked similarity between P450R-FMN and flavodoxins in the interactions betwee n the FMN and the protein, indicate a striking evolutionary conservation of the FMN binding site. The P450R-FMN molecule has an unusual surface charge distribution, leading to a very strong dipole, which may be involved in do cking cytochrome P450 into place for electron transfer near the FMN. Severa l acidic residues near the FMN are identified by mutagenesis experiments to be important for electron transfer to P4502D6 and to cytochrome c, a clear indication of the part of the molecular surface that is likely to be invol ved in substrate binding. Somewhat different parts are found to be involved in binding cytochrome P450 and cytochrome c.