Unique binding of nitric oxide to ferric nitric oxide reductase from fusarium oxysporum elucidated with infrared, resonance raman, and X-ray absorption spectroscopies

Nitric oxide reductase from the denitrifying fungus Fusarium oxysporum catalyzes the reduction of NO to N₂O [Nakahara, K., et al. J. Biol. Chem. 1993, 268, 8350-8355]. Since this enzyme belongs to the cytochrome P450 superfamily [Kizawa, H., et al. J. Biol. Chem. 1991, 266, 10632-10637], it is called cytochrome P450nor (P450nor), but does not exhibit monooxygenation activity. In the present study, we examine the coordination structure of the heme iron in P450nor in the ferric-NO form by using infrared, resonance Raman, and X-ray absorption (EXAFS = extended X-ray absorption fine structure) spectroscopies, since the ferric-NO complex is a first intermediate in the NO reduction cycle by P450nor [Shiro, Y, et al J. Biol. Chem. 1995, 270, 1617-1623]. We compared the data obtained with those for the d-camphor-bound form of Pseudomonas putida camphor hydroxylase cytochrome P450cam (P450cam), a typical model of the monooxygenase. From the vibrational spectroscopic measurements, we found that the Fe-bound N-O stretching frequency (v(N-O)) occurred at 1851 cm^-1 and the Fe-NO stretching frequency (Y(Fe-NO)) at 530 cm^-1 for P450nor, while those at 1806 and 522 cm^-1 were observed for P450cam, respectively. The assignments were confirmed by the ¹⁵NO substituting effect on these vibrational frequencies. These results indicated that NO binds to the ferric iron in P450nor stronger than in P450cam. Support for this was provided from the EXAFS study, which gave an Fe-N(NO) bond distance of 1.66 ± 0.02 Å for P450nor and 1.76 ± 0.02 Å for P450cam. These spectroscopic results suggest that, compared with P450cam, the lower steric hindrance and/or the difference in the electrostatic interactions of the ligand NO with its surroundings facilitates the donation of the NO 2pπ* electron to the iron 3dπ orbital, resulting in the strengthening of the Fe-NO and the N-O bonds of P450nor. The vibrational spectral observation of the ferrous-CO complex of P450nor supported this suggestion. This configuration can reduce the electron density on the NO ligand in P450nor, which is seemingly relevant to the NO reduction reactivity of P450nor.