MOLECULAR ORBITAL-BASED QUANTITATIVE STRUCTURE-ACTIVITY RELATIONSHIP FOR THE CYTOCHROME P450-CATALYZED 4-HYDROXYLATION OF HALOGENATED ANILINES

The cytochrome P450 (P450) catalyzed 4-hydroxylation of halogenated anilines was investigated with special emphasis on possible relationships between kinetic parameters and physicochemical and electronic characteristics of the substrates. The most important observation of the present study was a correlation (r = 0.96) between the natural logarithm of the apparent maximum reaction rate k(cat)(S) for 4-hydroxylation of the aniline substrates in a iodosobenzene-supported microsomal cytochrome P450-catalyzed reaction and the energy of the highest molecular orbital [E(HOMO)] of the anilines. This result is in accordance with a mechanism that proceeds by an initial electrophilic attack of the P450 (FeO)(3+) intermediate on the frontier pi electrons of the aniline substrates. In the iodosobenzene-supported aniline 4-hydroxylation this electrophilic attack is the rate-limiting step. In the NADPH/oxygen-supported cytochrome P450-catalyzed 4-hydroxylation of the anilines a correlation of the natural logarithm of k(cat)(S) with E(HOMO) was not observed and the k(cat)(S) values were lower than observed in the iodosobenzene-supported reaction. From this result it is concluded that, although the NADPH/oxygen-supported microsomal 4-hydroxylation of the halogenated anilines proceeds by the same cytochrome P450 (FeO)(3+) intermediate and, thus, by a similar electrophilic attack of the (FeO)(3+) on the pi electrons of the substrate, this attack is no longer the rate-limiting step of the reaction. Additional results of the present study demonstrate that the apparent Michaelis constant K-m(S) of the NADPH/oxygen-supported 4-hydroxylation of the anilines decreases with increasing hydrophobicity of the aniline derivatives. Because the spectral dissociation constant K-d(S) Of the aniline-cytochrome P450 complex appeared to be severalfold lower than the K-m(S), it was concluded that other parameters than binding influence the K-m(S) of the cytochrome P450-catalyzed aniline 4-hydroxylation. In conclusion, the present paper presents a MO-QSAR for the cytochrome P450-catalyzed conversion of a series of aniline derivatives. The implications of these findings for the catalytic cycle of cytochrome P450 are being discussed.