文摘
A new model for the P450 enzyme carrying a SO3- ligand coordinated to iron(III) (complex 2)reversibly binds NO to yield the nitrosyl adduct. The rate constant for NO binding to 2 in toluene is of thesame order of magnitude as that found for the nitrosylation of the native, substrate-bound form of P450cam(E·S-P450cam). Large and negative activation entropy and activation volume values for the binding of NOto complex 2 support a mechanism that is dominated by bond formation with concomitant iron spin changefrom S = 5/2 to S = 0, as proposed for the reaction between NO and E·S-P450cam. In contrast, the dissociationof NO from 2(NO) was found to be several orders of magnitude faster than the corresponding reaction forthe E·S-P450cam/NO system. In a coordinating solvent such as methanol, the alcohol coordinates to iron(III) of 2 at the distal position, generating a six-coordinate, high-spin species 5. The reaction of NO with 5in methanol was found to be much slower in comparison to the nitrosylation reaction of 2 in toluene. Thisbehavior can be explained in terms of a mechanism in which methanol must be displaced during Fe-NObond formation. The thermodynamic and kinetic data for NO binding to the new model complexes of P450(2 and 5) are discussed in reference to earlier results obtained for closely related nitrosylation reactions ofcytochrome P450cam (in the presence and in the absence of the substrate) and a thiolate-ligated iron(III)model complex.