Biological Oxidations with Heme Proteins. In: Montanari F, Casella L, editors. Metalloporphyrins Catalyzed Oxidations. Dordrecht: Springer Netherlands; 1994. pp. 269-306. [DOI: 10.1007/978-94-017-2247-6_9]

Reactivity study on microperoxidase-8

The catalytic activity of the microperoxidase-8/H(2)O(2) system toward tyramine and 3-(4-hydroxyphenyl)propionic acid has been determined in acetate buffer, pH 5.0. Operating with a strong excess of hydrogen peroxide, the rate-determining step of the reaction was substrate oxidation. Owing to the fast microperoxidase-8 degradation, only the very initial phase of the reactions were analyzed. The reaction rates follow a substrate saturation behavior, with turnover numbers [ k(cat)=26+/-1 s(-1) for 3-(4-hydroxyphenyl)propionic acid and k(cat)=22+/-1 s(-1) for tyramine] that were similar for the two substrates. In contrast, the K(M) values indicated a reduced affinity for the catalyst active species by the positively charged phenol, probably due to repulsive interaction with the protonated N-terminal microperoxidase-8 amino group. The reactivity of the catalyst active species was studied upon incubation of microperoxidase-8 with a small excess hydrogen peroxide, followed by reaction with the phenolic substrates. The kinetic analysis showed that more than two active species are accumulated. The species responsible for the faster reactions was present in solution as a minor fraction. The active intermediate which accumulated in a larger amount (intermediate III) has a reduced substrate oxidation activity. Comparison of this activity with the kinetic constants obtained under turnover experiments shows that intermediate III is not involved in the microperoxidase-8 catalytic cycle. The active species of the catalytic process are intermediates I and II, which in the absence of substrate rapidly convert to intermediate III.

Axial Imidazole Distortion Effects on the Catalytic and Binding Properties of Chelated Deuterohemin Complexes

The effect of strain in the axial coordination of imidazole to the heme has been studied in the chelate complexes deuterohemin-histidine (DH-His) and deuterohemin-alanylhistidine (DH-AlaHis). Molecular mechanics calculations indicate that three types of distortion of the axial ligand occur in DH-His, due to the relatively short length of the arm carrying the donor group: tilting off-axis, tipping, and inclination of the imidazole plane with respect to the axial Fe-N bond. The effects of tilting (Deltagamma approximately 10 degrees ) and inclination of the imidazole ring (Deltadelta approximately 17 degrees ) are dominant, while tipping is small and is probably of little importance here. By contrast, the axial imidazole coordination is normal in DH-AlaHis and other computed deuterohemin-dipeptide or -tripeptide complexes where histidine is the terminal residue, the only exception being DH-ProHis, where the rigidity of the proline ring reduces the flexibility of the chelating arm. The distortion in the axial iron-imidazole bond in DH-His has profound and negative influence on the binding and catalytic properties of this complex compared to DH-AlaHis. The former complex binds more weakly carbon monoxide, in its reduced form, and imidazole, in its oxidized form, than the latter. The catalytic efficiency in peroxidative oxidations is also reduced in DH-His with respect to DH-AlaHis. The activity of the latter complex is similar to that of microperoxidase-11, the peptide fragment incorporating the heme that results from hydrolytic cleavage of cytochrome c.