Fast photochemical reactions of cytochrome P450 at subzero temperatures

Cryoradiolysis and cryospectroscopy for studies of heme-oxygen intermediates in cytochromes p450

Cryogenic radiolytic reduction is one of the most straightforward and convenient methods of generation and stabilization of reactive iron-oxygen intermediates for mechanistic studies in chemistry and biochemistry. The method is based on one-electron reduction of the precursor complex in frozen solution via exposure to the ionizing radiation at cryogenic temperatures. Such approach allows for accumulation of the fleeting reactive complexes which otherwise could not be generated at sufficient amount for structural and mechanistic studies. Application of this method allowed for characterizing of peroxo-ferric and hydroperoxo-ferric intermediates, which are common for the oxygen activation mechanism in cytochromes P450, heme oxygenases, and nitric oxide synthases, as well as for the peroxide metabolism by peroxidases and catalases.

Activation volume and energetic properties of the binding of CO to hemoproteins

We have investigated the CO binding to various reduced hemoproteins by stopped-flow rapid mixing as a function of pressure (from 0.1 to 200 MPa) and temperature (from 4 to 35 degrees C). In particular, we studied several varieties of cytochrome P-450: CYP11A1 (scc), CYP2B4 (LM2), CYP3A6 (LM3c), and Cyp2a (7 alpha), as well as chloroperoxidase and lactoperoxidase, and compared the results to data reported for other hemoproteins. Whereas the CO binding activation enthalpy delta H++ and entropy delta S++ (correlated through a compensation effect) varied greatly between the hemoproteins, with no apparent relation to structural features, the pressure effect depended on the nature of the proximal axial heme ligand: the activation volume was very small for cysteine (S-) ligand hemoproteins (delta V++ = +1 to +6 ml mol-1), and markedly negative for histidine (N) ligand hemoproteins (delta V++ = -3 to -36 ml mol-1). Furthermore, the transition state volume of the histidine ligand class enzymes, but not that of the cysteine ligand enzymes, depended on the solvent composition. These results suggest that the CO-binding transition state of the S-ligand class has a molecular conformation similar to the ground state. In the histidine class, however, the transition state appears to involve protein conformational changes and/or solvation processes.

Heme P460 of hydroxylamine oxidoreductase of Nitrosomonas. Reaction with CO and H2O2

Hydroxylamine oxidoreductase (HAO) of Nitrosomonas catalyzes the dehydrogenation of NH2OH and subsequent addition of oxygen to form nitrite. HAO contains c hemes and the CO-binding heme P460 in a 7:1 ratio; dehydrogenation of NH2OH involves passage of electrons to P460 and then c hemes. We now report that electrons rapidly pass from c hemes of HAO to the P460 center and then to H2O2. This conclusion is supported by (a) inhibition of c heme oxidation with CO and (b) loss of H2O2-oxidizability of ferrous c hemes following specific destruction of heme P460. Reaction of ferrous P460 with H2O2 is rate-limiting. Activation of dioxygen for N-oxidation by ferrous HAO may involve the two-electron reduction of O2 by P460. The reaction of ferrous HAO with H2O2 was studied as it may reveal aspects of the mechanism of activation of dioxygen. Reaction of ferrous heme P460 with CO is slow and with low affinity as compared with other hemoproteins. Values for reaction of CO with enzyme were: k1, 1.1 X 10(-3) M-1 s-1 and Kd, 12 microM.