Formation of Both Primary and Secondary N-Alkylhemins during Hemin-Catalyzed Epoxidation of Terminal Alkenes

Abstract

The N-alkylhemin formation during iron(II1) porphyrin-catalyzed epoxidation of alkenes is studied in a homogeneous system using pentafluoroiodosobenzene (PFIB) as the oxidant. The system is a model for the enzyme cytochrome P-450. A second N-alkyl species other than the well-documented primary N-alkylhemin is observed during the oxidation of terminal alkenes. It is unstable and decomposes under the reaction conditions. It is also a catalyst for alkene epoxidation by PFIB. Its characteristics are consistent with a secondary N-alkylhemin. The kinetics were partially resolved, and rate constants were estimated for the formation of both N-alkylhemins, for the decomposition of the secondary N-alkylhemin, and for the epoxidation by the hemin and the N-alkylhemins. The partition ratios were measured by a kinetic method, which provides better insight for the mechanisms of both N-alkylation and epoxidation than the measurements by product analysis using heterogeneous systems. We suggest that alkenes reversibly inhibit cytochrome P-450 through the formation of the unstable N-alkylporphyrin species.Suicide inactivation of cytochromes P-450 during oxidation of alkenes and alkynes catalyzed by these enzymes involves the alkylation of the porphyrins at a pyrrolic nitrogen.' Most of the substrates found to cause suicide inactivation are terminal alkenes, and the isolated porphyrin product contains an Nsubstituent attached at the primary carbon (Figure 1).* Synthetic models of cytochromes P-450 also form N-alkylhemins with both terminal and nonterminal alkene^.^-^ Unlike the enzymes, however, the synthetic N-alkylhemins were found to catalyze the epoxidation of alkenes (Scheme l).5 R I lo N-Alkylhemin 2 O N-Alkylhemin The structure proposed for N-alkylhemins contains a fivemembered metallocycle as shown in Figure 2A. The existence *Please address reprint requests to Frater, Y.; Gibbs, A. H.; De Matteis, F.; Lamb, J. H.; Farmer, P. B.; Naylor, S. Biochem. J . 1991, 274, 843-848. (2) Kunze, K. L.; Mangold, B. L. K.; Wheeler, C.; Beilan, H. S.; Ortiz de Montellano, P. R. J . Biol. Chem. 1983, 258, 4202-4207. (3) (a) Mansuy, D.; Devocelle, L.; Artaud, I.; Battioni, J.-P. New J. Chem. 1985, 9, 711-716. (b) Artaud, I.; Devocelle, L.; Battioni, I.-P.; Girault, J. P.; Mansuy, D.

Structure of the Modified Heme in Allylbenzene-Inactivated Chloroperoxidase Determined by Q-Band CW and Pulsed ENDOR

“…In a number of heme enzyme and heme model systems, the catalytic epoxidation of monosubstituted alkenes is accompanied by an autoinactivation reaction that produces modified hemins containing the alkene skeleton. The formation of these “green pigments” was first observed with hepatic microsomal cytochrome P450 (P450) in vivo , and has since been demonstrated with several model hemins 5,6 and more recently in our laboratory with chloroperoxidase (CPO). 3a, Surprisingly, the heme−alkene adducts spontaneously revert to the native heme on standing, with both CPO 7 and model systems . A similar reversibility for the P450 inactivation reaction has been predicted,6c but has not been reported.…”

Polyvalent perfluoroorgano- and selected polyfluoroorgano-halogen(III and V) compounds

“…One indicator may be the strongly differing physical data, e.g. the temperature of decomposition [36,39], solubilities [49,108,109], and IR spectroscopic data [36,40] reported for C 6 F 5 IO together with the spectroscopically proofed presence of [(C 6 F 5 ) 2 I][IO 3 ] [40]. IO 3 À anions may be the result of the pronounced nucleofugality of the C 6 F 5 group in [C 6 F 5 (OH)IO] À as an intermediate in the presence of OH À .…”

Mössbauer and electron paramagnetic resonance studies of chloroperoxidase following mechanism-based inactivation with allylbenzene

“…In parallel with this gain of activity, the prosthetic group modification is lost, and the heme reverts to heme b. Spontaneous dealkylation of model N-alkylhemins related to Structure 2 has been also reported (44)(45)(46), though no mechanism has been proposed. In each case, the reaction amounts to an N-dealkylation of the modified heme.…”