Heme modification

Fig. 14. A mechanism to explain heme modification in the P. vitcde catalase and possibly E. coli HPII. For simplicity, the phenyl ring of T3rr415 is not shown, and only ring III of the heme and the heme iron are shown. Compound I is an oxyferryl species formed, along with water, in the reaction of one H2O2 with the heme. The iron is in a formal Fe oxidation state, but one oxidation equivalent is delocalized on the heme to create the 0x0-Fe" -heme cation, shown as the starting species, compound I. A water on the proximal side of the heme is added to the heme cation species of compound 1 shown in A to generate a radical ion in B. The electron flow toward the oxo-iron would generate the cation shown in (C), leading to the spirolactone product shown in D. In E, an alternate mechanism for the His-Tyr bond formation in HPII is presented that could occur independently of the heme modification reaction. Reprinted with permission of Cambridge University Press from Bravo et al. (93).

The strategy of modifying the prosthetic group can be divided into at least three approaches, as shown in Fig. 17 (1) modification of peripheral alkyl and/ or alkenyl side chains or two heme-propionates of protoheme IX (2) substitution of other metals such as Co, Mn, Cr, and so on, for the heme-iron and (3) preparation of a new prosthetic group with a non-porphyrin framework. These heme modifications must have a drastic influence on the Mb function, thus, the incorporation of an artificial prosthetic group into apoMb will give us a new protein with unique functions. 

Huang LS, Ortiz de Montellano PR (2006) Heme-protein covalent bonds in peroxidases and resistance to heme modification during halide oxidation. Arch Biochem Biophys 446 77-83... 

Prosthetic heme modification also occurs in some instances with the two-electron oxidation products formed by peroxidases when they oxidize halide and pseudohalide ions. Thus, the oxidation of bromide by HRP results in the addition of HOBr to one or both of the heme vinyl groups (Fig. 5.11) [62], Similar reactions are... 

Heme modification by the products of peroxidase catalysis has been observed with peroxidases other than HRP, but it does not occur with all peroxidases. Some peroxidases are resistant to these types of reactions. In particular, the mammalian peroxidases are resistant to heme modification by both the free radical and electrophilic metabolites [63]. This resistance is due, at least in part, to the covalent bonds that link the heme to the mature protein. A similar resistance to modification by the HOBr produced by HRP is observed when the reaction is carried out with the F41E mutant in which a covalent bond to the heme has been introduced [65]. However, resistance to radical products can occur even without the presence of covalent links between the heme and the protein. Thus, LiP has a heme that is resistant to modification by phenylhydrazine or azide, although the protein is apparently inactivated by modifications of the protein [66]. 

Huang L, Colas C, Ortiz de Montellano PR (2004) Oxidation of carboxylic acids by horseradish peroxidase results in prosthetic heme modification and inactivation. J Am Chem Soc 126 12865-12873... 

Huang L, Wojciechowski G, Ortiz de Montellano PR (2005) Prosthetic heme modification during halide ion oxidation. Demonstration of chloride oxidation by horseradish peroxidase. J Am Chem Soc 127 5345-5353... 

Autocatalytic radical reactions in physiological prosthetic heme modification 03CRV2305. 

Horseradish peroxidase can be inactivated by protein as well as heme modifications. The reaction of horseradish peroxidase with phenylhydrazine is instructive in this regard (Ator and Ortiz de Montellano, 1987). Enzyme inactivation correlates well with covalent binding of 2 equivalents of radiolabeled phenylhydrazine but is associated with conversion of only a small fraction of the prosthetic group to the meso-phenyl adduct. Covalent binding of a metabolite of phenylhydrazine to the protein is therefore primarily responsible for enzyme inactivation, although the identity of the reactive species and the site at which it reacts are not known. The factors that determine whether the heme group (e.g., methylhydrazine, sodium azide) or the protein (e.g., phenylhydrazine) is the primary site of the inactivation reaction also remain elusive. 

HPLC-analyses of heme extracts from PB-pretreated rabbit liver microsomes, incubated with or without PCP, identified PCP-modified heme fractions proposed to be iV-modified porphyrins, but the nature of these porphyrins was not established . It is therefore unclear whether heme modification or degradation was responsible for the loss of P450 activity or simply coincidental with it. However, a substantial discrepancy in the partition ratio obtained for PCP-mediated functional inactivation and that for heme loss suggested that this enzyme inactivation might also entail protein modification. 

Similar studies with f/-ans-4-hydroxy-2-none-nal (HNE, a cytotoxic byproduct of biological membrane lipid peroxidation), indicate that it is also metabolically activated by CYP2B1 and -2B4 to a reactive species that binds irreversibly to their prosthetic heme"". Unlike the mechanism-based inactivation by aromatic aldehydes, strucmral analyses of the corresponding heme adduct (MW 770) revealed that the reaction proceeds without deformylation and involves an acyl carbon radical that partitions between addition to the heme and formation of the carboxylic acid"". Together these findings suggest that the P450-mediated metabolic activation of aldehydes is a versatile process wherein the enzyme may be inactivated via mechanistically diverse heme modifications. 

Jonen, H.G., J. Werringloer, R.A. Prough, and R.W. Estabrook (1982). The reaction of phenyl-hydrazine with microsomal cytochrome P-450 Catalysis of heme modification. J. Biol. Chem. 257,4404-4411. 

Lin HL, D Agostino J, Kenaan C, Calinski D, Hol-lenbeig PF (2013) The effect of ritonavir on human CYP2B6 catalytic activity heme modification contributes to the mechanism-based inactivation of CY-P2B6 and CYP3A4 by ritonavir. Drug Metab Dispos 41 1813-1824... 

Peroxynitrite inaetivation of human cyto-ehrome P450s 2B6 and 2E1 heme modification and site-specific nitrotyrosine formation. Chem Res Toxicol 20 1612-1622... 

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