Hemoproteins reaction with iron

Figure 6 The most common paradigm for hemoprotein-catalyzed substrate oxidation involves heterolytic scission of the 0-0 bond of an iron-bound peroxo species to give an Fe(IV) = O ferryl intermediate and either a porphyrin radical or a protein radical. The peroxo intermediate is generated in the cytochromes P450 by in situ NAD(P)H-dependent reduction of O2, and in the peroxidases by reaction with H2O2.

There are some further aspects of the ionic iron-hydrogen peroxide system which have a possible bearing on hemoprotein reactions. Apart from the reactions of the HO radical listed above (1 and 2) and reactions with oxidizable or polymerizable substrates, there is some experimental evidence that two additional types of reaction may occur. The first... 

A less common reactive species is the Fe peroxo anion expected from two-electron reduction of O2 at a hemoprotein iron atom (Fig. 14, structure A). Protonation of this intermediate would yield the Fe —OOH precursor (Fig. 14, structure B) of the ferryl species. However, it is now clear that the Fe peroxo anion can directly react as a nucleophile with highly electrophilic substrates such as aldehydes. Addition of the peroxo anion to the aldehyde, followed by homolytic scission of the dioxygen bond, is now accepted as the mechanism for the carbon-carbon bond cleavage reactions catalyzed by several cytochrome P450 enzymes, including aromatase, lanosterol 14-demethylase, and sterol 17-lyase (133). A similar nucleophilic addition of the Fe peroxo anion to a carbon-nitrogen double bond has been invoked in the mechanism of the nitric oxide synthases (133). 

Synthesis and reactions of nitric oxide (NO). l-NMMA inhibits nitric oxide synthase. NO complexes with the iron in hemoproteins (eg, guanylyl cyclase), resulting in the activation of cGMP synthesis and cGMP target proteins such as protein kinase G. Under conditions of oxidative stress, NO can react with superoxide to nitrate tyrosine. 

The above mechanistic interpretation is in contrast with the one appearing in the coordination chemistry of NO on the very labile Fe(III) porphyrins and hemoproteins, which show water substitution-controlled kinetics at the iron(III) center (22,25). The latter Fe(III) moieties are, however, high-spin systems, whilst the cyano-complexes are low-spin. There is strong experimental evidence to support the dissociative mechanism with the Fe(III)-porphyrins, because the rates are of the same order as the water-exchange reactions measured in these systems (22d). Besides, the Fe(III) centers are less oxidizing than [Fein(CN)5H20]2- (21,25). 

Some small peptide-heme complexes have been prepared, including an undecapeptide (residues 11-21)668"669 and an octapeptide (residues 14-21). TTiese are useful models as they include the two cysteine residues that covalently link the heme to the peptide, and one of the axial ligands. The axial Met-80 residue is absent, but the position can be filled by methionine or by other ligands as required.670 Work with several octapeptide complexes shows that the rates of outer-sphere electron transfer appear to be independent of the axial ligand, and faster than the reaction for cytochrome c. Other comparisons show that the orientation of the axial methionine in cytochrome c and the contacts between heme and protein are important controlling factors in the electronic structure of the heme. Aqua and hydroxo complexes of iron(III) octapeptide complexes are also useful models for studying spin equilibria in iron(III) hemoproteins.671... 

Several diverse metal centres are involved in the catalysis of monooxygenation or hydroxylation reactions. The most important of these is cytochrome P-450, a hemoprotein with a cysteine residue as an axial ligand. Tyrosinase involves a coupled binuclear copper site, while dopamine jS-hydroxylase is also a copper protein but probably involves four binuclear copper sites, which are different from the tyrosinase sites. Putidamonooxin involves an iron-sulfur protein and a non-heme iron. In all cases a peroxo complex appears to be the active species. 

A quarter of a century has passed since the first contribution on catalase to The Enzymes Enzyme substrate compounds Mechanism of action of hydroperoxidases (I). In this perspective, we can identify a sequence of steps in the development of ideas on the mechanism of enzymic action and the nature of enzyme-substrate compounds. The identification of these compounds and the approach to enzymic reactions at concentrations stoichiometric with the substrate caused a principal transition of viewpoint on hemoprotein catalysis from free radical mechanisms (2) unrelated to an active center toward the acceptance of catalysis occurring at the iron atom of the porphyrin (S-5). The latter concept followed natu-... 

The reaction sequence at the heme active site starts with the binding of unactivated triplet dioxygen forming the so-called oxy-heme complexes. The iron center in 02-activating heme enz5maes is then thought to be converted into a peroxo anion species. It can be protonated to form a ferric hydroperoxo intermediate usually termed compormd 0 (183), which is a crucial reactive species in catalase and peroxidase enz5nne catalysis (Fig. 21). These hydroperoxo intermediates of hemoproteins are important... 

NO reacts with both ferric and ferrous centers in hemoproteins to form the respective iron(II) and iron(III) nitrosyl adducts, whose structural features are similar to those observed for iron (II) and iron(III) porphyrin nitrosyls. These analogies are also reflected in similar chemical reactivity observed for nitrosylated ferri- and ferroproteins and their respective porphyrin models. For example, NO-adducts of Fe(III) undergo reductive nitrosylation in the presence of an excess of NO, and a similar process is commonly observed for synthetic Fe(III) porphyrins. The first step of this reaction involves nucleophilic attack of OH on the nitrosyl ligand coordinated to the iron center, as presented in reaction (13) (33,60) ... 

---