Peroxidase iron properties

Typical examples of enzymes and their properties appear in the articles Copper Proteins Oxidases, Iron Heme Proteins, Peroxidases, Catalases Catalase-peroxidases Iron Heme... 

The co-operative effect. That a metal can become much more chemically active after chelation is evident enough in the oxygen-binding properties of haemoglobin and the oxidizing powers of the heme enzymes. Inorganic salts of iron have catalase- and peroxidase-like properties, but these are enormously increased upon incorporation in the porphyrin nucleus attached to a specific protein. Similarly, cuprous ions catalyse the aerial oxidation of ascorbic acid, but this effect is immensely magnified in the enzyme ascorbic oxidase (Table 10.4). 

This net transformation, however, encompasses a more complex mechanism that can be visualized in the generic catalytic cycle shown schematically in Fig. 5.2. Compounds I and II, the critical catalytic intermediates, are readily distinguished from the resting ferric state of the protein by their UV-visible absorption spectra (Table 5.1) [1-4]. Although the exact positions of the maxima show small variations, the spectroscopic properties of HRP are representative of those of all the peroxidases in which the heme iron atom is coordinated to a histidine nitrogen atom. The individual stages of the catalytic cycle are considered below. 

The first step of peroxidase catalysis involves binding of the peroxide, usually H2C>2, to the heme iron atom to produce a ferric hydroperoxide intermediate [Fe(IE)-OOH]. Kinetic data identify an intermediate prior to Compound I whose formation can be saturated at higher peroxide concentrations. This elusive intermediate, labeled Compound 0, was first observed by Back and Van Wart in the reaction of HRP with H2O2 [14]. They reported that it had absorption maxima at 330 and 410 nm and assigned these spectral properties to the ferric hydroperoxide species [Fe(III)-OOH]. They subsequently detected transient intermediates with similar spectra in the reactions of HRP with alkyl and acyl peroxides [15]. However, other studies questioned whether the species with a split Soret absorption detected by Back and Van Wart was actually the ferric hydroperoxide [16-18], Computational prediction of the spectrum expected for Compound 0 supported the structure proposed by Baek and Van Wart for their intermediate, whereas intermediates observed by others with a conventional, unsplit Soret band may be complexes of ferric HRP with undeprotonated H2O2, that is [Fe(III)-HOOH] [19]. Furthermore, computational analysis of the peroxidase catalytic sequence suggests that the formation of Compound 0 is preceded by formation of an intermediate in which the undeprotonated peroxide is coordinated to the heme iron [20], Indeed, formation of the [Fe(III)-HOOH] complex may be required to make the peroxide sufficiently acidic to be deprotonated by the distal histidine residue in the peroxidase active site [21],... 

Electron-transfer reactions between cytochrome c and cytochrome c peroxidase have been studied extensively because of the well-characterized structures and biophysical properties of the reactants [146-150]. It is well known that the resting ferric form of cytochrome c peroxidase is oxidized by hydrogen peroxide to compound I, which contains an oxyferryl heme moiety in which the iron atom has a formal oxidation state of 4-1- [146-150]. The other is a porphyrin n radical cation or organic radical (R +) localized on an amino acid residue of Trp-191 [151-154] this is formed by transfer of the oxidized equivalent to the amino acid side chain [150]. The site of electron transfer in the reduction of compound I has been controversial and two forms of compound II have been identified, (P)Fe =0 containing the oxyferryl heme Fe(IV) [155-158] and [(P)Fe ] + containing Fe(III) and the porphyrin % radical cation which oxidizes the amino acid side-chain to produce an organic radical [(P)Fe +, R" ] [159 165] as shown in Scheme 10. 

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