Chloroperoxidase axial ligand

Figure 1.1. Iron(lII) protoporphyrin IX with a cysteinate as the axial ligand (1), which is typical of cytochrome P450, chloroperoxidase (CPO), and nitric oxide synthase (NOS) enzymes. The active oxygen species of these proteins and related heme enzymes is an oxoiron(IV) porphyrin cation radical (2), often called compound I.

T. Ogura, T. Kitagawa et al. (2001). Effects of a thiolate axial ligand on the it -it electronic states of oxoferryl porphyrins A study of the optical and resonance Raman spectra of compounds I and 11 of chloroperoxidase. J. Biol. Inorg. Chem. 6, 46-54. 

Axial ligands of heme-containing enzymes, such as a cysteine thiolate in cytochromes P450 and chloroperoxidases, a histidine imidazole in peroxidases, and a tyrosine phenolate in catalases, are... 

Fig. 1 Reactions involving molecular oxygen and its reduced congeners in heme proteins. Reactions where equilibria are manifest are indicated as such. In globins, peroxidases, cytochrome oxidases, and heme oxygenases, the axial ligand X is a protein-derived histidine in catalases, X is a tyrosine phenolate in chloroperoxidases and cytochromes P450, X is a cysteine thiolate...

The absorption spectrum of the Compound I form of chloroperoxidase is different from that of horseradish peroxidase Compound I, and is more closely analogous to that of catalase Compound I. This suggests that it may also have a Aiu ground state [22, 50]. However, the EPR spectrum of chloroperoxidase Compound I indicates that there is electron density at the meso carbons this finding is inconsistent with a Ai ground electronic state [50, 93]. Thus, the different absorption spectral properties of the Compound I intermediates of peroxidases may not derive solely from differences in orbital symmetry. Rather, other factors such as the nature of the axial ligand or the macrocycle stereochemistry may be responsible for these spectral differences. 

Horseradish peroxidase is one of a class of heme enzymes (also including chloroperoxidase, vide supra) that catalyze the overall two electron oxidation of organic substrates with H2O2 as the oxidant or electron acceptor (Eq. 3) [50]. Unlike P-450 and chloroperoxidase, the axial fifth ligand to the heme iron of horseradish peroxidase is a nitrogen donor (histidine). 

Spectral similarities between P-450 and chloroperoxidase originally led to suggestions that both enzymes had thiolate ligation [20, 22, 42]. However, the two systems displayed clear differences in their catalytic activities. Furthermore, at the time when EXAFS studies of chloroperoxidase were initiated, it was not clear whether the enzyme had a free (non-disulfide linked) cysteine available to coordinate to the heme iron [100]. Also, the unusually low pH optimum of the chloroperoxidase halogenation reaction, pH 3.0 for peroxidative formation of a carbon-halogen bond [42], raised questions concerning possible protonation of the axial heme ligand(s). 

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