Heme peroxidases substrate specificity

The ability to both exist in a stable basal state and generate sufficiently oxidizing intermediates in order to specifically transform substrates is the challenge that heme peroxidases face. Redox potential (E°) play a critical role in determining a peroxidase ability to catalyze challenging oxidation reactions. However, it is not the only factor. As in other heme proteins, activity also depends on electrostatic interactions, substrate orientation, and active site topography [2],... 

The phenoloxidases are a related group of copper containing enzymes that catalyze the oxidation of phenols to quinones in animals and plants (5,6). Two distinct types of phenoloxidases that have substrate specificities and inhibitor sensitivities resembling typical tyrosinases and laccases are found in different types of insect cuticle (Z 8). Peroxidases, heme-containing enz)niies that also oxidize diphenols to quinones, may be present in cuticle and may play a role in sclerotization (9,10). 

Several enzymes with peroxidase-like action have also been used in immunoassays. Microperoxidases are catalytically active fragments obtained from cytochrome c by proteolytic action. They consist of the heme group covalently coupled to a short peptide alpha helix (26). The active site structure is similar to that of peroxidase Four of the six possible coordination bonds of the iron atom are occupied by bonding to the porphyrin while the fifth complexes with a histidine residue and the sixth is exposed to the environment and forms the catalytically active portion of the molecule. The reaction mechanism and spectrum of substrates is similar to HRP, although the specific activity is variable... 

The structure of beef liver catalase is shown in Fig. 2-16 [2], The heme site IS accessible by a channel 3 nm long. The specificity is largely determined by the ability of a substrate to form H-bonding interactions to some amino acids [2]. Unlike peroxidases, the fifth ligand of catalase is tyrosine. The phenyl ring is tilted 42° inwards to the heme plane with an iron-heme distance of 0.22 nm. This suggest that the phenolic side chain is deprotonated and has a localized charge. The reaction cycle with Fe(III) and Fe(IV) is shown in Eq. 2-6. 

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