CATALASE AND PEROXIDASE Description of the Enzymes

Catalase and peroxidase are heme enzymes that catalyze reactions of hydrogen peroxide.94,95 In catalase, the enzymatic reaction is the disproportionation of hydrogen peroxide (Reaction 5.82) and the function of the enzyme appears to be prevention of any buildup of that potentially dangerous oxidant (see the discussion of dioxygen toxicity in Section III).

Peroxidase reacts by mechanisms similar to catalase, but the reaction catalyzed is the oxidation of a wide variety of organic and inorganic substrates by hydrogen peroxide (Reaction 5.83).

(The catalase reaction can be seen to be a special case of Reaction 5.83 in which the substrate, AH2, is hydrogen peroxide.) Some examples of peroxidases that have been characterized are horseradish peroxidase, cytochrome c peroxidase, glutathione peroxidase, and myeloperoxidase. 

 

X-ray crystal structures have been determined for beef-liver catalase 80 and for horseradish peroxidase 96 in the resting, high-spin ferric state. In both, there is a single heme b group at the active site. In catalase, the axial ligands are a phenolate from a tyrosyl residue, bound to the heme on the side away from the active-site cavity, and water, bound to heme within the cavity and presumably replaced by hydrogen peroxide in the catalytic reaction. In horseradish peroxidase, the axial ligand is an imidazole from a histidyl residue. Also within the active-site cavity are histidine and aspartate or asparagine side chains that appear to be ideally situated to interact with hydrogen peroxide when it is bound to the iron. These residues are believed to play an important part in the mechanism by facilitating 0-0 bond cleavage (see Section VI.B below).
 

Three other forms of catalase and peroxidase can be generated, which are referred to as compounds I, II, and III. Compound I is generated by reaction of the ferric state of the enzymes with hydrogen peroxide.  Compound I is greenand has spectral characteristics very similar to the FeIV(p' -)(0) + complex prepared at low temperatures by reaction of ferric porphyrins with single-oxygenatom donors (see Section V.c.1.a). Titrations with reducing agents indicate that it is oxidized by two equivalents above the ferric form. It has been proposed (see 5.84) that the anionic nature of the tyrosinate axial ligand in catalase may serve to stabilize the highly oxidized iron center in compound I of that enzyme, 80 and furthermore that the histidyl imidazole ligand in peroxidase may deprotonate, forming imidazolate,52,97 or may be strongly hydrogen bonded,98 thus serving a similar stabilizing function for compound I in that enzyme.

Reduction of compound I by one electron produces compound II, which has the characteristics of a normal ferryl-porphyrin complex, analogous to 2, i.e., (L)FeIV(p)(O). Reaction of compound II with hydrogen peroxide produces compound III, which can also be prepared by reaction of the ferrous enzyme with dioxygen. It is an oxy form, analogous to oxymyoglobin, and does not appear to have a physiological function. The reactions producing these three forms and their proposed formulations are summarized in Reactions (5.85) to (5.88).