Hydrogen peroxide oxidoreductase

The reactants of enzyme-catalyzed reactions are termed substrates and each enzyme is quite specific in character, acting on a particular substrate or substrates to produce a particular product or products. The names of enzymes usually indicate the substrate involved. For example, hydrogen peroxide oxidoreductase is an enzyme that uses hydrogen peroxide as its substrate to carry out the oxidation of organic substrates. Such formal names are often abbreviated such as in the contraction of hydrogen peroxide oxidoreductase to peroxidase. However, in the interest of avoiding... 

Peroxidase, also known as hydrogen peroxide oxidoreductase (EC 1.11.1.7), was the first enzyme that was investigated for its ability to accomplish the treatment of aqueous aromatic compounds [7], The system of reactions involved in the catalytic cycle of several peroxidases has been studied extensively and is summarized in Fig. 2. The catalytic cycle involves a progression of the enzyme through three oxidation states, i.e., native state (HRP), Compound I (HRP ) and Compound II (HRP i), following its reaction with hydrogen peroxide and its subsequent reactions with an aromatic substrate (AH2). The products of this cycle are water and highly reactive free radicals (AH ). It is these free radicals that spontaneously combine to produce insoluble precipitates. 

Peroxidase (donor hydrogen-peroxide oxidoreductase, EC 1.11.1.7) is a prominent biocatalyst that polymerizes a wide range of aromatic compounds, generally through the following consecutive steps (Yamazaki and Yokota, 1973 Sakurada et al., 1990) ... 

Catalase oxidoreductase (1) Aspergillus niger var. (2) bovine liver (3) Micrococcus lysodeikticus hydrogen peroxide hydrogen peroxide oxidoreductase 1.11.1.6... 

Yeast cytochrome c peroxidase (ferrocytochrome c hydrogen-peroxide oxidoreductase, EC 1.11.1.5) which catalyzes the oxidation of ferrocytochrome c to ferricytochrome c in the presence of hydroperoxide, was dls-... 

Catalase is a heme protein belonging to the class of oxidoreductases with ferripro-toporphyrin-IX at the redox center, and it catalyzes the disproportionation of hydrogen peroxide into oxygen and water without the formation of free radicals. 

Four methods have been developed for enzyme immobilization (1) physical adsorption onto an inert, insoluble, solid support such as a polymer (2) chemical covalent attachment to an insoluble polymeric support (3) encapsulation within a membranous microsphere such as a liposome and (4) entrapment within a gel matrix. The choice of immobilization method is dependent on several factors, including the enzyme used, the process to be carried out, and the reaction conditions. In this experiment, an enzyme, horseradish peroxidase (donor H202 oxidoreductase EC 1.11.1.7), will be imprisoned within a polyacrylamide gel matrix. This method of entrapment has been chosen because it is rapid, inexpensive, and allows kinetic characterization of the immobilized enzyme. Immobilized peroxidase catalyzes a reaction that has commercial potential and interest, the reductive cleavage of hydrogen peroxide, H202, by an electron donor, AH2 ... 

The mechanism(s) by which Xanthine oxidoreductase exerts its prooxidant effect(s) is not fully understood. Hydrogen peroxide, resulting from oxidation of a suitable substrate by Xanthine oxidoreductase, could oxidize milk lipids. However, normal milk contains little or no substrate for the enzyme. A possible mechanism involving interaction between native and denatured Xanthine oxidoreductase in MFGM and lactoperoxidase or copper in milk serum has been proposed (Hill, 1979 Allen and Wreiden, 1982b). 

Superoxide dismutases are enzymes that catalyze the dismutation of superoxide radicals to hydrogen peroxide and molecular oxygen. Superoxide dismutases either have Cu and Zn (cytosolic isoform) or Mn " " (mitochondrial isoform). Reduction of hydrogen peroxides in cells is accomplished via catalases to water and oxygen. Cellular organelles called peroxisomes contain large quantities of catalase. In addition, NADPH quinone oxidoreductases (DT-diaphorase) also act as antioxidant enzymes by catalyzing two-electron reduction of quinones. 

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