Heme in Catalysis

Heme (Fe +) and hemin (Fe +) proteins are widely distributed in food. Lipid peroxidation in animal tissue is accelerated by hemoglobin, myoglobin and cytochrome C. These reactions are often responsible for rancidity or aroma defects occurring during storage of fish, poultry and cooked meat. In plant food the most important heme(in) proteins are peroxidase and catalase. Cytochrome P450 is a particularly powerful catalyst for lipid peroxidation, although it is not yet clear to what extent the compound affects food shelf life in situ . 

During heme catalysis, a Fe + protoporphyrin complex (P-Fe +), like in myoglobin, will be oxidized by air to P-Fe + as indicated in Formula 3.66. The formed superoxide radical anion O2, whose properties are discussed 

Oxene is the active form of the hemin catalyst. It oxidizes two fatty acid hydroperoxide molecules to peroxy radicals that will then initiate lipid peroxidation. 

In comparison with iron ions, some heme(in) corr5 ounds degrade the hydroperoxides more rapidly hy several orders of magnitude (cf. Table 3.29). Therefore they are more effective as initiators of lipid peroxidation. Their activity is also negligibly influenced by a decrease in the pH-value. 

However, the activity of a heme(in) protein towards hydroperoxides is influenced by its steric accessibility to fatty acid hydroperoxides. Hydroperoxide binding to the Fe-porphyrin moiety of native catalase and peroxidase molecules is obviously not without interferences. The prosthetic group is free to promote hydroperoxide decomposition only after heat denaturation of the enzymes. Indeed, a model experiment with peroxidase showed that the peroxidation of linoleic acid increased by a factor of 10 when the enzyme was heated for 1 minute to 140 °C. As expected, the enzymatic activity of peroxidase decreased and was only 14%. Similar results were obtained in reaction systems containing catalase. 

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Pufahl, R. A., and Marietta, M. A. (1993). Oxidation of N -Hydroxy-L-arginine by nitric oxide synthase Evidence for the involvement of the heme in catalysis. Biochem. Biophys. Res. Commun. 193, 963-970. 

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