Cytochrome c exhibits peroxidase-like activity and is able to catalyze the oxidation of a variety of organic substrates, including aromatic, organosulfur, and lipid compounds. As all peroxidases, cytochrome c is inactivates by the presence of hydrogen peroxide. During this inactivation, the heme prostetic group is destroyed.

Variants of the yeast iso-1-cytochrome constructed by site-directed mutageneris improved stability to inactivation by hydrogen peroxide. No heme destruction was detected in a triple variant (N521/Y67F/C102T) in the presence of ctalytic hydrogen peroxide concentrations (1mM) even following loss of catalytic activity, while both double variants Y67F/C102T and N52I/C102T exhibited a grester rate of peroxide-induced heme destruction that did the wild-type protein.

Heme destruction and catalytic inactivation are two independent processes. An internal water molecule (Wat 166) showed to be important in the heme destruction process. The absence of protein radicals in the resistant variant suggest that its presence is necessary in the heme destruction process, but presumably it is not involved in the reactions leading to protein inactivation.