Catalase-peroxidases (KatGs) use a peroxidase scaffold to support robust catalase activity, an ability no other member of its superfamily possesses. Because catalase turnover requires H(2)O(2) oxidation, whereas peroxidase turnover requires oxidation of an exogenous electron donor, it has been anticipated that the latter should inhibit catalase activity. To the contrary, we report peroxidatic electron donors stimulated catalase activity up to 14-fold, particularly under conditions favorable to peroxidase activity (i.e., acidic pH and low H(2)O(2) concentrations). We observed a "low-" and "high-K(M)" component for catalase activity at pH 5.0. Electron donors increased the apparent k(cat) for the "low-K(M)" component. During stimulated catalase activity, less than 0.008 equivalents of oxidized donor accumulated for every H(2)O(2) consumed. Several classical peroxidatic electron donors were effective stimulators of catalase activity, but pyrogallol and ascorbate showed little effect. Stopped-flow evaluation showed that a Fe(III)-O(2)(·-)-like intermediate dominated during donor-stimulated catalatic turnover, and this intermediate converted directly to the ferric state upon depletion of H(2)O(2). In this respect, the Fe(III)-O(2)(·-) -like species was more prominent and persistent than in the absence of the donor. These results point toward a much more central role for peroxidase substrates in the unusual catalase mechanism of KatG.