Haem peroxidases catalyse the H2O2-dependent oxidation of a variety of, usually organic, substrates. Mechanistically, these enzymes are very well characterized: they share a common catalytic cycle that involves formation of a two-electron oxidized intermediate (Compound I) followed by reduction of Compound I by substrate. The substrate specificity is more diverse, however. Most peroxidases oxidize small organic substrates, but there are prominent exceptions to this and the structural features that control substrate specificity remain poorly defined. APX (ascorbate peroxidase) catalyses the H2O2-dependent oxidation of l-ascorbate and has properties that place it at the interface between the class I (e.g. cytochrome c peroxidase) and classical class III (e.g. horseradish peroxidase) peroxidase enzymes. We present a unified analysis of the catalytic and substrate-binding properties of APX, including the crystal structure of the APX-ascorbate complex. Our results provide new rationalization of the unusual functional features of the related cytochrome c peroxidase enzyme, which has been a benchmark for peroxidase-mediated catalysis for more than 20 years.

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