Oxidation of indole-3-acetic acid by dioxygen catalysed by plant peroxidases: specificity for the enzyme structure

Indole-3-acetic acid (IAA) can be oxidized via two mechanisms: a convention al hydrogen-peroxide-dependent pathway, and one that is hydrogen-peroxide-i ndependent and requires oxygen. It has been shown here for the first time t hat only plant peroxidases are able to catalyse the reaction of IAA oxidati on with molecular oxygen. Cytochrome c peroxidase (CcP), fungal peroxidases (manganese-dependent peroxidase, lignin peroxidase and Arthromyces ramosus peroxidase) and microperoxidase were essentially inactive towards IAA in t he absence of added H2O2. An analysis of amino acid sequences allowed five structurally similar fragments to be identified in auxin-binding proteins a nd plant peroxidases. The corresponding fragments in CcP and fungal peroxid ases showed no similarity with auxin-binding proteins. Five structurally si milar fragments form a subdomain including the catalytic centre and two res idues highly conserved among 'classical' plant peroxidases only, namely His -40 and Trp-117. The subdomain identified above with the two residues might be responsible for the oxidation of the physiological substrate of classic al plant peroxidases, IAA.