Ascorbate peroxidase (APX) isoforms localized in the stroma and thylakoid of the chloroplast play a principle role in detoxifying hydrogen peroxide (H2O2) generated in photosystem I; however, once the ascorbate is depleted, the enzyme is attacked by H2O2 and rapidly loses its activity. Here, we report that radical transfer across the porphyrin moiety and amino acid residues in the reaction intermediate and H2O2-mediated enzyme inactivation involve cooperative interactions of the Cys26, Trp35, and Cys126 residues of stromal APX. The wild-type enzyme had a half-time of inactivation of < 10 s, while the triple mutant of the three residues retained 50% of the initial activity after H2O2 treatment for 3 min. The H2O2 tolerance of this mutant was comparable to that of the H2O2-tolerant APX isoform localized in the cytosol. (c) 2008 Elsevier Inc. All rights reserved.