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Peroxiredoxins (Prx's) are a superfamily of thiol-specific antioxidant proteins present in all organisms and involved in the hydroperoxide detoxification of the cell. The catalytic cysteine of Prx's reduces hydroperoxides and is transformed into a transient sulfenic acid (Cys-SOH). At high hydroperoxide concentration, the sulfenic acid can be overoxidized into a sulfonate, or even a sulfonate. We present here the first peroxiredoxin characterization by solution NMR of the Saccharomyces cerevisiae alkylhydroperoxide reductase (Ahpl) in its reduced and in vitro overoxidized forms. NMR [sup15]N relaxation data and ultracentrifugation experiments indicate that the protein behaves principally as a homodimer (2 × 19 kDa) in solution, regardless of the redox state. In vitro treatment of Ahp 1 by a large excess of tBuOOH leads to an inactive form, with the catalytic cysteine overoxidized into sulfonate, as demonstrated by [sup13]C NMR. Depending on the amino acid sequence of their active site, Prx' s are classified into five different families. In this classification, Ahpl is a member of the scarcely studied D-type Prx' s. Ahpl is unique among the D-type Prx's in its ability to form an intermolecular disulfide. The peptidic sequence of Ahpl was analyzed and compared to other D-type Prx sequences. [ABSTRACT FROM AUTHOR]