Peroxiredoxins (Prxs) make up a ubiquitous class (proposed EC 1.11.1.15) of cysteine-dependent peroxidases with roles in oxidant protection and signal transduction. An intriguing biophysical property of typical 2-Cys Prxs is the redox-dependent modulation of their oligomeric state between decamers and dimers at physiological concentrations. The functional consequences of this linkage are unknown, but on the basis of structural considerations, we hypothesized that decamer-building (dimer-dimer) interactions serve to stabilize a loop that forms the peroxidatic active site. Here, we address this important issue by studying mutations of Thr77 at the decamer-building interface of AhpC from Salmonella typhimurium. Ultracentrifugation studies revealed that two of the substitutions (T77I and T77D) successfully disrupted the decamer, while the third (T77V) actually enhanced decamer stability. Crystal structures of the decameric forms of all three mutant proteins provide a rationale for their properties. A new assay allowed the first ever measurement of the true k(cat) and K(m) values of wild-type AhpC with H(2)O(2), placing the catalytic efficiency at 4 x 10(7) M(-)(1) s(-)(1). T77V had slightly higher activity than wild-type enzyme, and both T77I and T77D exhibited ca. 100-fold lower catalytic efficiency, indicating that the decameric structure is quite important for, but not essential to, activity. The interplay between decamer formation and active site loop dynamics is emphasized by a decreased susceptibility of T77I and T77D to peroxide-mediated inactivation, and by an increase in the crystallographic B-factors in the active site loop, rather than at the site of the mutation, in the T77D variant.