Pectate lyase C (pelC) was the first protein in which the parallel beta-helix structure was recognized. The unique features of parallel beta-helix-containing proteins-a relatively simple topology and unusual interactions among side chains-make pelC an interesting protein to study with respect to protein folding. In this paper, we report studies of the unfolding equilibrium of pelC. PelC is unfolded reversibly by gdn-HCl at pH 7 and 5, as monitored by far- and near-UV CD and fluorescence. The coincidence of these spectroscopically detected transitions is consistent with a two-state transition at pH 7, but the three probes are not coincident at pH 5. No evidence was found for a loosely folded intermediate in the transition region at pH 5. At pH 7, the for unfolding is 12.2 kcal/mol, with the midpoint of the transition at 0.99 M gdn-HCl and m = 12.3 kcal/(mol.M). Thus, pelC is unusually stable and has an m value that is much larger than for typical globular proteins. Thermal denaturation of pelC has been studied by differential scanning calorimetry (DSC) and by CD. Although thermal denaturation is not reversible, valid thermodynamic data can be obtained for the unfolding transition. DeltaH(van't Hoff)/DeltaH(cal) is less than 1 for pHs between 5 and 8, with a maximum value of 0.91 at pH 7 decreasing to 0.85 at pH 8 and to 0.68 at pH 5. At all pHs studied, the excess heat capacity can be deconvoluted into two components corresponding to two-state transitions that are nearly coincident at pH 7, but deviate more at higher and lower pH. Thus, pelC appears to consist of two domains that interact strongly and unfold in a cooperative fashion at pH 7, but the cooperativity decreases at higher and lower pH. The crystal structure of pelC shows no obvious domain structure, however.