A low molecular mass pectate lyase from Fusarium mondiforme was unfolded reversibly by urea and Gdn-HCl at its optimum pH of 8.5, as monitored by intrinsic fluorescence, circular dichroism, and enzymatic activity measurements. Equilibrium unfolding studies yielded a DeltaG(H2O) of 1.741 kcal/mol, D-1/2 of 2.3 M, and m value of 0.755 kcal/mol M with urea and a DeltaG(H2O) of 1.927 kcal/ mol, D-1/2 of 1.52 M, and m value of 1.27 kcal/mol M with Gdn-HCl as the denaturant. Thermal denaturation of the pectate lyase at, pH 8.5, was also reversible even after exposure to 75 degreesC for 10 min. Thermodynamic parameters calculated from thermal denaturation curves at pH values from 5,0 to 8.5 yielded a DeltaC(p) of 0.864 kcal/(mol K). The DeltaG(25 degreesC) at, pH 8.5, was 2.06 kcal/mol and was in good agreement with the DeltaG(H2O) values obtained from chemical denaturation curves. There was no exposure of hydrophobic pockets during chemical or thermal denaturation as indicated by the inability of ANS to bind the pectate lyase. (C) 2004 Elsevier Inc. All rights reserved.