Stability and Oligosaccharide Binding of the N1 Cellulose-Binding Domain ofCellulomonas fimiEndoglucanase CenC

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Abstract
Differential scanning calorimetry has been used to study the thermal stability and oligosaccharide-binding thermodynamics of the N-terminal cellulose-binding domain of Cellulomonas fimi β-1,4-glucanase CenC (CBDN1). CBDN1 has a relatively low maximum stability (ΔGmax = 33 kJ/mol = 216 J/residue at 1 °C and pH 6.1) compared to other small single-domain globular proteins. The unfolding is fully reversible between pH 5.5 and 9 and in accordance with the two-state equilibrium model between pH 5.5 and 11. When the single disulfide bond in CBDN1 is reduced, the protein remains unfolded at all conditions, as judged by NMR spectroscopy. This indicates that the intramolecular cross-link makes a major contribution to the stability of CBDN1. The measured heat capacity change of unfolding (ΔCp = 7.5 kJ mol-1 K-1) agrees well with that calculated from the predicted changes in the solvent accessible nonpolar and polar surface areas upon unfolding. Extrapolation of the specific enthalpy and entropy of unfolding to their respective convergence temperature indicates that per residue unfolding energies for CBDN1, an isolated domain, are in accordance with those found by Privalov ( 1) for many single-domain globular proteins. DSC thermograms of the unfolding of CBDN1 in the presence of various concentrations of cellopentaose were fit to a thermodynamic model describing the linkage between protein−ligand binding and protein unfolding. A global two-dimensional minimization routine is used to regress the binding enthalpy, binding constant, and unfolding thermodynamics for the CBDN1−cellopentaose system. Extrapolated binding constants are in quantitative agreement with those determined by isothermal titration calorimetry at 35 °C.

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