AS-48: a circular protein with an extremely stable globular structure.

The unfolding thermodynamics of the circular enterocin protein AS-48, produced by Enterococcus faecalis, has been characterized by differential scanning calorimetry. The native structure of the 70-residue protein is extremely thermally stable. Thus, at pH 2.5 and low ionic strength thermal denaturation occurs under equilibrium at 102 degrees C, while the unfolded state irreversibly aggregates at neutral and alkaline pH. Calorimetric data analysis shows that the specific enthalpy change upon unfolding is unusually small and the heat capacity change is quite normal for a protein of this size, whereas the Gibbs energy change at 25 degrees C is relatively high. At least part of this high stability might be put down to entropic constraints induced by the circular organization of the polypeptide chain.