Thermodynamics of staphylococcal nuclease denaturation. II. The A‐state

Staphylococcal nuclease, at low pH and in the presence of high salt concentrations, has previously been proposed to exist in a partially folded or molten globule form called the “A‐state” (Fink et al., 1993, Protein Sci 2:1155–1160). We have found that the A‐state of nuclease at pH 2.1 in the presence of moderate to high salt concentrations and at low temperature exists in a substantially folded form structurally more similar to a native state. The A‐state has the far‐UV circular dichroism spectra characteristic of the native protein, which indicates that it has a large degree of secondary structure. Upon heating, the A‐state denatures with a sigmoidal change in far‐UV ellipticity and an observable peak in a differential scanning calorimeter trace, indicating that it is thermodynamically distinct from the denatured state. Three different mutations in a residue normally buried in the protein's core stabilize or destabilize the A‐state in the same way as they affect the denaturation of the native state. The A‐state must, therefore, contain at least some tertiary packing of side chains. Unlike the native state, which shows cold denaturation at low temperatures, the A‐state is most stable at temperatures below 0°C.