Further evidence on the equilibrium "pre-molten globule state": four-state guanidinium chloride-induced unfolding of carbonic anhydrase B at low temperature.

Equilibrium guanidinium chloride-induced unfolding of bovine carbonic anhydrase NB has been investigated by a combination of optical methods with size-exclusion chromatography. It has been shown that, as in the case of staphylococcal beta-lactamase, bovine carbonic anhydrase B unfolds at low temperature through two equilibrium intermediates; the molten globule and the pre-molten globule states. This pre-molten globule state has a hydrodynamic volume no more than twofold larger than that of the native state, i.e. is relatively compact. It has a pronounced far UV CD spectrum, suggesting the presence of a substantial secondary structure. It binds 8-anilinonaphthalene-1-sulphonate (though weaker than the molten globule state), which suggests the formation of solvent-exposed clusters of non-polar groups. Thus, this novel state of protein molecules shares a number of properties with the "burst" kinetic intermediate of protein folding and can be considered as its equilibrium counterpart.