Structural energetics of the molten globule state.

Certain partly ordered protein conformations, commonly called "molten globule states," are widely believed to represent protein folding intermediates. Recent structural studies of molten globule states of different proteins have revealed features which appear to be general in scope. The emerging consensus is that these partly ordered forms exhibit a high content of secondary structure, considerable compactness, nonspecific tertiary structure, and significant structural flexibility. These characteristics may be used to define a general state of protein folding called "the molten globule state," which is structurally and thermodynamically distinct from both the native state and the denatured state. Despite extensive knowledge of structural features of a few molten globule states, a cogent thermodynamic argument for their stability has not yet been advanced. The prevailing opinion of the last decade was that there is little or no enthalpy difference or heat capacity difference between the molten globule state and the unfolded state. This view, however, appears to be at variance with the existing database of protein structural energetics and with recent estimates of the energetics of denaturation of alpha-lactalbumin, cytochrome c, apomyoglobin, and T4 lysozyme. We discuss these four proteins at length. The results of structural studies, together with the existing thermodynamic values for fundamental interactions in proteins, provide the foundation for a structural thermodynamic framework which can account for the observed behavior of molten globule states. Within this framework, we analyze the physical basis for both the high stability of several molten globule states and the low probability of other potential folding intermediates. Additionally, we consider, in terms of reduced enthalpy changes and disrupted cooperative interactions, the thermodynamic basis for the apparent absence of a thermally induced, cooperative unfolding transition for some molten globule states.