THERMODYNAMIC PREDICTION OF STRUCTURAL DETERMINANTS OF THE MOLTEN GLOBULE STATE OF BARNASE

Recently, it has been demonstrated that the enthalpy and heat capacity changes for protein folding/unfolding can be predicted rather accurat ely from the crystallographic or NMR solution structure of a protein. (K.P. Murphy, V. Bhakuni, D. Xie and E. Freire, Mol, Biol. 227 (1992) 293-306.) Under some conditions proteins do not unfold completely, giv ing rise to states in which the molecule remains in a compact configur ation after denaturation. These compact denatured or molten globule st ates retain a hydrophobic core, exhibit residual structure and a compa ctness close to that of the native state. This phenomenon is reflected in the thermodynamics of the process. By using the structural paramet rization of the energetics, it is possible to develop an algorithm aim ed at selecting partly folded states that conform to the experimental thermodynamic constraints of the molten globule. We have applied our m olten globule search algorithm to the globular protein barnase. This a pproach has allowed a structure based selection of a unique family of structural states that satisfy the experimental criteria of the molten globule. The prediction of the molten globule search algorithm indica tes that the first helix together with most of the beta-sheet structur e (beta 2, beta 3-5) and loop 5 constitute the main determinants of th e molten globule intermediate, in agreement with the NMR data. These r esults open the prospect for an automated search of the structural det erminants of the molten globule state of proteins and suggest that sol vation parameters can be effectively used to probe structural states o f proteins.