On the thermal unfolding character of globular proteins

A theoretical model is presented to study the stepwise thermal unfolding of globular proteins using the stabilizing/destabilizing characters of amino acid residues in protein crystals. A multiple regression relation connectin g the melting temperature and the amounts of stabilizing and destabilizing groups of residues in a protein, when used for the thermal behavior of pept ide segments, provides reliable results on the stepwise unfolding nature of the protein. In ribonuclease A, the shell residues 16-22 are predicted to unfold earlier in the temperature range 30-45 degrees C; the beta-sheet str uctures undergo thermal denaturation as a single cooperative unit and there is evidence indicating the segment 106-118 as a nucleation site. In ribonu clease S, the S-peptide unfolds earlier than S-protein. The predicted avera ge and the range of melting temperatures, and the folding pathways of a set of globular proteins, agree very well with the experimental results. The r esults obtained in the present study indicate that (i) most of the nucleati on parts possess high relative thermal stability, (ii) the unfolded state r etains some residual structure, and (iii) some segments undergo gradual and overlapping thermal denaturation.