Oxidation of tryptophans in an interhelical hydrophobic cluster of myoglobin alters the thermodynamics of the denaturation transition.
Model folding studies of sperm whale myoglobin have illustrated the presence of hydrophobic interfacial regions between elements of secondary structure. The specific oxidation of two tryptophan residues, in the A-H helix contact of sperm whale myoglobin, to the less hydrophobic oxindolylalanine residues is utilized to probe the contribution of hydrophobic packing density in this contact region. The acid denaturation of the modified protein is no longer a simple two-state process exhibiting the presence of stable intermediates. The relative stability of the intermediate is shown to be +5.3 kcal/mol less stable than native myoglobin. This value is consistent with the predicted relative stability, based upon electrostatic model calculations, of the docking of the A helix with a des-A helix myoglobin. The presence of stable intermediate structures in the denaturation pathway of the modified protein is consistent with the proposed role of hydrophobic interactions in damping structural fluctuations and statistical mechanical models of noncooperative protein unfolding. These results demonstrate the relationship between large-scale fluctuations and the frictional forces governing small-scale motions within the protein core.
D000818:Animals; D001059:Apoproteins; D006418:Heme; D006863:Hydrogen-Ion Concentration; D046911:Macromolecular Substances; D008958:Models, Molecular; D009211:Myoglobin; D010084:Oxidation-Reduction; D011199:Potentiometry; D011487:Protein Conformation; D011489:Protein Denaturation; D013816:Thermodynamics; D014364:Tryptophan; D014907:Whales
D001059:Apoproteins; D046911:Macromolecular Substances; D009211:Myoglobin; C010857:apomyoglobin; D006418:Heme; D014364:Tryptophan