生物化学（英文版）Chapter4 Proteins Three-Dimensional Structures and Function-part 2.ppt

Denaturation - disruption of native conformation of a protein, with loss of biological activity Energy required is small, perhaps only equivalent to 3-4 hydrogen bonds Proteins is commonly denatured by heating Denaturation takes place over a relatively small range of temperature. Under physiological conditions, most proteins are stable at temperatures up to 50℃ to 60 ℃. However some are not. 4.6 Protein denaturation and renaturation Heat denaturation of ribonuclease A Unfolding monitored by changes in ultraviolet (blue), viscosity (red), optical rotation (green) Tm What is Tm ? Proteins can also be denatured by two types of chemicalschaotropic agents and detergents. For example, urea, guanidinium salts, SDS. Why can the chaotropic agents and detergents result in denaturation of proteins ? The native conformation of some proteins is stabilized by disulfide bonds. Disulfide bridges in bovine ribonuclease A (a) Location of disulfide bridges (b) Stereo view of Cys-26 and Cys-84 a) There are four disulfide bridges in bovine ribonuclease A. (b) The bridge between Cys-26 and Cys 84 is shown in stereo. Can proteins be renatured ? Anfinsen’s experiments (next page). Conclusion: The conformation of proteins are determined by their primary structure. Protein disulfide isomerase (PDI) and E.coli disulfide isomerase (DsbA) can help misfolded proteins to form correct disulfide bonds. Anfinsen’s experimentsDenaturation and renaturation of ribonuclease A Treatement of native ribonuclease A (top) with urea in the presence of 2-mercaptoethanol unfolds the protein and disrupts disulfide bonds to produce reduced, reversibly denatured ribonuclease A (bottom). When the denatured protein is returned to physiological conditions in the absence of 2-mercaptoethanol, it refolds into its native conformation and the correct disulfide bonds form. Protein folding and stability Folded proteins occupy a low-energy well that makes the native structure much more stable than alternative conformatio