Denaturation of Protein
The phenomenon of disorganization of native protein structure is known as denaturation. Denaturation results in the loss of secondary, tertiary, and quaternary structure of proteins by breaking down bonds, such as hydrogen bonds and covalent bonds (disulfide bonds (S-S)). This involves a change in the physical, chemical, and biological properties of protein molecules.
Agents of Denaturation of Protein
Physical agents: Heat, violent shaking, X-rays, UV radiation etc.
Chemical agents: Acids, alkalies, organic solvents (ether, alcohol), salts of heavy metals(Pb, Hg), urea, salicylate, detergents (e.g. sodium dodecyl sulfate).
Characteristics of Denaturation of Protein
1. The native helical structure of protein is lost.
2. The primary structure of a protein with peptide linkages remains intact i.e., peptide bonds are not hydrolysed.
3. The protein loses its biological activity.
4. Denatured protein becomes insoluble in the solvent in which it was originally soluble.
5 The viscosity of denatured protein (solution) increases while its surface tension decreases.
6. Denaturation is associated with increase in ionizable and sulfhydryl groups of protein. This is due to loss of hydrogen and disulfide bonds.
7. Denatured protein is more easily digested. This is due to increased exposure of peptide bonds to enzymes. Cooking causes protein denaturation and, therefore, cooked food (protein) is more easily digested. Further, denaturation of dietary protein by gastric HCl enchances protein digestion by pepsin.
8. Denaturation is usually irreversible. For instance, omelet can be prepared from an egg (protein-albumin) but the reversal is not possible.
9. Careful denaturation is sometimes reversible (known as renaturation). Hemoglobin undergoes denaturation in the presence of salicylate. By removal of salicylate, hemoglobin is renatured.
10. Denatured protein cannot be crystallized.
Coagulation: The term ‘coagulum’ refers to a semi-solid viscous precipitate of protein. Irreversible denaturation results in coagulation. Coagulation is optimum and requires lowest temperature at isoelectric pH. Albumins and globulins (to a lesser extent) are coagulable proteins. Heat coagulation test is commonly used to detect the presence of albumin in urine.
Flocculation: It is the process of protein precipitation at isoelectric pH. The precipitate is referred to as flocculum. Casein (milk protein) can be easily precipitated when adjusted to isoelectric pH (4.6) by dilute acetic acid. Flocculation is reversible. On application of heat, flocculum can be converted into an irreversible mass, coagulum.
What happens when a protein is denatured?
When a protein is denatured, the secondary and tertiary structures get destroyed and only the primary structure is retained. Covalent bonds are broken and interaction between amino-acid chains gets disrupted. its normal shape is deformed, and it loses its ability to perform its function.
| Agents | Effect |
|---|---|
| Mechanical | Forces such as agitation or stirring are able to disrupt the structure of a protein by breaking down the weak bonds. For example, whisking egg whites can create a light foam which can be used in baking. |
| Heat | Increased heat or temperatures increases kinetic energy causing molecules to vibrate quickly and breaks the bonds, causing the protein structure to unravel. |
| Acids & Bases | Acids and bases can change the pH of the proteins. This will break the hydrogen bonding and also salt bridges. This will lead to the denaturation of the protein structure. |
| Heavy Metals (Hg2+, Pb2+, Cd2+) | The presence of heavy metals can break disulfide bonds in the secondary and tertiary structures. |
| Organic Solvents (alcohols, ketones) | Organic solvents will break the hydrophobic interactions between nonpolar side chains of amino acids |
Source: Biochemistry by Dr. U. Satyanarayana and Dr. U. Chakrapani
