In this article, author has explained the causes, process and characteristics of denaturation of proteins.
The disorganization of the native structure of proteins is known as denaturation.
It involves the breaking of many weak linkages and bonds. Denaturation destroys the secondary, tertiary, and quaternary structures of proteins. It affects the chemical, physical and biological properties of proteins. It also causes the loss of functions of proteins.
Causes of denaturation of proteins
- Temperature and pH affect the structure and stability of proteins.
- Due to changes in temperature, pH, and chemical activities, the hydrogen bonds of proteins get disturbed. This causes the unfolding of the globular structure of protein and the uncoiling of the helical structure.
- When the helical structure is uncoiled, it affects the chemistry of proteins which causes the loss of biological activity of proteins. When proteins lost their activity, they are known as denatured proteins.
- After denaturation, secondary and tertiary structures are destroyed. Only primary structure is retained.
- Covalent bonds are broken and interaction between amino acids is lost.
Agents of denaturation
Physical agents: Heat, violent shaking, X-rays, and UV radiation are physical agents that cause denaturation.
Chemical agents: Acids, alkalies, organic solvents like ether and alcohol, salts of heavy metals, urea, salicylate, and detergents (sodium dodecyl sulfate) are chemical agents that cause the denaturation of proteins.
Process of denaturation
- Heating can lead to the process of denaturation. Heat (above 500C) and UV radiation increase the kinetic energy of atoms of the protein. They vibrate more rapidly, disrupt the hydrogen bonding and dispersion forces, and ultimately destroy the structure.
- Organic compounds such as ethyl alcohol engage in hydrogen bonding and disrupt intramolecular hydrogen bonding which results in denaturation.
- Salts of heavy metals such as mercury, lead, and silver form the bond with the carboxyl group of acidic amino acids and with the SH group of cysteine. This disrupts ionic bonds and disulfide bonds.
- Alkaloid reagents such as tannic acid form bonds with the positively charged amino groups of basic amino acids. This disrupts the ionic bonds.
- Heavy physical activity can also denature the proteins.
Characteristics of denaturation
- The helical structure of the protein is destroyed.
- Peptide bonds are not hydrolyzed therefore the primary structure of proteins remains intact.
- The biological activity of the protein is lost.
- After denaturation, the viscosity increases while its surface tension decreases.
- When proteins are denatured they become insoluble in the solvents in which they were soluble.
- The hydrogen and disulfide bonds are lost therefore ionizable and sulfhydryl groups of protein as a result of denaturation.
- Denatured protein can be digested easily because peptide bonds are more exposed to the enzymes.
- Cooking causes the denaturation of proteins therefore cooked protein is more easily digested.
- The denaturation of protein by gastric HCL increases the digestion of protein by pepsin.
- The process of denaturation is irreversible. For example, an omelet can be made of the egg but not the egg from an omelet.
- Some proteins can be renaturation. For example, hemoglobin when treated with salicylate gets denatured but when salicylate is removed, hemoglobin is renatured.
- The denatured proteins cannot be crystallized.
- The process of coagulation requires the lowest temperature at isoelectric pH. The coagulable proteins are albumin and globulins. The presence of albumin in the urine is detected by the heat coagulation process.
- Flocculation: the process of protein precipitation at isoelectric pH is known as flocculation. The precipitates are known as flocculum. The milk protein casein can be precipitated at isoelectric pH by dilute acetic acid. Flocculation is reversible. On application of heat, flocculum is converted into an irreversible mass called coagulum.