9.6: Denaturation of Proteins
- Page ID
- 433017
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- Identify ways in which proteins are denatured.
Protein Denaturation
The highly organized structures of proteins are truly masterworks of chemical architecture. But highly organized structures tend to have a certain delicacy, and this is true of proteins. Denaturation is the term used for any change in the three-dimensional structure of a protein that renders it incapable of performing its assigned function. A denatured protein cannot do its job because there is a change in the secondary, tertiary, or quaternary structure. Denaturation often leads to the precipitation or coagulation of a protein. Anyone who has fried an egg has observed denaturation. The clear egg white turns opaque as the albumin denatures and coagulates. This precess cannot be reversed.
Figure \(\PageIndex{1}\): Denaturation of a Protein.
There are a variety of ways to denature proteins including those below.
- Heat above 50oC
- Treatment with acids and bases (changes in pH)
- Exposure to Alcohol
- Concentrated Brine Solution (i.e. NaCl)
- Reaction with Reducing agents and Oxidizing agents
- Soaps and Detergents
- Heavy metal ions such as Ag+, Cu2+, Pb2+ and Hg2+
- Agitation
The table 9.6.1 lists the denaturing agents and the disruption of chemical bonds or IMF's.
| Method | Effect on Protein Structure |
|---|---|
| Heat above 50°C, ultraviolet (UV) radiation, or agitation | Heat, UV radiation, Agitation supplies kinetic energy to protein molecules, causing their atoms to vibrate more rapidly and disrupting IMF's such as hydrogen bonding and Londons dispersion forces. |
| Acids or Bases | Acids react with amino acid side chains that are negatively charged and bases react with the amino acid side chains that are positively charged resulting in the disruption of the salt bridges. |
| Use of organic compounds, such as isopropyl alcohol (rubbing alcohol) | These compounds are capable of engaging in intermolecular hydrogen bonding with protein molecules, disrupting intramolecular hydrogen bonding within the protein. |
| Salts of heavy metal ions, such as mercury, copper, silver, and lead ions | These ions form strong bonds with the carboxylate anions of aspartic acid and glutamic acid or SH groups of cysteine, disrupting ionic bonds and disulfide linkages. |
| Soaps and Detergents | These agents disrupt the hydrophobic interactions in the protein since soaps and detergents are amphipathic. |
If you have ever had a hair permanent or chemically straightened your hair, the process involved the denaturation of proteins. The reducing agent (usually an ammonium compound) breaks the disulfide bonds in the hair. The hair is then curled or straightened which aligns the amino acids in a different pattern. An oxidizing agent is applied and the disulfide bonds reform between different amino acids. The change is permanent for the hair that you have at the time but new hair growing in will have the structure of the original proteins and your hair is back to its normal state.
The primary structures of proteins are quite sturdy. In general, fairly vigorous conditions are needed to hydrolyze peptide bonds. So the 1o structure is not disrupted on denaturation. At the secondary, tertiary, and quaternary levels, however, proteins are quite vulnerable to denaturation, though they vary in their vulnerability to denaturation. The delicately folded globular proteins are much easier to denature than are the tough, fibrous proteins of hair and skin.