Denaturation and renaturation of proteins

Denaturation is the loss of the specific three dimensional shape of a protein molecule. The change may be temporary or permanent, but the amino acid sequence of the protein remains unaffected. If denaturation occurs, the molecule unfolds and can no longer perform its normal biological function.

A number of agents may cause denaturation as follows :

(a) Heat or radiation : e.g. infra-red or ultra violet light. Kinetic energy is supplied to the protein causing its atoms to vibrate violently, so disrupting the weak hydrogen and ionic bonds. Coagulation of the protein then occurs.

(b) Strong acids and alkalis and high concentrations of salts : Ionic bonds are disrupted and the protein is coagulated. Breakage of peptide bonds may occur if the protein is allowed to remain mixed with the

reagent for a long period of time.

(c) Heavy metals : The positively charged ions of heavy metals (cations) form strong bonds with the negatively charged carboxyl groups on the R groups of proteins and often disrupt ionic bonds. They also reduce the protein’s electrical polarity (its overall charge) and thus increase its insolubility. This causes the protein to precipitate out of solution.

(d) Organic solvents and detergents : These reagents disrupt hydrophobic interactions and form bonds with hydrophobic (non-polar) groups. This is turn causes the disruption of hydrogen bonding. When alcohol is used as a disinfectant it functions to denature the protein of any bacteria present.

Sometimes a protein will spontaneously refold into its original structure after denaturation, providing conditions are suitable. This is called renaturation, and is good evidence that tertiary structure can be determined purely by primary structure and that biological structures can spontaneously assemble according to a few general principles.