Denaturation describes physical changes in the three dimensional structure of proteins. These changes occur when the proteins are exposed to disruptive chemical or physical forces.
These structural changes not only affect the physical form of the proteins but also their chemical function. One common example of a denatured protein is egg albumin which changes from a gelatinous liquid to a solid when heated.
Proteins are large molecules composed of amino acids which are arranged in a variety of complex structures. The primary protein structure is the simple linear sequence of amino acids within the protein. The secondary structure is determined by the configuration of certain subgroups of amino acids within the primary structure. These subgroups can be configured in three different ways depending on the type of protein. The alpha helix form resembles a spiral staircase. It is found in structural proteins, such as the keratins in hair and skin. The beta-pleated sheet form is, as the name implies, a flat, zig-zag shape, and it is found in silk fiber proteins. The third type of secondary structure is the random coil, which does not have a defined shape and which is found in proteins such as collagen. The random coil structure links together the alpha helices and beta sheets so that proteins may contain all three secondary structures.
Proteins also adopt a tertiary structure which is achieved by looping and folding the chain over itself. This folded structure occurs because certain portions of the molecules have an affinity for water and other portions do not. Therefore, proteins will fold or bend into shapes so that the water loving (hydrophilic) groups are on the outside of the molecule and the hydrophobic groups, which tend to exclude water, are buried in the internal parts of the molecule. This folding process causes the proteins to take on complex helical patterns, which result in stable structures and give the protein specific chemical and physical properties. For example, the shape (conformation) of enzymes creates specific catalytic areas, causing different enzymes to have different capacities for catalyzing chemical reactions. Similarly, the structure of hemoglobin is responsible for that molecule's ability to carry oxygen and carbon dioxide.
When a protein is denatured, the molecule's tertiary structure is corrupted. This disruption affects the molecule's secondary (helical) structure without altering its primary structure. In other words, denaturation does not break any of the primary chemical bonds that link one amino acid to another but it changes the way the protein folds in upon itself. Denaturation occurs when proteins are exposed to strong acids or bases, high concentrations of inorganic salts, or organic solvents such as alcohol. In addition, heat or irradiation can cause denaturation. When the three-dimensional structure of the protein is disrupted, the molecule's biological activity is affected. Therefore, enzymes do not have the same catalytic function when they are denatured. Some proteins may be renatured by exposing the denatured protein to a solution that approximates normal physiological conditions.
Denaturation can have many detrimental side effects. In biological systems, denatured proteins can result in illness or even death. Protein denaturation is linked to diseases such as prion encephalopathies, Alzheimer disease, and dementias. Denaturation can also have negative effects certain industrial processes. For example, in dairy processing certain milk proteins are denatured as a result of separation techniques and heat treatments. These treatments change the way the proteins, minerals and ions in the milk interact and may affect the milk's nutritional content. Research has shown that whey protein denaturation can be reduced by the addition of small amounts of nonfat dry milk. Lowering the calcium ion concentration and reducing the pH of the milk protein concentrate also reduces the degree of denaturation.
Not all denaturation processes are harmful. For example, the egg protein albumin is easily denatured to form a gelatinous solid capable of absorbing foreign material. This property makes denatured albumin suitable for certain important industrial applications such as sugar refining and the manufacture of adhesives, varnishes, and inks.