DENATURATION OF PROTEINS
Because protein structure is so important to life’s functions, the loss of protein structure can be crucial. If a protein loses only its natural three-dimensional conformation, the process is referred to as denaturation. In contrast, the hydrolysis of peptide bonds ultimately converts proteins into their constitutent amino acids. Denaturation often precedes hydrolysis.
Denaturation involves the alteration or disruption of the secondary, tertiary, or quaternary--but not primary—structure of proteins. Because a protein’s function depends on its natural conformation, biological activity is lost with denaturation. This process may involve changes ranging from the subtle and reversible alterations caused by a slight shift in pH to the extreme alterations involved in tanning a skin to form leather.
Denaturation involves the alteration or disruption of the secondary, tertiary, or quaternary--but not primary—structure of proteins. Because a protein’s function depends on its natural conformation, biological activity is lost with denaturation. This process may involve changes ranging from the subtle and reversible alterations caused by a slight shift in pH to the extreme alterations involved in tanning a skin to form leather.
Environmental changes may easily disrupt natural protein structure, which is held together predominantly by noncovalent, relatively weak bonds. As gentle an act as pouring a protein solution can cause denaturation. Purified proteins must often be stored under ice-cold conditions because room temperature denatures them. It is not surprising that a wide variety of chemical and physical agents can also denature proteins, including strong acids and strong bases, salts (especially those of heavy metals), certain specific reagents such as tannic acid and picric acid, alcohol and other organic solvents, detergents, mechanical action such as whipping, high temperature, and ultraviolet radiation. Denatured proteins are generally less soluble than natural proteins and often coagulate or precipitate from solution. Cooks have taken advantage of this for many years. When egg white, which is a concentrated solution of egg albumin protein, is stirred vigorously (as with an egg beater), an unsweetened meringue forms; the albumin denatures and coagulates. A cooked egg solidifies partially because egg proteins, including albumin, are denatured by heat.
In clinical laboratories, the analysis of blood serum for small molecules such as glucose and uric acid is hampered by the presence of serum protein. This problem is resolved by first treating the serum with an acid to denature and precipitate the protein. The precipitate is removed, and the protein-free liquid is then analyzed.
Loss of protein structure also occurs with hydrolysis of the peptide bonds to produce free amino acids. This chemical reaction destroys the protein’s primary structure. Proteins can be hydrolyzed by boiling in a solution containing a strong acid such as HCl or a strong base such as NaOH. At ordinary temperatures, proteins can be hydrolyzed with the use of enzymes. These molecules, called proteolytic enzymes, are themselves proteins that function to catalyze or speed up the hydrolysis reaction. The essential reaction of hydrolysis is the breaking of a peptide linkage and the addition of the elements of water:
Loss of protein structure also occurs with hydrolysis of the peptide bonds to produce free amino acids. This chemical reaction destroys the protein’s primary structure. Proteins can be hydrolyzed by boiling in a solution containing a strong acid such as HCl or a strong base such as NaOH. At ordinary temperatures, proteins can be hydrolyzed with the use of enzymes. These molecules, called proteolytic enzymes, are themselves proteins that function to catalyze or speed up the hydrolysis reaction. The essential reaction of hydrolysis is the breaking of a peptide linkage and the addition of the elements of water:
Any molecule with one or more peptide bonds, from the smallest dipeptide to the largest protein molecule, can be hydrolyzed. During hydrolysis, proteins are broken down into smaller and smaller fragments until all component amino acids are liberated. Dietary protein must have its primary structure completely destroyed before it can provide nutrition for the body. Thus, digestion involves both denaturation and hydrolysis. Stomach acid causes most proteins to denature. Then proteolytic enzymes in the stomach and the small intestine hydrolyze the proteins to smaller and smaller fragments, until the free amino acids are formed. They can then be absorbed through the intestinal membranes into the bloodstream.