Denaturation (biochemistry)

Irreversible egg protein denaturation and loss of solubility, caused by the high temperature (while cooking it)

Proteins are long strands of amino acids linked together in specific sequences. A protein is created by ribosomes that "read" codons in the gene and assemble the requisite amino acid combination from the genetic instruction, in a process known as translation. The newly created protein strand then undergoes post-translational modification in which additional atoms or molecules are added, for example copper, zinc, iron. Once this post-translational modification process has been completed, the protein begins to fold (spontaneously, and sometimes with enzymatic assistance), curling up on itself so that hydrophobic elements of the protein are buried deep inside the structure and hydrophilic elements end up on the outside. The final shape of a protein determines how it interacts with its environment. If the shape of the protein is altered through some form of external stress (for example, by applying heat, acid or alkali) then it will no longer be able to carry out its cellular function. This is called denaturation of the protein.

Denatured proteins can exhibit a wide range of characteristics, from loss of solubility to communal aggregation.

How denaturation occurs at levels of protein structure

• In quaternary structure denaturation, protein sub-units are dissociated and/or the spatial arrangement of protein subunits is disrupted.
• Tertiary structure denaturation involves the disruption of:

• Covalent interactions between amino acid side chains (such as disulfide bridges between cysteine groups)
• Noncovalent dipole-dipole interactions between polar amino acid side chains (and the surrounding solvent)
• Van der Waals (induced dipole) interactions between nonpolar amino acid side chains.

• In secondary structure denaturation, proteins lose all regular repeating patterns such as alpha-helices and beta-pleated sheets, and adopt a random coil configuration.
• Primary structure, such as the sequence of amino acids held together by covalent peptide bonds, is not disrupted by denaturation.

Loss of function

Most biological proteins lose their biological function when denatured, For example, enzymes lose their catalytic activity, because the substrates can no longer bind to the active site, and because amino acid residues involved in stabilizing substrates' transition states are no longer positioned to be able to do so.

Reversibility and irreversibility

In many proteins (unlike egg whites), denaturation is reversible (the proteins can regain their native state when the denaturing influence is removed). This was important historically, as it led to the notion that all the information needed for proteins to assume their native state was encoded in the primary structure of the protein, and hence in the DNA that codes for the protein.

Some common examples

When food is cooked, some of its proteins become denatured. This is why boiled eggs become hard and cooked meat becomes firm.

A classic example of denaturing in proteins comes from egg whites, which are largely egg albumins in water. Fresh from the eggs, egg whites are transparent and liquid. But by cooking they are turned opaque and white, and form an interconnected solid mass. The same transformation can be effected with a denaturing chemical. Pouring egg whites into a beaker of acetone will also turn egg whites opaque and solid. The skin which forms on curdled milk is another common example of denatured protein.

An example of reversible denaturing in proteins is the modern permanent wave technique for curling or straightening hair.