Denaturation (Biochemistry)
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In biochemistry, denaturation is a structural change in biomolecules such as nucleic acids and proteins, such that they are no longer in their native state (the shape which allows for optimal biological activity).
This change is usually caused by heat, acids, bases, detergents, alcohols, heavy metal salts, reducing agents or certain chemicals such as urea.
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Protein Denaturation
Proteins denature when they lose their three-dimensional structure (or "conformation") and thus their characteristic folded structure. Proteins may be denatured at the secondary, tertiary and quaternary structural levels, but not at the primary structural level.
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, i.e., the sequence of amino acids held together by covalent peptide bonds, are not destroyed by denaturation.
Loss of function
Most 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
A classic example of denaturing in proteins comes from the whites of eggs, which are largely egg albumens. 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.
Nucleic Acid Denaturation
The denaturation of nucleic acids such as DNA, also called melting, is the separation of a double strand into two single strands, which occurs when the hydrogen bonds between the strands are broken. Nucleic acid strands realign when "normal" conditions are restored. This process is known as annealing. If the conditions are restored too quickly, the nucleic acid strands may realign imperfectly.