Protein denaturation by pH, heat, detergent and reducing agents
Denaturation is the loss of a protein's folded three-dimensional shape without any break in its chain of amino acids, as with a sustained change in pH or extreme heat.
Remember
Denaturation unfolds a protein by disrupting the forces that hold its fold in place, but the peptide bonds (the strong links between amino acids) stay intact, so the amino acid sequence survives.
Denaturation means that a protein loses its folded, three-dimensional shape while the order of its amino acids stays the same. That order is the primary structure, and it survives because denaturation leaves the chain intact. A denatured protein usually loses its function, and it often clumps together, as when heat sets egg white. Denaturation can sometimes be reversed once the cause is removed, but it may be permanent.
Heat denatures a protein once it makes the molecules vibrate hard enough to break the weak bonds that hold the fold. These are mainly hydrogen bonds (small attractions between slightly charged atoms) and hydrophobic interactions (the clustering of water-hating side chains, the R groups of amino acids). Extreme heat is used to sterilise surgical instruments (destroy the microorganisms on them), because denaturing the proteins of a microorganism destroys it.
A sustained change in pH (how acidic or alkaline a solution is) alters the ionisation of the side chains, which is their gain or loss of charge. The pattern of positive and negative charges shifts, so the ionic bonds (attractions between oppositely charged side chains) and the hydrogen bonds loosen or break, and the chain unfolds.
A detergent denatures by disrupting the hydrophobic interactions. Its molecules have a water-loving head and an oily tail. The tail pushes into the water-hating core of the folded protein and pulls the fold apart.
A reducing agent denatures by breaking disulphide bonds, which are links between the sulphur atoms of two side chains and help hold the tertiary structure (the overall three-dimensional fold). A protease (an enzyme that cuts peptide bonds) cuts the polypeptide chain and so destroys the primary structure. Denaturation leaves the chain intact.
Key facts
- Denaturation
- the folded shape is lost, while the order of the amino acids (the primary structure) survives.
- Heat
- makes molecules vibrate enough to break the weak bonds that hold the fold, mainly hydrogen bonds and hydrophobic interactions.
- pH
- a marked change alters the charge on side chains, which loosens the ionic and hydrogen bonds.
- Detergent
- the oily tails push into the water-hating core and pull the fold apart.
- Reducing agents
- they target disulphide bonds, which are links between the sulphur atoms of two side chains.
- Peptide bonds
- these strong links between amino acids survive denaturation, whereas a protease cuts them.
Exam traps
- Cutting peptide bonds with a protease is not denaturation, because it destroys the primary structure, whereas denaturation changes the shape and leaves the chain intact.
- Heat does not selectively break disulphide bonds, because it denatures proteins mainly by disrupting weak hydrogen bonds and hydrophobic interactions.
- Extra disulphide bridges stabilise a folded protein rather than unfolding it, and they do not work by changing ionisation.
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