Topic 9 of 12
Proteins and denaturation
Folding, denaturation and hydrolysis are different processes.
A-Level 8873, revised syllabus (2026-2027)
Change a protein shape without cutting its backbone
Folding, denaturation and hydrolysis are different processes.
The amino-acid sequence provides a covalently bonded backbone, but many functions depend on the chain adopting a particular three-dimensional shape. Interactions between different parts of the chain, or between different chains, help stabilise that shape. An enzyme's active site depends on this arrangement, linking protein structure to the specificity explained in Reaction Kinetics.
| Interaction | Structural origin | How conditions can disturb it |
|---|---|---|
| Hydrogen bonding | Attractions involving suitable N-H or O-H groups and lone pairs on nearby O or N atoms, including peptide C=O groups. | Heating and changed interactions with water can disrupt the pattern stabilising the folded arrangement. |
| Intermolecular-type attractions | Permanent dipole interactions and instantaneous dipole-induced dipole attractions act between suitable regions within a folded chain and between chains. | A changed arrangement alters the contacts and their collective stabilisation. These are not new peptide bonds. |
| Ionic linkages | Attraction between oppositely charged groups, such as -NH3+ and -COO-. | Changing pH can change whether these groups are charged, disrupting the original ionic attractions. |
Denaturation is loss of the specific three-dimensional structure responsible for normal function. High temperature can disrupt the interactions maintaining that arrangement; extreme pH changes protonation and therefore ionic attractions and other interactions. A protein can unfold, refold incorrectly or aggregate with other chains. The usual denaturation explanation does not require cleavage of the peptide backbone.
The chain remains connected when its fold is disrupted
The top green line is a folded protein backbone stabilised by several dashed noncovalent contacts. The lower green line remains continuous after these contacts are disrupted. The drawing is schematic and does not represent a particular protein or the detailed atoms of a peptide chain.
Low temperature usually slows enzyme-catalysed reactions by reducing molecular motion; it does not automatically denature the enzyme. At excessively high temperature the enzyme may lose its active-site shape, so fewer enzyme-substrate complexes form even though molecules move faster. Returning to the original temperature or pH does not guarantee recovery after aggregation or other irreversible changes.
| Observation | Explanation |
|---|---|
| Egg white becomes opaque and sets on heating. | Proteins lose their original folded arrangements. Exposed regions associate with other chains, forming a new extended network or aggregates that scatter light and trap water. This does not mean cooking has hydrolysed every protein into amino acids. |
| Vinegar causes milk to form curds. | Added acid changes protein charge and disrupts the balance maintaining dispersion. With reduced repulsion and changed interactions, proteins associate and precipitate into visible curds. The changed pH affects three-dimensional organisation and association; identifying the proteins by name is not required. |
Protein hydrolysis is a different chemical change: heat with aqueous acid or aqueous alkali breaks peptide links, ultimately giving amino-acid-derived products after complete hydrolysis. Acidic conditions give protonated amino groups, H3N+-CH(R)-COOH. Alkaline conditions give carboxylate groups, H2N-CH(R)-COO-, with the appropriate counter-ions. The amino-acid skeletons and side chains are recovered, subject to the supplied conditions.
Worked example
Distinguish two treatments of a protein
Treatment A briefly heats a protein in water until it aggregates. Treatment B heats it with aqueous acid until its peptide links are completely hydrolysed. Compare the products.
- In A, changed noncovalent interactions destroy the original fold and allow new associations. Long covalently bonded chains can remain.
- In B, C-N bonds of peptide links are cleaved by hydrolysis. The original long chain is broken into amino-acid-derived molecules.
- In the acidic final mixture, amino groups are mainly protonated, so show the appropriate positive charges when drawing products.
Denaturation describes loss of the functional fold; complete hydrolysis describes cleavage of the chain into its monomer-derived products.