Lesson 9 of 12
Proteins and denaturation
Folding, denaturation and hydrolysis are different processes.
A-Level 8873, revised syllabus (2026-2027)
I can describe protein hydrolysis.Syllabus
Syllabus 8873, 9(k). Describe protein hydrolysis.
I can explain interactions stabilising three-dimensional protein structure.Syllabus
Syllabus 8873, 9(l). Explain interactions stabilising three-dimensional protein structure.
I can explain protein denaturation.Syllabus
Syllabus 8873, 9(m). Explain protein denaturation.
I can apply protein shape changes to real phenomena.Syllabus
Syllabus 8873, 9(n). Apply protein shape changes to real phenomena.
Change a protein shape without cutting its backbone
Folding, denaturation and hydrolysis are different processes.
Make a guess. You are not marked.
A protein is gently heated until it denatures. What happens to its peptide bonds?
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They stay intact; weaker interactions break
Gentle heating breaks weaker interactions such as hydrogen bonds, so the protein unfolds. Breaking peptide bonds needs hydrolysis.
The order of amino acids gives a chain held together by covalent bonds. But many proteins only work when the chain folds into a particular three-dimensional shape. Attractions between different parts of the chain, or between chains, hold that shape. An enzyme's active site depends on this shape, which links 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 the loss of the particular three-dimensional shape that a protein needs to work. High temperature can break the attractions that hold this shape. Extreme pH changes which groups carry a charge, so ionic attractions and other interactions change. The protein can unfold, refold wrongly or clump with other chains. Denaturation does not break the peptide bonds of the 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 reactions because molecules move more slowly. It does not denature the enzyme. At too high a temperature, the enzyme can lose the shape of its active site. Then fewer enzyme-substrate complexes form, even though molecules move faster. Going back to the original temperature or pH does not always bring the activity back, for example if the protein has clumped.
| 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. Heating with aqueous acid or aqueous alkali breaks the peptide links, and complete hydrolysis gives amino acids. In acid, the amino groups are protonated: H3N+-CH(R)-COOH. In alkali, the acid groups become carboxylate ions: H2N-CH(R)-COO-, with suitable counter-ions. The carbon chains and side groups of the amino acids are recovered.
Worked exampleDistinguish 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.
Try it first. Then show the worked stepsHide the worked steps
- 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.
Check your understanding
Why can adding acid break an ionic link between -NH3+ and -COO- groups in a protein?
Show the answer
The -COO- gains H+ to become -COOH, so its charge and the attraction are lost.
The ionic link is an attraction between opposite charges. Protonating the carboxylate removes the negative charge, so the shape held by that link can change.