AQA · A-Level · Biology
Course version: 7402 · Version from Sept 2016
The role of hydrogen bonds, ionic bonds and disulfide bridges in the…
Full specification wording
General properties of proteins: The role of hydrogen bonds, ionic bonds and disulfide bridges in the structure of proteins.
Three interactions hold a folded protein together, and each behaves differently.
Checked against the AQA 7402 specification (opens in a new tab) on .
- Point 5 of 9
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Three interactions hold a folded protein together, and each behaves differently. Hydrogen bonds form between the slightly positive and slightly negative parts of the backbone in secondary structure, producing the alpha helix and the beta pleated sheet, and also form between polar R groups in tertiary structure. Each is weak, but there are so many that collectively they matter. Ionic bonds form between oppositely charged R groups; because they depend on charge, a change in pH adds or removes hydrogen ions, alters those charges and breaks the bonds. Disulfide bridges are covalent bonds between the sulfur-containing R groups of two cysteine residues, and they are far stronger than the other two, which is why proteins rich in them, such as keratin and many extracellular enzymes, resist heat and pH change.
What earns the marks
- Hydrogen bonds forming the secondary structure, the alpha helix or beta pleated sheet.
- Tertiary structure being formed by interactions between R groups.
- Naming hydrogen bonds, ionic bonds, disulfide bridges or hydrophobic interactions as those R group interactions.
- Disulfide bridges being covalent bonds between two cysteine R groups and stronger than hydrogen or ionic bonds.
- Heat or pH breaking hydrogen and ionic bonds, so the tertiary structure changes and the protein is denatured.
Common mistakes and how to put them right
Saying disulfide bridges can form between any two R groups rather than between two cysteines.
Describing ionic bonds as covalent, or disulfide bridges as weak.
Claiming peptide bonds hold the tertiary structure together.
Stating that a single hydrogen bond is strong, instead of many weak bonds acting together.
Saying denaturation breaks peptide bonds and changes the primary structure.
Exam tips
- Match the disruption to the bond: pH changes break ionic bonds, heat breaks hydrogen and ionic bonds, and disulfide bridges resist both.
- Say 'interactions between R groups' when describing tertiary structure - it is the phrase the mark scheme uses.
- Do not offer peptide bonds as an answer about tertiary or quaternary structure; they belong to primary structure only.
Try the lesson
The start of the General properties of proteins: The role of hydrogen bonds, ionic bonds and disulfide bridges in the structure of proteins. lesson, made from checked teaching notes for this specification point.
Have a go first
Which type of bond holds the α-helix of a polypeptide in its coiled shape?
Listen
Three interactions hold a folded protein together, and each behaves differently. Hydrogen bonds form between the slightly positive and slightly negative parts of the backbone in secondary structure, producing the alpha helix and the beta pleated sheet, and also form between polar R groups in tertiary structure. Each is weak, but there are so many that collectively they matter. Ionic bonds form between oppositely charged R groups; because they depend on charge, a change in pH adds or removes hydrogen ions, alters those charges and breaks the bonds. Disulfide bridges are covalent bonds between the sulfur-containing R groups of two cysteine residues, and they are far stronger than the other two, which is why proteins rich in them, such as keratin and many extracellular enzymes, resist heat and pH change. A strong answer earns marks for these points. hydrogen bonds forming the secondary structure, the alpha helix or beta pleated sheet. tertiary structure being formed by interactions between R groups. naming hydrogen bonds, ionic bonds, disulfide bridges or hydrophobic interactions as those R group interactions. disulfide bridges being covalent bonds between two cysteine R groups and stronger than hydrogen or ionic bonds. Exam tip: Match the disruption to the bond: pH changes break ionic bonds, heat breaks hydrogen and ionic bonds, and disulfide bridges resist both.
Key words
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hydrogen bonds
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ionic bonds
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disulfide bridges
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denaturation
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See every definition
- hydrogen bonds
- Form between the slightly positive and slightly negative parts of the backbone in secondary structure, and between polar R groups in tertiary structure; each is weak but collectively they matter.
- ionic bonds
- Form between oppositely charged R groups, and a change in pH adds or removes hydrogen ions, alters those charges and breaks the bonds.
- disulfide bridges
- Covalent bonds between the sulfur-containing R groups of two cysteine residues, far stronger than the other two, so proteins rich in them resist heat and pH change.
- denaturation
- Heat or pH breaks hydrogen and ionic bonds, so the tertiary structure changes and the protein is denatured.
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