AQA · A-Level · Biology
Course version: 7402 · Version from Sept 2016
A functional protein may contain one or more polypeptides.
Full specification wording
General properties of proteins: A functional protein may contain one or more polypeptides.
Some proteins are a single folded polypeptide; many are two or more polypeptides held together, which is quaternary structure.
Checked against the AQA 7402 specification (opens in a new tab) on .
- Point 4 of 9
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Some proteins are a single folded polypeptide; many are two or more polypeptides held together, which is quaternary structure. Lysozyme and amylase are single-chain, insulin is two chains, haemoglobin is four polypeptides - two alpha and two beta chains - each holding an iron-containing haem prosthetic group, and collagen is three chains wound into a triple helix. The polypeptides in a quaternary structure are held together by exactly the same interactions that hold tertiary structure together: hydrogen bonds, ionic bonds, disulfide bridges and hydrophobic interactions between R groups. They are not held by peptide bonds, and examiners specifically refuse to credit peptide bonds in this context. A protein may also include non-protein prosthetic groups, such as the haem in haemoglobin or the carbohydrate in a glycoprotein, and is then described as conjugated.
What earns the marks
- Quaternary structure meaning more than one polypeptide chain in the functional protein.
- The chains being held together by bonds or interactions between polypeptides, not by peptide bonds.
- Naming those interactions as hydrogen bonds, ionic bonds, disulfide bridges or hydrophobic interactions.
- A correct named example, such as haemoglobin with four polypeptides or insulin with two.
- Identifying a prosthetic group, such as haem, as part of the functional protein.
Common mistakes and how to put them right
Saying the polypeptides of a quaternary structure are joined by peptide bonds, which is not credited.
Claiming every protein has a quaternary structure.
Giving haemoglobin two polypeptide chains instead of four.
Omitting the haem groups when describing haemoglobin as a functional protein.
Treating a polypeptide and a functional protein as identical.
Exam tips
- Never write peptide bond when describing what holds polypeptides together in quaternary structure - it is an explicit exclusion.
- Keep two examples ready, one single-chain and one multi-chain, so you can answer either direction of the question.
- Mention prosthetic groups where relevant; haemoglobin without haem is an incomplete description.
Try the lesson
The start of the General properties of proteins: A functional protein may contain one or more polypeptides. lesson, made from checked teaching notes for this specification point.
Have a go first
Which statement accurately describes the polypeptide composition of functional proteins?
Listen
Some proteins are a single folded polypeptide; many are two or more polypeptides held together, which is quaternary structure. Lysozyme and amylase are single-chain, insulin is two chains, haemoglobin is four polypeptides - two alpha and two beta chains - each holding an iron-containing haem prosthetic group, and collagen is three chains wound into a triple helix. The polypeptides in a quaternary structure are held together by exactly the same interactions that hold tertiary structure together: hydrogen bonds, ionic bonds, disulfide bridges and hydrophobic interactions between R groups. They are not held by peptide bonds, and examiners specifically refuse to credit peptide bonds in this context. A protein may also include non-protein prosthetic groups, such as the haem in haemoglobin or the carbohydrate in a glycoprotein, and is then described as conjugated. A strong answer earns marks for these points. quaternary structure meaning more than one polypeptide chain in the functional protein. the chains being held together by bonds or interactions between polypeptides, not by peptide bonds. naming those interactions as hydrogen bonds, ionic bonds, disulfide bridges or hydrophobic interactions. a correct named example, such as haemoglobin with four polypeptides or insulin with two. Exam tip: Never write peptide bond when describing what holds polypeptides together in quaternary structure - it is an explicit exclusion.
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Key words
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quaternary structure
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prosthetic groups
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functional protein
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interactions between polypeptides
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See every definition
- quaternary structure
- More than one polypeptide chain in the functional protein, held together by interactions between polypeptides.
- prosthetic groups
- Non-protein groups that a protein may include, such as the haem in haemoglobin or the carbohydrate in a glycoprotein.
- functional protein
- A protein that may contain one or more polypeptides, and may also include non-protein prosthetic groups.
- interactions between polypeptides
- Hydrogen bonds, ionic bonds, disulfide bridges and hydrophobic interactions between R groups, not peptide bonds.
Label the diagram
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1Box 1
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3Box 3
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