How does the structure of a protein determine the way it binds and functions?
Protein structure and binding: amino acids and peptide bonds, the four levels of structure, R group interactions, prosthetic groups, ligand binding and conformational change, allosteric regulation, cooperativity, and modification by phosphorylation.
An SQA Advanced Higher Biology answer on proteins, covering amino acids and peptide bonds, primary to quaternary structure, the R group interactions that stabilise tertiary structure, prosthetic groups, ligand binding and conformational change, allosteric regulation, cooperativity, and reversible modification by phosphorylation.
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What this key area is asking
The SQA wants you to explain how a protein is built from amino acids, describe its four levels of structure and the interactions that stabilise them, and explain how binding a ligand changes a protein's shape and activity. The key ideas are conformational change, allosteric regulation, cooperativity and modification by phosphorylation, all of which recur in signalling and the control of cell division.
Amino acids and peptide bonds
The four levels of structure
The tertiary shape is stabilised by interactions between R groups:
- Hydrogen bonds between polar R groups.
- Ionic bonds between oppositely charged (acidic and basic) R groups.
- Hydrophobic interactions that bury non-polar R groups away from water in the core of the protein.
- Disulfide bridges, strong covalent links between two cysteine R groups.
Many proteins also contain a prosthetic group, a non-protein component bound tightly to the chain and essential for function, such as the iron-containing haem group in haemoglobin.
Ligand binding and conformational change
Allosteric regulation and cooperativity
Haemoglobin shows both ideas. Its four subunits bind oxygen cooperatively, so the first oxygen makes the next bind more readily, giving the characteristic S-shaped (sigmoid) oxygen dissociation curve.
Modification by phosphorylation
Examples in context
Example 1. End-product inhibition. In a pathway making an amino acid, the final product binds an allosteric site on the first enzyme and switches it off. As product accumulates it shuts down its own production, and as it is used up the enzyme reactivates, an efficient feedback control built entirely on conformational change.
Example 2. Insulin signalling and phosphorylation. When insulin binds its receptor, it triggers a cascade of phosphorylation events that change the shape and activity of many proteins, ultimately moving glucose transporters to the cell surface. The example shows reversible phosphorylation acting as a fast molecular switch.
Try this
Q1. Name the strongest type of bond that stabilises tertiary structure. [1 mark]
- Cue. The disulfide bridge, a covalent bond between two cysteine R groups.
Q2. Explain what is meant by a conformational change and why it matters. [2 marks]
- Cue. A change in a protein's shape on ligand binding; it alters the protein's activity, which is how enzymes, receptors and transporters work.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA AH style5 marksDescribe the four levels of protein structure and the bonds or interactions that stabilise the tertiary structure.Show worked answer →
A 5-mark answer needs all four levels plus the tertiary interactions.
Primary structure is the sequence of amino acids joined by peptide bonds. Secondary structure is the regular folding of the polypeptide backbone into alpha helices and beta-pleated sheets, held by hydrogen bonds. Tertiary structure is the overall three-dimensional shape of a single polypeptide. Quaternary structure is the arrangement of two or more polypeptide subunits, sometimes with a prosthetic group, in the functional protein.
The tertiary structure is stabilised by interactions between R groups: hydrogen bonds, ionic bonds, hydrophobic interactions and disulfide bridges.
Markers reward the four levels named correctly and at least two named tertiary interactions, up to the five marks.
SQA AH style4 marksExplain how allosteric regulation and cooperativity control the activity of a protein.Show worked answer →
A 4-mark answer needs both ideas explained through conformational change.
An allosteric protein has a binding site separate from its active site. When a modulator binds this site it changes the protein's conformation, either increasing activity (a positive modulator) or decreasing it (a negative modulator). This is how end-product inhibition switches off a metabolic pathway.
Cooperativity occurs in proteins with several subunits. The binding of a ligand to one subunit changes the conformation of the others so that their affinity for the ligand increases, as in oxygen binding to haemoglobin.
Markers reward (1) modulator binds a site away from the active site, (2) it changes conformation and activity, (3) cooperativity needs multiple subunits, and (4) one binding raises affinity at the others.
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Sources & how we know this
- SQA Advanced Higher Biology Course Specification — SQA (2019)