How does the structure of proteins determine their function?
Amino acids are the monomers from which proteins are made. The general structure of an amino acid as RCH(NH2)COOH. A condensation reaction between two amino acids forms a peptide bond. The relationship between primary, secondary, tertiary and quaternary structure, and protein function. The biuret test for proteins.
A focused answer to the AQA A-Level Biology 3.1 specification points on proteins. Covers amino acid structure, peptide bonds and condensation, the primary, secondary, tertiary and quaternary levels of structure, how structure determines function, and the biuret test.
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What this dot point is asking
AQA wants you to give the general structure of an amino acid, explain how condensation forms peptide bonds, define and distinguish the primary, secondary, tertiary and quaternary levels of protein structure, explain how structure determines function, and carry out the biuret test.
Amino acids - the monomers
The R group is what makes each amino acid distinct and is the reason proteins fold into specific shapes.
The peptide bond
A condensation reaction between the carboxyl group of one amino acid and the amine group of another forms a peptide bond and releases one molecule of water. The product of two amino acids is a dipeptide; many amino acids form a polypeptide.
The reverse, hydrolysis, breaks a peptide bond by adding water (this happens during digestion).
The four levels of protein structure
- Primary structure
- The sequence and number of amino acids in the polypeptide chain, held together by peptide bonds. This is the blueprint for everything that follows.
- Secondary structure
- The polypeptide folds locally into an alpha-helix or beta-pleated sheet, held by hydrogen bonds between the slightly negative oxygen of one carboxyl group and the slightly positive hydrogen of an amine group along the backbone.
- Tertiary structure
- The overall 3D shape of a single polypeptide, formed when the chain folds further. It is held by interactions between R groups:
- Hydrogen bonds (weakest, easily broken)
- Ionic bonds (between charged R groups)
- Disulfide bridges (strong covalent bonds between two cysteine R groups)
- Hydrophobic interactions (non-polar R groups cluster inside)
Quaternary structure. Two or more polypeptide chains held together, sometimes with a prosthetic group. Example: haemoglobin has four polypeptide chains, each with an iron-containing haem group.
Structure determines function
A protein's precise 3D shape gives it its function. Examples to know:
- Enzymes have a specific active site shaped to fit one substrate (tertiary structure).
- Antibodies have variable regions shaped to bind a specific antigen.
- Haemoglobin (quaternary) carries oxygen via its four haem groups.
- Collagen is a structural protein with a tough, rope-like quaternary structure.
If the shape changes - for example by denaturation from high temperature or extreme pH breaking hydrogen and ionic bonds - the protein can no longer carry out its function.
The biuret test for proteins
To test for protein (specifically for peptide bonds):
- Add sodium hydroxide solution to make the sample alkaline.
- Add a few drops of dilute copper(II) sulfate solution. (These two together are the biuret reagent.)
- No heating is needed. A positive result is a colour change from blue to purple/lilac. If no protein is present, the solution stays blue.
Try this
Q1. Give the general structure of an amino acid and name the bond formed between two of them. [2 marks]
- Cue. RCH(NH2)COOH - central carbon with amine, carboxyl, hydrogen and R group; peptide bond (formed by condensation).
Q2. Explain why the primary structure of a protein is so important. [3 marks]
- Cue. Primary = sequence of amino acids → determines which R groups are present and where → determines where hydrogen/ionic/disulfide bonds form → determines tertiary 3D shape and function.
Q3. Describe the biuret test and state how it would differ for a sample containing protein versus one without. [3 marks]
- Cue. Add NaOH then dilute copper(II) sulfate, no heat. Protein present: blue → purple/lilac. No protein: stays blue.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA Paper 1 (style)6 marksDescribe the four levels of protein structure and explain how the primary structure determines the final three-dimensional shape of a protein.Show worked answer →
A 6-mark answer needs all four levels defined plus the causal chain from primary to tertiary shape.
- Primary structure
- The sequence and number of amino acids in the polypeptide, joined by peptide bonds.
- Secondary structure
- Local folding into alpha-helices or beta-pleated sheets, held by hydrogen bonds between the amino and carboxyl groups of the backbone.
- Tertiary structure
- The overall 3D shape of a single polypeptide, held by hydrogen bonds, ionic bonds and disulfide bridges (and hydrophobic interactions) between R groups.
- Quaternary structure
- Two or more polypeptide chains (and sometimes prosthetic groups, e.g. haem in haemoglobin) held together in a functional protein.
- Causal link
- The primary structure (the specific sequence of amino acids) determines which R groups are present and where, so it determines where hydrogen, ionic and disulfide bonds form. This determines the secondary and tertiary folding, and hence the final 3D shape. Change one amino acid and the bonds - and therefore the shape and function - can change.
Markers reward defining all four levels correctly, naming the bonds at each level, and explaining that the primary sequence dictates the position of R-group bonds and so the final shape.
AQA Paper 1 (style)2 marksDescribe how you would test a solution for the presence of protein and state the result of a positive test.Show worked answer →
A 2-mark answer needs the reagent(s) and the colour change.
- Add biuret reagent (sodium hydroxide then dilute copper(II) sulfate solution) to the sample at room temperature - no heating is required.
- A positive result is a colour change from blue to purple/lilac. If no protein is present, the solution stays blue.
Markers reward correct reagent and the blue-to-purple colour change.
Related dot points
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