2.3.2—Protein structure levels
- Syllabus
- 9700–2028–2029
- Objective
- 2.3.2
- Level
- AS
Protein structure is a hierarchy from sequence to subunit assembly
The four levels describe increasing organisation of a protein, not four separate molecules. A protein’s amino-acid sequence can fold locally and globally, and some proteins then assemble from multiple polypeptide subunits.
| Level | What it describes | Main stabilising feature | One chain or multiple? |
|---|---|---|---|
| Primary | The amino-acid sequence | Covalent peptide bonds along the chain | One polypeptide sequence |
| Secondary | Local α-helix or β-pleated-sheet regions | Hydrogen bonds between groups in the polypeptide backbone | One chain folding locally |
| Tertiary | The overall three-dimensional shape of one polypeptide | R-group interactions: hydrophobic, hydrogen, ionic and covalent disulfide links | One polypeptide chain |
| Quaternary | The functional arrangement of two or more polypeptide subunits | Interactions holding the subunits together and coordinating their assembly | Multiple polypeptide chains |
The sequence is the starting information: changing it can change later folding and function. Secondary structure is local backbone folding; tertiary structure is the complete shape of one chain; quaternary structure exists only when multiple chains work together. Haemoglobin is a useful example because its four globin subunits give it quaternary structure.
Do not say that every protein has quaternary structure, or that each level is a new molecule. A single-chain protein can have primary, secondary and tertiary structure without a quaternary level; also distinguish backbone hydrogen bonds in secondary structure from R-group interactions in tertiary structure.
ConceptA-Level CAIE Biology AS