Primary structure is the amino acid sequence of a polypeptide; secondary structure is its local folding; tertiary structure is the overall three-dimensional conformation of one polypeptide; and quaternary structure is the arrangement of multiple polypeptide subunits. This is Additional Higher Level (AHL) content in IB Biology B1.2 Proteins.
| Level | Structure and stabilization | Key example or feature |
|---|---|---|
| Primary | A specific linear sequence of amino acids joined by covalent peptide bonds | A change in the amino acid sequence can alter every higher level of structure. |
| Secondary | Local, regular folding into alpha helices or beta-pleated sheets, stabilized by hydrogen bonds between groups in the polypeptide backbone | The hydrogen bonds occur in regular positions along the backbone. |
| Tertiary | The complete three-dimensional conformation of one polypeptide, produced by interactions between amino acid R-groups | Stabilized by hydrogen bonds, ionic bonds, covalent disulfide bonds and hydrophobic interactions. |
| Quaternary | The association and spatial arrangement of two or more polypeptide subunits | Collagen contains three polypeptide chains; haemoglobin contains four subunits and non-polypeptide haem groups. |
The levels are hierarchical. The primary structure determines where particular R-groups occur. This influences local folding and the interactions that produce the protein’s tertiary conformation, so a mutation that changes one amino acid may alter protein function.
In water-soluble globular proteins, hydrophobic R-groups generally cluster inside the protein, while hydrophilic R-groups face the surrounding water. Integral membrane proteins instead contain exposed hydrophobic regions that interact with the membrane’s hydrophobic core.
