Understanding:
• The sequence and number of amino acids in a polypeptide is the primary structure
Primary (1º) Structure
- The first level of structural organisation in a protein is the order / sequence of amino acids which comprise the polypeptide chain
- The primary structure is formed by covalent peptide bonds between the amine and carboxyl groups of adjacent amino acids
- Primary structure controls all subsequent levels of protein organisation because it determines the nature of the interactions between R groups of different amino acids
Understanding:
• The secondary structure is the formation of α-helices and β-pleated sheets stabilised by hydrogen bonding
Secondary (2º) Structure
- The secondary structure is the way a polypeptide folds in a repeating arrangement to form α-helices and β-pleated sheets
- This folding is a result of hydrogen bonding between the amine and carboxyl groups of non-adjacent amino acids
- Sequences that do not form either an alpha helix or beta-pleated sheet will exist as a random coil
- Secondary structure provides the polypeptide chain with a level of mechanical stability (due to the presence of hydrogen bonds)
- In pictures, alpha helices are represented as spirals (purple ; left) and beta-pleated sheets as arrows (blue ; right)
Understanding:
• The tertiary structure is the further folding of the polypeptide stabilised by interactions between R groups
Tertiary (3º) Structure
- The tertiary structure is the way the polypeptide chain coils and turns to form a complex molecular shape (i.e. the 3D shape)
- It is caused by interactions between R groups; including H-bonds, disulfide bridges, ionic bonds and hydrophobic interactions
- Relative amino acid positions are important (e.g. non-polar amino acids usually avoid exposure to aqueous solutions)
- Tertiary structure may be important for the function of the protein (e.g. specificity of active site in enzymes)
Understanding:
• The quaternary structure exists in proteins with more than one polypeptide chain
Quaternary (4º) Structure
- Multiple polypeptides or prosthetic groups may interact to form a single, larger, biologically active protein (quaternary structure)
- A prosthetic group is an inorganic compound involved in protein structure or function (e.g. the heme group in haemoglobin)
- A protein containing a prosthetic group is called a conjugated protein
- Quaternary structures may be held together by a variety of bonds (similar to tertiary structure)
Summary of the Four Levels of Protein Structure
