Protein Structure


ninja icon


•  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

primary structure

ninja icon


•  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)

alpha helices beta sheets

ninja icon


•  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)

tertiary structure

ninja icon


•  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)

quaternary structure

Summary of the Four Levels of Protein Structure 

levels of protein structure