A level Biology exam revision resources written by A level Examiners


A complete protein molecule is made up of at least one polypeptide chain which has a specific shape. The structure of protein molecules is amazingly complex. Their biological function often depends on their specific shape, and this shape depends in turn on how their amino acids are joined together. To help understand protein structure it is explained at different levels: These are a proteins

  • Primary structure
  • Secondary structure
  • Tertiary structure
  • (and sometimes quaternary structure)

The primary structure is the sequence of amino acids in the polypeptide chain. It is the most fundamental level of structure and actually determines the rest of the protein structure.

The secondary structure is the arrangement of the polypeptide chain. There are two common secondary structures that the chain can have.

  • It can be arranged in a coil, known as the Biochemistry-helix
  • It can be arranged in a zigzag fashion, known as the Biochemistry-pleated sheets
Figure 4

Both these structures hold their shape because hydrogen bonds can form between amino acids and strengthen the structure. A single polypeptide chain can have both Biochemistry-helix sections and Biochemistry-pleated sheet sections at different places along its length.

The tertiary structure is the three dimensional structure formed by the folding up of a whole polypeptide chain. Every protein has its own unique tertiary structure. The tertiary structure is held together by bonds between the R groups of the amino acids in polypeptide chains. Most of these bonds are weak hydrogen bonds, but there are also stronger sulphur bridges and ionic bonds which hold the tertiary structure together (especially common in thermostable proteins). The importance of the tertiary structure is seen in enzymes, where the active site's shape is determined by the tertiary structure, and it is the active site that the reactant molecules fit into, in order for the enzyme to catalyse the reaction.

If a protein is made from more than one polypeptide chain the way these are arranged together is known as the quaternary structure.

The whole protein can also be classified by shape into one of two categories, summarised in the table below.


Share this knowledge


Back To Top Back To Top