Structure of proteins

A protein's shape decides what it can do. Its structure is described at four levels.

• Primary structure: the order (sequence) of amino acids along the chain, like the order of letters in a word. The first amino acid is the N-terminal one, and the last is the C-terminal one.
• Secondary structure: parts of the chain fold into a regular shape: a coil (helix) or a folded sheet (the beta-pleated sheet), held by hydrogen bonds. In proteins, only right-handed helices are found.
• Tertiary structure: the whole chain then folds on itself into a three-dimensional shape, like a hollow ball of wool. This shape is essential for many proteins to work.
• Quaternary structure: some proteins are made of several folded chains (subunits). How these are arranged together is the quaternary structure.

Example: adult human haemoglobin, which carries oxygen in red blood cells, is made of four subunits: two α and two β chains.