1. Protein Structure  The structure of proteins can be described at 4 levels – primary, secondary, tertiary and quaternary.  Primary structure  The sequence of amino acids in the peptide chain  The order is unique for each protein and this order determines the properties of the protein  There are 20 different kinds of amino acids so 20 x 20 = 400 possible dipeptides, 20 x 20 x 20 = 8 000 possible tripeptides etc.

2.  Secondary structure  The folding of a section of the polypeptide chain. There are two forms  An alpha (α) helix: the peptide chain forms a spiral or helix  A beta (β) sheet: two or more sections of the polypeptide chain run next to each other  A single polypeptide chain can have both of these forms of folding  Tertiary structure  The way an entire single polypeptide chain folds up  The short sections of alpha helix or beta sheet will fold upon themselves to form an overall three-dimensional shape.  Quaternary structure  The association of two or more polypeptides in some proteins and the way these intertwine and fold.  This level does not apply if a protein is made up of a single polypeptide only.