Each type of amino acid differs in the composition of the variable side chain (there are 20 common amino acids)

• These side chains have distinct chemical properties and cause the protein to fold and function differently according to their specific position within the polypeptide chain
  

• Side chains can be hydrophobic (non-polar) or hydrophilic (polar or charged / acidic or basic)
  

As most natural polypeptide chains contain between 50 – 2000 amino acid residues, organisms are capable of producing a huge range of possible polypeptides with a wide variety of functional properties