Although the structure–activity relationship of many bioactive peptides has not yet been fully established, several structural features have been identified which appear to influence the biological action of peptides. With angiotensin converting enzyme inhibitory peptides, for example, binding of the peptide to ACE is strongly influenced by the presence of tyrosine, phenylalanine, tryptophan, proline, lysine, isoleucine, valine, leucine and arginine.