Structure of a specific acyl-enzyme complex formed between β-casomorphin-7 and porcine pancreatic elastase

Mass spectrometric screening reveals that an unmodified natural heptapeptide—human β-casomorphin-7, an internal sequence of human β-casein that possesses opioid-like activity—reacts with porcine pancreatic elastase to form an unusually stable acyl-enzyme complex at low pH. X-ray crystallographic analysis (to 1.9 Å resolution) at pH 5 shows continuous electron density linking the C-terminal isoleucine of β-casomorphin-7 to Ser 195 through an ester bond. The structure reveals a well defined water molecule (Wat 317), equidistant between the carbon of the ester carbonyl and Nε2 of His 57. Deprotonation of Wat 317 will produce a hydroxide ion positioned to attack the ester carbonyl through the favoured Bürgi-Dunitz trajectory.