| Literature DB >> 9187653 |
R C Wilmouth1, I J Clifton, C V Robinson, P L Roach, R T Aplin, N J Westwood, J Hajdu, C J Schofield.
Abstract
Mass spectrometric screening reveals that an unmodified natural heptapeptide--human beta-casomorphin-7, an internal sequence of human beta-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 A resolution) at pH 5 shows continuous electron density linking the C-terminal isoleucine of beta-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 N epsilon 2 of His 57. Deprotonation of Wat 317 will produce a hydroxide ion positioned to attack the ester carbonyl through the favoured Bürgi-Dunitz trajectory.Entities:
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Year: 1997 PMID： 9187653 DOI： 10.1038/nsb0697-456
Source DB: PubMed Journal: Nat Struct Biol ISSN： 1072-8368