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Title
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Structure of a specific acyl-enzyme complex formed between beta-casomorphin-7 and porcine pancreatic elastase.
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Authors
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R.C.Wilmouth,
I.J.Clifton,
C.V.Robinson,
P.L.Roach,
R.T.Aplin,
N.J.Westwood,
J.Hajdu,
C.J.Schofield.
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Ref.
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Nat Struct Biol, 1997,
4,
456-462.
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PubMed id
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Abstract
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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.
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