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Carboxypeptidase A mechanisms.

W N Lipscomb

    Proceedings of the National Academy of Sciences of the United States of America
    |July 1, 1980
    PubMed
    Summary

    Carboxypeptidase A binds ketonic substrates similarly to peptides, suggesting ester cleavage occurs at the primary peptide site. Enzyme behavior in solution differs from one crystal form but not another.

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    Area of Science:

    • Enzymology
    • Protein Structure and Function
    • Biochemistry

    Background:

    • Carboxypeptidase A (CPA) is a key enzyme involved in protein and peptide hydrolysis.
    • Understanding CPA's substrate binding and catalytic mechanisms is crucial for enzyme engineering and drug design.
    • Ketonic substrates serve as analogues for ester hydrolysis, offering insights into CPA's broader substrate specificity.

    Purpose of the Study:

    • To elucidate the binding mode of a ketonic substrate, an ester analogue, to carboxypeptidase A.
    • To compare the binding interactions of the ketonic substrate with those of known peptide substrates like Gly-Tyr.
    • To investigate the implications of substrate binding on the enzyme's catalytic site and mechanism.

    Main Methods:

    • X-ray crystallography to determine the enzyme-substrate complex structure.
    • Analysis of substrate positioning within the active site, including interactions with key residues (Arg-145) and the catalytic zinc ion.
    • Comparison of binding modes across different enzyme states (solution vs. crystalline phases).

    Main Results:

    • The ketonic substrate binds to carboxypeptidase A at the S'1 site, mirroring the binding of Gly-Tyr.
    • The substrate's side chain occupies the enzyme's pocket, its carboxylate forms a salt bridge with Arg-145, and the carbonyl group coordinates with the active site zinc.
    • Observed differences in enzyme behavior between solution and one crystalline phase were not replicated in another crystalline phase.

    Conclusions:

    • The binding mode suggests that ester substrates are likely cleaved at the canonical peptide cleavage site of carboxypeptidase A.
    • While the site of cleavage is likely conserved, the precise rate-determining steps or detailed mechanisms may differ between peptide and ester hydrolysis.
    • The conformational behavior of carboxypeptidase A can be state-dependent, with distinct properties observed in solution versus different crystalline environments.

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