Catalytic Hydrolysis of Acylcyanamide Substrates by Carboxypeptidase A Defines the Stereochemistry of Carbonyl Hydration at the Enzyme Active Site
✍ Scribed by W.L. Mock; X.D. Xu
- Publisher
- Elsevier Science
- Year
- 1994
- Tongue
- English
- Weight
- 737 KB
- Volume
- 22
- Category
- Article
- ISSN
- 0045-2068
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✦ Synopsis
New peptide and ester substrate analogues for carboxypeptidase A have been prepared which differ by the possession of an acylcyanamide residue of comparable acidity in place of the C-terminal carboxylic acid group that is normally required by the enzyme. The surrogates are cleaved respectively (2 \times 10^{5}) and (10^{4}) times more slowly than corresponding regular substrates (\left(k_{\mathrm{cat}} / K_{m}\right)). The suggested interpretation is that the carboxylate functionality of normal substrates, in addition to conferring specificity, participates in the enzymic reaction mechanism as a general base, and that this becomes disallowed in the new analogues by specific-binding interactions which induce the cyano substituent to interfere with hydration of the scissile linkage. That reasoning controverts widely accepted ideas regarding the stereochemical course of (\mathrm{H}_{2} \mathrm{O}) addition to the enzymic (\mathrm{Zn}^{2+})-activated peptide linkage as well as the role of carboxypeptidase active site residue Glu 270 in the catalytic mechanism. 1994 Academic Press, Inc.