Crystal structure and mechanism of human carboxypeptidase O: Insights into its specific activity for acidic residues

Maria C. Garcia-Guerrero, Javier Garcia-Pardo, Esther Berenguer, Roberto Fernandez-Alvarez, Gifty B. Barfi, Peter J. Lyons, Francesc X. Aviles, Robert Huber, Julia Lorenzo, David Reverter

Research output: Contribution to journalArticleResearchpeer-review

11 Citations (Scopus)


© 2018 National Academy of Sciences. All Rights Reserved. Human metallocarboxypeptidase O (hCPO) is a recently discovered digestive enzyme localized to the apical membrane of intestinal epithelial cells. Unlike pancreatic metallocarboxypeptidases, hCPO is glycosylated and produced as an active enzyme with distinctive substrate specificity toward C-terminal (C-t) acidic residues. Here we present the crystal structure of hCPO at 1.85-Å resolution, both alone and in complex with a carboxypeptidase inhibitor (NvCI) from the marine snail Nerita versicolor. The structure provides detailed information regarding determinants of enzyme specificity, in particular Arg275, placed at the bottom of the substrate-binding pocket. This residue, located at “canonical” position 255, where it is Ile in human pancreatic carboxypeptidases A1 (hCPA1) and A2 (hCPA2) and Asp in B (hCPB), plays a dominant role in determining the preference of hCPO for acidic C-t residues. Site-directed mutagenesis to Asp and Ala changes the specificity to C-t basic and hydrophobic residues, respectively. The single-site mutants thus faithfully mimic the enzymatic properties of CPB and CPA, respectively. hCPO also shows a preference for Glu over Asp, probably as a consequence of a tighter fitting of the Glu side chain in its S1′ substrate-binding pocket. This unique preference of hCPO, together with hCPA1, hCPA2, and hCPB, completes the array of C-t cleavages enabling the digestion of the dietary proteins within the intestine. Finally, in addition to activity toward small synthetic substrates and peptides, hCPO can also trim C-t extensions of proteins, such as epidermal growth factor, suggesting a role in the maturation and degradation of growth factors and bioactive peptides.
Original languageEnglish
Pages (from-to)E3932-E3939
JournalProceedings of the National Academy of Sciences of the United States of America
Issue number17
Publication statusPublished - 24 Apr 2018


  • Acidic protease
  • Carboxypeptidase
  • Crystal structure
  • Protein digestion


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