PMID: 17157876

 

    Legend: Sugar

Title : Crystal structure of the human carboxypeptidase N ( kininase I ) catalytic domain

Abstract :
  1. Human carboxypeptidase N ( CPN ), a member of the CPN /E subfamily of "regulatory" metallo-carboxypeptidases, is an extracellular glycoprotein synthesized in the liver and secreted into the blood, where it controls the activity of vasoactive peptide hormones, growth factors and cytokines by specifically removing C-terminal basic residues
  2. Normally, CPN circulates in blood plasma as a hetero-tetramer consisting of two 83 kDa ( CPN2 ) domains each flanked by a 48 to 55 kDa catalytic ( CPN1 ) domain
  3. We have prepared and crystallized the recombinant C-terminally truncated catalytic domain of human CPN1 , and have determined and refined its 2.1 A crystal structure
  4. The structural analysis reveals that CPN1 has a pear-like shape, consisting of a 319 residue N-terminal catalytic domain and an abutting, cylindrically shaped 79 residue C-terminal beta-sandwich transthyretin ( TT ) domain , more resembling CPD-2 than CPM
  5. Like these other CPN /E members, two surface loops surrounding the active-site groove restrict access to the catalytic center, offering an explanation for why some larger protein carboxypeptidase inhibitors do not inhibit CPN
  6. Modeling of the Pro-Phe-Arg C-terminal end of the natural substrate bradykinin into the active site shows that the S1' pocket of CPN1 might better accommodate P1'- Lys than Arg residues , in agreement with CPN 's preference for cleaving off C-terminal Lys residues
  7. Three Thr residues at the distal TT edge of CPN1 are O-linked to N-acetyl glucosamine sugars; equivalent sites in the membrane-anchored CPM are occupied by basic residues probably involved in membrane interaction
  8. In tetrameric CPN , each CPN1 subunit might interact with the central leucine-rich repeat tandem leucine-rich repeat tandem of the cognate CPN2 subunit via a unique hydrophobic surface patch wrapping around the catalytic domain- TT interface, exposing the two active centers