1M4L: Structure Of Native Carboxypeptidase A At 1.25 Resolution

The crystal structure of the bovine zinc metalloproteinase carboxypeptidase A (CPA) has been refined to 1.25 A resolution based on room-temperature X-ray synchrotron data. The significantly improved structure of CPA at this resolution (anisotropic temperature factors, R factor = 10.4%, R(free) = 14.5%) allowed the modelling of conformational disorders of side chains, improved the description of the protein solvent network (375 water molecules) and provided a more accurate picture of the interactions between the active-site zinc and its ligands. The calculation of standard uncertainties in individual atom positions of the refined model of CPA allowed the deduction of the protonation state of some key residues in the active site and confirmed that Glu72 and Glu270 are negatively charged in the resting state of the enzyme at pH 7.5. These results were further validated by theoretical calculations that showed significant reduction of the pK(a) of these side chains relative to solution values. The distance between the zinc-bound solvent molecule and the metal ion is strongly suggestive of a neutral water molecule and not a hydroxide ion in the resting state of the enzyme. These findings could support both the general acid/general base mechanism, as well as the anhydride mechanism suggested for CPA.
PDB ID: 1M4LDownload
MMDB ID: 21724
PDB Deposition Date: 2002/7/3
Updated in MMDB: 2007/11
Experimental Method:
x-ray diffraction
Resolution: 1.25  Å
Source Organism:
Similar Structures:
Biological Unit for 1M4L: monomeric; determined by author
Molecular Components in 1M4L
Label Count Molecule
Protein (1 molecule)
Carboxypeptidase a(Gene symbol: CPA1)
Molecule annotation
Chemical (1 molecule)
* Click molecule labels to explore molecular sequence information.

Citing MMDB