1. Carboxypeptidase A
Sanketh Proddutur

2. What are Carboxypeptidases ?
Carboxypeptidases are enzymes that break down proteins into their constituent amino acids.
Carboxypeptidase A(CPA) and Carboxypeptidase B (CPB) are involved in digestion.
CPA is further divided into A1 and A2 isoforms

3. The A1 isoform showing a preference for aliphatic C-terminal residues of peptide substrates
While the A2 isoform selectively acts on the bulkier aromatic C-terminal residues
The other carboxypeptidases are largely involved in regulatory operations.

4. How does it matter?
Amino acids are the building blocks of all the proteins in the body.
This includes enzymes, hormones and antibodies.
CPA and other digestive enzymes obtain these amino acids, some of which cannot be synthesized by the body.

5. Enzymatic action
Hydrolyzes the peptide bonds at the carboxy teminus of proteins

6. Where is it located?(1AYE)
CPA is synthesized in the pancreas and activated in the duodenum typically by trypsin.
Structures composed of 9 alpha helices enclosing 8 beta sheet domain.
The active site is block by a N terminal globular domain.

7. Active site (2CTB)

8. Mechanism
The catalytic mechanism of CPA is proposed to proceed via the promoted water mechanism
A theoretical construct of the mechanism is as follows

9. Mechanism (contnd)
The peptide with terminal Phe residue binds to the enzyme. Arg 127 coordinates the H-bond network. Water is promoted to attack the C of the amide bond.

10. Mechanism (contnd)
Formation of the tetrahedral intermediate: the developing oxyanion moves to the positively charged zinc ion for greater stabilization. H bond network of newly acquired OH group

11. Mechanism (contnd)
Hydrolysis products formed: terminal Phe retained

12. Mechanism analogue (1CBX)

13. References
1. Vendrell, J., E. Querol, and F. X. Avilles. (2000) Metallocarboxypeptidases and their protein inhibitors. Structure, function and biomedical properties. Biochim Biophys Acta. 1477(1-2):284-98
2. Garcia-Saez, I., D. Reverter, J. Vendrell, F.X. Aviles and M. Coll. (1997) The three-dimensional structure of human procarboxypeptidase A2. EMBO J. 16(23):
3. Mark McDowall March. The structure of Carboxypeptidase A and the architecture of the active site. Homepage Accessed April 18, 2006.
4. Garcia-Saez I., D. Reverter , J. Vendrell , F.X. Aviles , M. Coll. (1997) The three-dimensional structure of human procarboxypeptidase A2. EMBO J. 16(23):

14. References (contnd)
5. Christianson, D. W., P.R. David  and W.N. Lipscomb. (1987) Mechanism of carboxypeptidase A: hydration of a ketonic substrate analogue.  Proc Natl Acad Sci U S A. 84(6):
6.  Mangani, S., P. Carloni , P. Orioli. (1992) Crystal structure of the complex between carboxypeptidase A and the biproduct analog inhibitor L-benzylsuccinate at 2.0 A resolution. J.Mol.Biol. 223: 573-8
7. Christianson, D.W. and Lipscomb, W.N. (1986). X-ray crystallographic investigation of substrate binding to varboxypeptidase A at subzero temperature. Proc Natl Acad Sci U S A. 83(20):
8. Christianson, D.W., and W.N. Lipscomb (1985) Proc. Natl. Acad. Sci. U S A. 82,

15. References
9. Teplyakov, A.,  Wilson, K.S.,  Orioli, P.,  Mangani, S. (1993) The High resolution crystal strcuture of the complex between carboxypeptidase and L-phenyl lactate.  Acta Crystallogr D Biol Crystallogr.  49(pt. 6) :
10. J. Cooper. February Protein Geometry. Homepage: Accessed April 18, 2006.
11. Yvette Petty. June DNA STRUCTURE, REPLICATION, TRANSCRIPTION, AND PROTEIN SYNTHESIS. Homepage: Accessed April 18, 2006.
12. Wikipedia. April Wikipedia homepage: Accessed April 19, 2006.

16. Questions?