Carboxypeptidase A: Difference between revisions

CPA from ''B. taurus'' has been  co-crystallized with two Zn²⁺ ions (Figure 1).  This structure has been deposited in the PDB database under the label [http://www.rcsb.org/pdb/explore/explore.do?structureId=1cpx 1CPX], which is a β-form of CPA.  1CPX has an interesting distinction from some other crystallized CPA proteins in that its crystallographic data has revealed a different conformation of the Tyr248 residue.[[Image:1CPX-Tyr248.png|thumb|Figure 2: Important Tyr248 residue of 1CPX is pointing toward the hydrophobic binding pocket.]]<ref name="CPA1">Bukrinsky JT, Bjerrum MJ, Kadziola A. 1998. Native carboxypeptidase A in a new crystal environment reveals a different conformation of the important tyrosine 248. ''Biochemistry''. 37(47):16555-16564. [http://pubs.acs.org/doi/abs/10.1021/bi981678i DOI: 10.1021/bi981678i]</ref>  Previous literature has suggested that the conserved Tyrosine residue among CPA proteins has been involved in an [http://en.wikipedia.org/wiki/Enzyme_catalysis#Induced_fit induced fit mechanism] because Tyr248 typically has been found <scene name='69/694222/Tyr248_5cpa/1'>pointing outward</scene> toward solution when a substrate is not bound in the active site.<ref name="CPA1">Bukrinsky JT, Bjerrum MJ, Kadziola A. 1998. Native carboxypeptidase A in a new crystal environment reveals a different conformation of the important tyrosine 248. ''Biochemistry''. 37(47):16555-16564. [http://pubs.acs.org/doi/abs/10.1021/bi981678i DOI: 10.1021/bi981678i]</ref>  However, crystallographic data for 1CPX shows Tyr248 <scene name='69/694222/Tyr248_1cpx/1'>pointing toward the active site</scene> (Figure 2) without a polypeptide substrate bound.  Therefore, this conflicts with the previously proposed induced fit mechanism for CPA proteins and suggests that Tyr248 is liganded to the catalytic Zn²⁺ ion in 1CPX.<ref name="CPA1">Bukrinsky JT, Bjerrum MJ, Kadziola A. 1998. Native carboxypeptidase A in a new crystal environment reveals a different conformation of the important tyrosine 248. ''Biochemistry''. 37(47):16555-16564. [http://pubs.acs.org/doi/abs/10.1021/bi981678i DOI: 10.1021/bi981678i]</ref>  Not only does Tyr248 point toward the active site, but in doing so, Tyr248 <scene name='69/694222/Tyr248_pointing_toward/2'>caps the hydrophobic binding pocket</scene> (shown in blue) with its interactions with Ile247 and a hydrogen bond.<ref name="CPA1">Bukrinsky JT, Bjerrum MJ, Kadziola A. 1998. Native carboxypeptidase A in a new crystal environment reveals a different conformation of the important tyrosine 248. ''Biochemistry''. 37(47):16555-16564. [http://pubs.acs.org/doi/abs/10.1021/bi981678i DOI: 10.1021/bi981678i]</ref>  This point of view is able to clearly show how Tyr248 is thought to be a ligand with the catalytic Zn²⁺ ion.