Carboxypeptidase A: Difference between revisions
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<StructureSection load='1cpx' size='340' side='right' caption='Bovine carboxypeptidase A (CPA)' scene='69/694222/1cpx_default/3'> | <StructureSection load='1cpx' size='340' side='right' caption='Bovine carboxypeptidase A (CPA)' scene='69/694222/1cpx_default/3'> | ||
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As previously stated, <scene name='69/694222/1cpx_default/3'>CPA</scene> from ''B. taurus'' has the ability to bind two Zn<sup>2+</sup> ions in its active site. The binding of only one Zn<sup>2+</sup> ion is [http://en.wikipedia.org/wiki/Catalysis catalytic], while the binding of a second is [http://en.wikipedia.org/wiki/Reaction_inhibitor inhibitory]. These Zn<sup>2+</sup> ions are connected to each other via a hydroxy-bridge (Figure 4) with a distance of 3.48 [http://en.wikipedia.org/wiki/%C3%85ngstr%C3%B6m Å].<ref name="CPA1" /> The catalytic Zn<sup>2+</sup> ion maintains its tetrahedral binding configuration just as if the inhibitory Zn<sup>2+</sup> ion was not bound. In the CPA structure containing only the catalytic Zn<sup>2+</sup> ion (3CPA), a water molecule complexed to the zinc is able to be deprotonated by <scene name='69/694222/3cpas1subsiteglu270/3'>Glu270</scene>, allowing normal initiation of hydrolysis. Again, this water molecule was not crystallized in the structure of 3CPA, but it is shown in Figure 3. However, when <scene name='69/694222/Glu270wiz/8'>the inhibitory zinc ion</scene> is also present ([http://www.rcsb.org/pdb/explore/explore.do?structureId=1cpx 1CPX]), it occupies the physical space that would normally be occupied by the water molecule. Thus, the inhibitory Zn<sup>2+</sup> ion interacts with the carboxylate group of Glu270. The Glu270 (shown in yellow) now simply stabilizes the second Zn<sup>2+</sup> ion and is unable to perform its usual base catalyst role; the catalytic Zn<sup>2+</sup> ion (shown in cyan) is still being stabilized in place by His69, Glu72, and His196 (shown in orange). | As previously stated, <scene name='69/694222/1cpx_default/3'>CPA</scene> from ''B. taurus'' has the ability to bind two Zn<sup>2+</sup> ions in its active site. The binding of only one Zn<sup>2+</sup> ion is [http://en.wikipedia.org/wiki/Catalysis catalytic], while the binding of a second is [http://en.wikipedia.org/wiki/Reaction_inhibitor inhibitory]. These Zn<sup>2+</sup> ions are connected to each other via a hydroxy-bridge (Figure 4) with a distance of 3.48 [http://en.wikipedia.org/wiki/%C3%85ngstr%C3%B6m Å].<ref name="CPA1" /> The catalytic Zn<sup>2+</sup> ion maintains its tetrahedral binding configuration just as if the inhibitory Zn<sup>2+</sup> ion was not bound. In the CPA structure containing only the catalytic Zn<sup>2+</sup> ion (3CPA), a water molecule complexed to the zinc is able to be deprotonated by <scene name='69/694222/3cpas1subsiteglu270/3'>Glu270</scene>, allowing normal initiation of hydrolysis. Again, this water molecule was not crystallized in the structure of 3CPA, but it is shown in Figure 3. However, when <scene name='69/694222/Glu270wiz/8'>the inhibitory zinc ion</scene> is also present ([http://www.rcsb.org/pdb/explore/explore.do?structureId=1cpx 1CPX]), it occupies the physical space that would normally be occupied by the water molecule. Thus, the inhibitory Zn<sup>2+</sup> ion interacts with the carboxylate group of Glu270. The Glu270 (shown in yellow) now simply stabilizes the second Zn<sup>2+</sup> ion and is unable to perform its usual base catalyst role; the catalytic Zn<sup>2+</sup> ion (shown in cyan) is still being stabilized in place by His69, Glu72, and His196 (shown in orange). | ||
Carboxypeptidase A has been chemically modified and kinetically assayed to determine its Zn<sup>2+</sup> ion | Carboxypeptidase A has been chemically modified and kinetically assayed to determine its Zn<sup>2+</sup> ion binding affinities. Literature shows the K<sub>d</sub> value of the catalytic Zn<sup>2+</sup> ion to be two orders of magnitude less than the K<sub>d</sub> value of the inhibitory Zn<sup>2+</sup> ion (K<sub>d</sub> = 2.6x10<sup>-6</sup>M for the catalytic Zn<sup>2+</sup> ion and 5.5x10<sup>-4</sup>M for inhibitory Zn<sup>2+</sup> ion; pH = 8.2). This signifies that the catalytic Zn<sup>2+</sup> ion is approximately one hundred times more likely to bind to CPA compared to the inhibitory Zn<sup>2+</sup> ion.<ref name=“Binding”>Hirose, J., Noji, M., Kidani, Y., Wilkins, R. 1985. Interaction of zinc ions with arsanilazotyrosine-248 carboxypeptidase A.''Biochemistry''. 24(14):3495-3502. [http://pubs.acs.org/doi/abs/10.1021/bi00335a016 DOI:10.1021/bi00335a016]</ref> | ||
==Other Inhibitors== | ==Other Inhibitors== | ||
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Further detailed studies of anions have indicated that the nature of anion inhibition in the binding site is partly [http://en.wikipedia.org/wiki/Competitive_inhibition competitive].<ref name="CPA1" /> In particular, the sulfate anion (SO<sub>4</sub><sup>2-</sup>) has been of interest to researchers. In a crystallized structure of carboxypeptidase T (PDB code: [http://www.rcsb.org/pdb/explore/explore.do?structureId=1ord 1ORD]), a SO<sub>4</sub><sup>2-</sup> anion was found occupying a portion of a region that corresponds to the amino acid residues Arg127, Asn144, Arg145, and Tyr248 of the S1 subsite of carboxypeptidase A.<ref name="CPA1" /> In this case, it is understood that the SO<sub>4</sub><sup>2-</sup> anion prevents the recognition of the carboxylate group at the C-terminus of the polypeptide substrate. | Further detailed studies of anions have indicated that the nature of anion inhibition in the binding site is partly [http://en.wikipedia.org/wiki/Competitive_inhibition competitive].<ref name="CPA1" /> In particular, the sulfate anion (SO<sub>4</sub><sup>2-</sup>) has been of interest to researchers. In a crystallized structure of carboxypeptidase T (PDB code: [http://www.rcsb.org/pdb/explore/explore.do?structureId=1ord 1ORD]), a SO<sub>4</sub><sup>2-</sup> anion was found occupying a portion of a region that corresponds to the amino acid residues Arg127, Asn144, Arg145, and Tyr248 of the S1 subsite of carboxypeptidase A.<ref name="CPA1" /> In this case, it is understood that the SO<sub>4</sub><sup>2-</sup> anion prevents the recognition of the carboxylate group at the C-terminus of the polypeptide substrate. | ||
==3D structures of carboxypeptidase A== | |||
See [[Carboxypeptidase]] | |||
</StructureSection> | </StructureSection> | ||