Carboxypeptidase A: Difference between revisions

As previously stated, CPA from ''B. taurus'' has the ability to bind two Zn²⁺ ions in its active site.  The binding of only one Zn²⁺ 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 Zn²⁺ 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" />  [[Image:1CPXhydroxybridge.png|150 px|right|thumb|Figure 4: Hydroxy-bridge between catalytic and inhibitory zinc ions.  The catalytic Zn²⁺ ion (shown in orange on the right) is bridged to the inhibitory Zn²⁺ ion (shown in orange on the left) by a OH^- (shown in red).]]  In the CPA structure containing only the catalytic Zn²⁺ ion <scene name='69/694222/3cpas1subsiteglu270/1'>(3CPA)</scene>, a water molecule complexed to the zinc is able to be deprotonated by Glu270 to allow for normal initiation of hydrolysis (Figure 3).  However, when <scene name='69/694222/Glu270wiz/7'>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 Zn²⁺ ion interacts with the carboxylate group of Glu270.  The Glu270 now simply stabilizes the second Zn²⁺ and is unable to perform its usual base catalyst role while the catalytic Zn²⁺ ion (shown in green) is still being stabilized in place by His69, Glu72, and His196 (shown in orange).