Caspase-3 Regulatory Mechanisms: Difference between revisions

== Introduction ==

Any apoptotic signal received by the cell causes the activation of initiator caspases (-8 and -9) by associating with other protein platforms to form a functional holoenzyme.  These initiator caspases then cleave the executioner caspases -3, -6, and -7.  Caspase-3 specifically functions to cleave downstream apoptotic targets as well as both caspase-6 and -7, which in turn cleave their respective targets to induce cell death.  Aside from being able to activate caspase-6 and -7, caspase-3 also regulates caspase-9 activity, operating via a feedback loop.  This dual action of caspase-3 confers its distinct regulatory mechanisms, resulting in a wider extent of its effects in the apoptotic cascade.

== Overview of Caspase-3 Structure ==

===Dimer Formation ===

The active pocket of caspase-3 is defined by <scene name='Sandox_Bay_Serrano/Monomer/2'>Cys-163 and His-121</scene>.  Binding of a <scene name='Sandox_Bay_Serrano/Scene01_substrate/4'>substrate</scene>, such as DEVD-CHO to the active site of the enzyme induces a conformational change that allows the L2 and L2' loops to interlock and stabilize the active site <scene name='Sandox_Bay_Serrano/Scene01_substrate/3'></scene>.  Like caspase-7, caspase-3 recognizes a Asp-X-X-Asp sequence as a cleavage site in its protein substrates.

== Caspase-3 Loop Bundle and Active Site==

In order to be active and cleave specific apoptotic targets, Caspase-3 must be able to first bind substrate. There are several essential interactions responsible for securing the substrate before cleavage. The binding pocket at <scene name='Caspase-3_Regulatory_Mechanisms/P2/2'>P2</scene> is a hydrophobic patch made up of Y204, W206, and F250 (dark blue residues). This creates a hydrophobic pocket for the P2 residue (in this case, valine), helping it stick to the protein. At <scene name='Caspase-3_Regulatory_Mechanisms/P4/2'>P4</scene> there are contacts that contribute to the specificity of caspase-3. Asparagine 208 hydrogen bonds with an aspartate at P4 along with the backbone nitrogen of F250, creating a preference for a carboxylic acid at the P4 site.  

== Caspase-3 Regulation==

=== Exosite and Allosteric Site===

X-linked inhibitor of apoptosis proteins (XIAP) contains the second baculovirus IAP repeat domain (BIR2) targeting caspase-3 and caspase-7. The <scene name='Caspase-3_Regulatory_Mechanisms/Bir2/2'>BIR2</scene> domain sits directly in the active site of caspase-3 and completely inhibits the protein. The region binding the active site runs in the opposite direction of normal caspase-3 substrates, thus occupying P1 through P4 but avoiding cleavage by the protease. This unique method of inhibition is a critical regulatory mechanism used in cells to control apoptotic caspase activity.

=References=

Hardy, J. A., J. Lam, et al. (2004). "Discovery of an allosteric site in the caspases." Proc Natl Acad Sci U S A 101(34): 12461-12466.