Caspase 3

Caspase-3 is a is_associated_with::caspase protein that interacts with is_associated_with::caspase-8 and is_associated_with::caspase-9. It is encoded by the CASP3 gene. CASP3 is_associated_with::orthologs have been identified in numerous is_associated_with::mammals for which complete genome data are available. Unique orthologs are also present in is_associated_with::birds, is_associated_with::lizards, is_associated_with::lissamphibians, and is_associated_with::teleosts.

The CASP3 is_associated_with::protein is a member of the cysteine-aspartic acid protease (is_associated_with::caspase) family. Sequential activation of caspases plays a central role in the execution-phase of cell apoptosis. Caspases exist as inactive is_associated_with::proenzymes that undergo proteolytic processing at conserved aspartic residues to produce two subunits, large and small, that dimerize to form the active is_associated_with::enzyme. This protein cleaves and activates caspases 6 and 7; and the protein itself is processed and activated by caspases 8, 9, and 10. It is the predominant caspase involved in the cleavage of amyloid-beta 4A precursor protein, which is associated with neuronal death in is_associated_with::Alzheimer's disease. Alternative splicing of this gene results in two transcript variants that encode the same protein.

Caspase-3 shares many of the typical characteristics common to all currently-known caspases. For example, its active site contains a is_associated_with::cysteine residue (Cys-163) and is_associated_with::histidine residue (His-121) that stabilize the is_associated_with::peptide bond cleavage of a protein sequence to the carboxy-terminal side of an is_associated_with::aspartic acid when it is part of a particular 4-amino acid sequence. This specificity allows caspases to be incredibly selective, with a 20,000-fold preference for aspartic acid over is_associated_with::glutamic acid. A key feature of caspases in the cell is that they are present as is_associated_with::zymogens, termed procaspases, which are inactive until a biochemical change causes their activation. Each procaspase has an N-terminal large subunit of about 20 kDa followed by a smaller subunit of about 10 kDa, called p20 and p10, respectively.

Substrate specificity
Under normal circumstances, caspases recognize tetra-peptide sequences on their substrates and hydrolyze peptide bonds after is_associated_with::aspartic acid residues. Caspase 3 and is_associated_with::caspase 7 share similar substrate specificity by recognizing tetra-peptide motif Asp-x-x-Asp. The C-terminal Asp is absolutely required while variations at other three positions can be tolerated. Caspase substrate specificity has been widely used in caspase based inhibitor and drug design.

Structure
Caspase-3, in particular, (also known as CPP32/Yama/apopain)  is formed from a 32 kDa zymogen that is cleaved into 17 kDa and 12 kDa subunits. When the procaspase is cleaved at a particular residue, the active heterotetramer can then be formed by hydrophobic interactions, causing four anti-parallel beta-sheets from p17 and two from p12 to come together to make a heterodimer, which in turn interacts with another heterodimer to form the full 12-stranded is_associated_with::beta-sheet structure surrounded by is_associated_with::alpha-helices that is unique to caspases. When the heterodimers align head-to-tail with each other, an active site is positioned at each end of the molecule formed by residues from both participating subunits, though the necessary Cys-285 and His-237 residues are found on the p17 (larger) subunit.



Mechanism
The catalytic site of caspase-3 involves the sulfohydryl group of Cys-285 and the is_associated_with::imidazole ring of His-237. His-237 stabilizes the is_associated_with::carbonyl group of the key aspartate residue, while Cys-285 attacks to ultimately cleave the peptide bond. Cys-285 and Gly-238 also function to stabilize the tetrahedral is_associated_with::transition state of the substrate-enzyme complex through is_associated_with::hydrogen bonding. is_associated_with::In vitro, caspase-3 has been found to prefer the peptide sequence DEVDG (Asp-Glu-Val-Asp-Gly) with cleavage occurring on the carboxy side of the second aspartic acid residue (between D and G). Caspase-3 is active over a broad is_associated_with::pH range that is slightly higher (more basic) than many of the other executioner caspases. This broad range indicates that caspase-3 will be fully active under normal and apoptotic cell conditions.



Activation
Caspase-3 is activated in the apoptotic cell both by extrinsic (death ligand) and intrinsic (mitochondrial) pathways. The zymogen feature of caspase-3 is necessary because if unregulated, caspase activity would kill cells indiscriminately. As an executioner caspase, the caspase-3 zymogen has virtually no activity until it is cleaved by an initiator caspase after apoptotic signaling events have occurred. One such signaling event is the introduction of is_associated_with::granzyme B, which can activate initiator caspases, into cells targeted for apoptosis by killer is_associated_with::T cells. This extrinsic activation then triggers the hallmark caspase cascade characteristic of the apoptotic pathway, in which caspase-3 plays a dominant role. In intrinsic activation, is_associated_with::cytochrome c from the is_associated_with::mitochondria works in combination with is_associated_with::caspase-9, apoptosis-activating factor 1 (is_associated_with::Apaf-1), and ATP to process procaspase-3. These molecules are sufficient to activate caspase-3 in vitro, but other regulatory proteins are necessary is_associated_with::in vivo. Mangosteen (Garcinia Mangostana) extract has been shown to inhibit the activation of caspase 3 in B-amyloid treated human neuronal cells.

Inhibition
One means of caspase inhibition is through the IAP (inhibitor of apoptosis) protein family, which includes c-IAP1, c-IAP2, is_associated_with::XIAP, and ML-IAP. XIAP binds and inhibits initiator caspase-9, which is directly involved in the activation of executioner caspase-3. During the caspase cascade, however, caspase-3 functions to inhibit XIAP activity by cleaving caspase-9 at a specific site, preventing XIAP from being able to bind to inhibit caspase-9 activity.

Interactions
Caspase 3 has been shown to interact with:


 * CASP8
 * is_associated_with::NMT2
 * is_associated_with::CFLAR
 * DCC
 * is_associated_with::GroEL
 * is_associated_with::HCLS1
 * is_associated_with::Survivin
 * is_associated_with::TRAF3
 * is_associated_with::XIAP
 * is_associated_with::NFE2L2

Biological function
Caspase-3 has been found to be necessary for normal is_associated_with::brain development as well as its typical role in apoptosis, where it is responsible for is_associated_with::chromatin condensation and is_associated_with::DNA fragmentation. Elevated levels of a fragment of Caspase-3, p17, in the bloodstream is a sign of a recent is_associated_with::myocardial infarction. It is now being shown that caspase-3 may play a role in embryonic and hematopoietic is_associated_with::stem cell differentiation.