Caspase-3 is a caspase protein that interacts with caspase-8 and caspase-9.
It is encoded by the CASP3 gene. CASP3 orthologs have been identified in
numerous mammals for which complete genome data are available. Unique
orthologs are also present in birds, lizards, lissamphibians, and teleosts.
The CASP3 protein is a member of the cysteine-aspartic acid protease family.
Sequential activation of caspases plays a central role in the execution-phase of
cell apoptosis. Caspases exist as inactive proenzymes that undergo
proteolytic processing at conserved aspartic residues to produce two
subunits, large and small, that dimerize to form the active 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 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 cysteine
residue and histidine residue that stabilize the peptide bond cleavage of a
protein sequence to the carboxy-terminal side of an 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
glutamic acid. A key feature of caspases in the cell is that they are present as
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 aspartic acid residues.
Caspase 3 and 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, 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 beta-sheet structure surrounded by
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 subunit.
Mechanism The catalytic site of caspase-3 involves
the sulfohydryl group of Cys-285 and the imidazole ring of His-237. His-237
stabilizes the 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 transition state of the substrate-enzyme complex
through hydrogen bonding. In vitro, caspase-3 has been found to prefer the
peptide sequence DEVDG with cleavage occurring on the carboxy side of the
second aspartic acid residue. Caspase-3 is active over a broad pH range that is
slightly higher 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 and intrinsic
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 granzyme B, which
can activate initiator caspases, into cells targeted for apoptosis by killer 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, cytochrome c from the mitochondria works
in combination with caspase-9, apoptosis-activating factor 1, and ATP
to process procaspase-3. These molecules are sufficient to activate caspase-3 in
vitro, but other regulatory proteins are necessary in vivo. Mangosteen 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 protein family, which
includes c-IAP1, c-IAP2, 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: Biological function
Caspase-3 has been found to be necessary for normal brain development as well as
its typical role in apoptosis, where it is responsible for chromatin
condensation and DNA fragmentation. Elevated levels of a fragment of
Caspase-3, p17, in the bloodstream is a sign of a recent myocardial infarction.
It is now being shown that caspase-3 may play a role in embryonic and
hematopoietic stem cell differentiation. See also
The Proteolysis Map Caspase
References Further reading
External links The MEROPS online database for
peptidases and their inhibitors: C14.003 Media related to caspase 3 at Wikimedia
Commons Apoptosis & Caspase 3 - The Proteolysis
Map-animation