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.[6] Alternative splicing of this gene results in two transcript variants that encode the same protein.[7]
Caspase-3 shares many of the typical characteristics common to all currently-known caspases. For example, its active site contains a
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.[12]
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.[13] The C-terminal Asp is absolutely required while variations at other three positions can be tolerated.[14] Caspase substrate specificity has been widely used in caspase based inhibitor and drug design.[15]
Structure
Caspase-3, in particular, (also known as CPP32/Yama/apopain)
alpha-helices that is unique to caspases.[12][19] 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-163 and His-121 residues are found on the p17 (larger) subunit.[19]
The p12 (pink) and p17 (light blue) subunits of caspase-3 with the beta-sheet structures of each in red and blue, respectively; image generated in Pymol from 1rhm.pdb
Mechanism
The catalytic site of caspase-3 involves the thiol group of Cys-163 and the
hydrogen bonding.[19]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).[11][19][20] Caspase-3 is active over a broad 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.[21]
Cys-285 (yellow) and His-237 (green and dark blue) in the active site of caspase-3, p12 subunit in pink and p17 subunit in light blue; image generated in Pymol from 1rhr.pdb
Activation
Caspase-3 is activated in the apoptotic cell both by extrinsic (death ligand) and intrinsic (mitochondrial) pathways.
Apaf-1), and ATP to process procaspase-3.[20][26][27] These molecules are sufficient to activate caspase-3 in vitro, but other regulatory proteins are necessary in vivo.[27]
Mangosteen (Garcinia mangostana) extract has been shown to inhibit the activation of caspase 3 in B-amyloid treated human neuronal cells.[28]
Inhibition
One means of caspase inhibition is through the IAP (inhibitor of apoptosis) protein family, which includes c-IAP1, c-IAP2, XIAP, and ML-IAP.[19] XIAP binds and inhibits initiator caspase-9, which is directly involved in the activation of executioner caspase-3.[27] 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.[29]
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.[20] Elevated levels of a fragment of Caspase-3, p17, in the bloodstream is a sign of a recent myocardial infarction.[51] It is now being shown that caspase-3 may play a role in embryonic and hematopoietic stem cell differentiation.[52]
Zhao LJ, Zhu H (December 2004). "Structure and function of HIV-1 auxiliary regulatory protein Vpr: novel clues to drug design". Current Drug Targets. Immune, Endocrine and Metabolic Disorders. 4 (4): 265–75.
Sykes MC, Mowbray AL, Jo H (February 2007). "Reversible glutathiolation of caspase-3 by glutaredoxin as a novel redox signaling mechanism in tumor necrosis factor-alpha-induced cell death". Circulation Research. 100 (2): 152–4.
