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| [[Image:Apoptosiscascade.jpg]] | | [[Image:Apoptosiscascade.jpg]] |
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| Caspase 8
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| Overview
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| Caspase 8 is a member of the caspase family, a family of cysteine proteases that play an important role in inflammation and apoptosis or programmed cell death. The caspases are essential for apoptosis in cells during development and during later stages of life. Failure of apoptosis can lead to tumor formation and the development of autoimmune diseases. In addition, excess apoptosis has been implicated in various disease states, including ischemia and Alzheimer’s. Caspase 8 is just one of 11 caspases that have been indentified in humans. Caspase 8 is an “initiator caspase” which cleave inactive pro-forms of the effectors caspases and subsequently activating them. Caspases exist as inactive proenzymes that are composed of a prodomain, and a large and small protease subunit. The activation of caspase requires proteolysis at an internal aspartic residue which results in the generation of a heterodimeric enzyme with a large and small subunit.
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| Caspase 8 is induced by tumor necrosis (TNF)-related apoptosis-inducing ligand (TRAIL). TRAIL induces apoptosis via death receptors (DR4 and DR5). After ligand binding the death receptor Fas recruits the adaptor protein FADD. FADD then binds and activates procaspase-8. Upon activation, Caspase 8 is then able activate caspase 3 and other downstream effectors. The end result is apoptosis.
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| Structure
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| Caspase 8 is a 58 kilodalton protein that shares similarities with other members of the caspase family. The protein is composed of two subunits, referred to as p18 and p11. These two subunits form a heterodimer. The protein has a α/ß folding motif that has a central six stranded beta sheet. Five of the strands are parallel and one is anti-parallel. The antiparallel strand is on the edge of the ß sheet. There are also six alpha helices in the protein structure. Three of these alpha helices are located on one side of the ß sheet and the other two on the other side. There is a turn of helix (α1’) which is part of a large loop (loop 1). This is along the binding pocket region of the p18 subunit. There is a two-stranded antiparallel ß sheet found at the top of the main ß sheet which forms the base of the binding pocket.
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| The active site
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| The catalytic triad in caspase 8 comprises Cys360, His317, and Arg258. The carboxyl group of P1 aspartate forms a salt bridge with Arg413 and Arg260. It also forms hydrogen bonds to Gln358. The P1 α-carbonyl group rotates and the oxygen atom rehybridizes to become a hydroxyl group. This then forms a hydrogen bond with the imidazole group of His317. A clear interaction exists between the carbonyl oxygen of Arg258 and the Nє of His317. The S2 pocket Cγ atom of the Threonine sidechain of P2 lies in a hydrophobic pocket that is formed from the sidechains of Val410 and Tyr412. The Oγ is surrounded by water molecules. The S3 pocket consists of a glutamate at P3 that sits in a cleft made by Arg413, Arg258, and Pro415, and Asn261. Arg413 forms a salt bridge to P3 glutamate and P1 aspartate sidechains. Arg413 also forms hydrogen bonds between its main chain atoms and the mainchain if the peptide inhibitor. The S4 pocket is especially important in selectivity. The acetyl of the inhibitor hydrogen bonds to the carboxyl group of P3 glutamate through a hydrogen bond. The hydrogen bond participants then move away to accommodate a non-polar residue. The faces of two aromatic residues, Trp420 and Tyr412 help form part of the hydrophobic S4 pocket.
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| Heterotetramer
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| Two p18-p11 heterodimers form a tetramer. This extends the six strands of the ß sheet to 12 strands. In the heterotetramer, the first segment of the P11 subunit and the last segment of the p18 subunit extend from the structure in an anti-parallel fashion and interact with the other heterodimer. Residues Lys367-Asp374 of one heterodimer extends into the other heterodimer and interact with residues Thr390 and Asp395.
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Revision as of 02:18, 24 April 2009
University of Vermont College of Medicine
M.D. 2009
CRYSTAL STRUCTURE OF THE COMPLEX OF CASPASE-8 WITH THE TETRAPEPTIDE INHIBITOR ACE-IETD-ALDEHYDECRYSTAL STRUCTURE OF THE COMPLEX OF CASPASE-8 WITH THE TETRAPEPTIDE INHIBITOR ACE-IETD-ALDEHYDE
File:Apoptosiscascade.jpg
File:Caspase8cascade.gif
ReferencesReferences
