The name caspase is derived from the specificity of these cysteine proteases to cleave their substrates after an aspartic acid. All caspases are synthesized as inactive zymogens called procaspases. At the onset of apoptosis, caspases undergo intramolecular cleavage and often this is followed by a second cleavage to remove the prodomain from the protease domain. The caspases form two primary groups, initiator caspases that include caspase-2, -8, -9, and -10, and effector caspases. Initiator caspases are the proximal death-inducing caspases that are activated in response to apoptotic stimuli; their primary function is to activate downstream effector caspases by catalyzing a single proteolytic cleavage. Activation of an effector caspase zymogen involves a specific intrachain cleavage, which is mediated by a specific initiator caspase. As a consequence of the intrachain cleavage, the catalytic activity of the effector caspase is enhanced by several orders of magnitude, thus magnifying the cell death inducing effect. Classically, PCD is induced by either extrinsic or intrinsic pathways.
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