Inhibitor SourceBook™ Second Edition

Apoptosis
Caspase Inhibitors
Activation of caspases is one of the most widely
recognized features of apoptosis. Caspases are cysteinedependent,
aspartate-specific proteases. They exist as
latent precursors, which, when activated by limited
proteolysis, initiate the death program by destroying
key components of the cellular infrastructure and
activating factors that mediate damage to the cells.
The mechanism of caspase activation appears to be
conserved in evolution. Thus far, 14 members of the
caspase family have been identified, 11 of which are
present in humans. Caspases have been categorized into
upstream initiators and downstream executioners. A
distinctive feature of caspases is the absolute requirement
of an aspartic acid residue in the substrate P 1 position.
The P 4 residue is important in substrate recognition
and specificity. Generally, catalysis involves a cysteine
protease mechanism. The tetrapeptide corresponding to
the substrate P 4 - P 1 residues is sufficient to recognize
both caspase-1 and caspase-3. This also forms the basis of
designing novel inhibitors of caspases.
Caspase activation is generally considered as the
"point of no return” in apoptotic pathways. Caspases
are activated by two major pathways: the receptormediated
(Fas ligand or TNF-a-mediated) pathway and
Calbiochem • Inhibitor SourceBook
Apoptosis
the mitochondrial pathway. The receptor-mediated
pathway leads to the activation of pro-caspase-8. In the
mitochondrial pathway, pro-apoptotic members of the
Bcl-2 family associate with mitochondria and direct the
release of cytochrome c (Cyt c) and other proteins, which
activate pro-caspase-9.
Caspase inhibitors act by binding to the active site of
caspases either in a reversible or irreversible manner.
Inhibitor design includes a peptide recognition
sequence attached to a functional group such as
an aldehyde (CHO), chloromethylketone (CMK), or
fluoromethylketone (FMK). The peptide recognition
sequence corresponding to that found in endogenous
substrates determines the specificity of a particular
caspase. For example, compounds with the Ac-YVAD-
CHO sequence are potent inhibitors of caspases-1
(K i ≈ 10 nM), and exhibit very weak inhibitory effect on
caspases-3 and -7 (K i ≥ 50 mM). Exclusion of the tyrosine
residue from the inhibitor peptide results in a potent but
less specific inhibitor. For example, Z-VAD-FMK inhibits
not only caspases-1 and -4, but also caspases-3 and -7.
More online... www.calbiochem.com/inhibitors/caspase
Technical Support
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