CASPASES
Structure and Enzymatic Activity of Caspases
Caspases are expressed as inactive proenzymes (zymogens) and have to be
activated by cleavage at internal conserved Asp residues. The proenzymes
contain a N-terminal prodomain of various size, which in the case of
Caspase-8 and Caspase-10 contains the DEDs (Death Effector Domains) that
mediate the interaction with FADD/MORT1. This N-terminal prodomain is
separated from the central large caspase-subunit (about 20 kDa: so called
p20 subunit) by one or two Asp cleavage site(s). The large caspase-subunit
itself is separated from the C-terminal small subunit (about 10 kDa: so
called p10 subunit) by one Asp cleavage site or a linker peptide (two Asp
cleavage sites).
Caspases can activate themselves by autoproteolysis, or can be processed
by other active caspases. At least in the case of Caspase-1, Caspase-8 and
probably Caspase-10, the large prodomains are involved in the activation
process of the caspases, while the function of the other prodomains is not
yet known. The proteolytic cleavage leads to the formation of the active
caspases which consist of the p10 and p20 subunits. The X-ray structure of
Caspase-1 and Caspase-3 in complex with specific tetrapeptide inhibitors
was elucidated. Their three-dimensional structures are comparable: the
active proteins are tetramers of two p20 subunits surrounding two adjacent
p10 subunits. Both, the p20 and p10 subunits are essential for catalytic
activity. The active site pentapeptide QAC(R,G,Q)G is in the p20 subunit
and forms the primary recognition pocket (S1) for the Asp residue, but
several residues in the p10 and p20 subunits contribute to the specific
binding of the substrate (tetrapeptide inhibitor) and form the recognition
pockets S2-S4. The central cystein residue in the active site pentapeptide
and a histidine residue form a catalytic diad, i.e. they directly contribute
to the formation of the tetrahedral intermediate state in the hydrolytical
cleaving mechanism of a substrate peptide by the caspases. While the
active site pentapeptide (S1) is common to all caspases (they all are
specific to Asp residue in the P1 position), the caspases differ in
their recognition sites S2-S4 what explains their selectivity towards
different cleavage motifs (= different residues in P2-P4 positions)
and by this their substrate-specificity.