Conformational similarity in the activation of caspase-3 and -7 revealed by the unliganded and inhibited structures of caspase-7

Abstract

Caspase-mediated apoptosis has important roles in normal cell differentiation and aging and in many diseases including cancer, neuromuscular disorders and neurodegenerative diseases. Therefore, modulation of caspase activity and conformational states is of therapeutic importance. We report crystal structures of a new unliganded conformation of caspase-7 and the inhibited caspase-7 with the tetrapeptide Ac-YVAD-Cho. Different conformational states and mechanisms for substrate recognition have been proposed based on unliganded structures of the redundant apoptotic executioner caspase-3 and -7. The current study shows that the executioner caspase-3 and -7 have similar conformations for the unliganded active site as well as the inhibitor-bound active site. The new unliganded caspase-7 structure exhibits the tyrosine flipping mechanism in which the Tyr230 has rotated to block entry to the S2 binding site similar to the active site conformation of unliganded caspase-3. The inhibited structure of caspase-7/YVAD shows that the P4 Tyr binds the S4 region specific to polar residues at the expense of a main chain hydrogen bond between the P4 amide and carbonyl oxygen of caspase-7 Gln 276, which is similar to the caspase-3 complex. This new knowledge of the structures and conformational states of unliganded and inhibited caspases will be important for the design of drugs to modulate caspase activity and apoptosis.

Granulysin Delivered by Cytotoxic Cells Damages Endoplasmic Reticulum and Activates Caspase-7 in Target Cells

“…Caspase-7 is another executioner caspase that is closely related to caspase-3, with highly similar structure and function (34). We observed that NK cells expressing granulysin in the absence of granzyme B mediated enhanced activation of caspase-7, implicating it in granulysin-induced cell-mediated apoptosis.…”

A New Apo-Caspase-6 Crystal Form Reveals the Active Conformation of the Apoenzyme

“…It has been proposed that the presence of inhibitor or substrate triggers flipping of the L2′ loop. 34 In support of this hypothesis, apo-crystal structures of caspase-3 and caspase-7 obtained in the presence of inhibitor 35,36 showed that the L2′ loop folded against the L2 and L4 loops. This was in contrast to the zymogen-like conformation of the L2′ loop in the previously reported apocaspase-7 structure crystallized in the absence of inhibitor.…”

Studies of the molecular mechanism of caspase-8 activation by solution NMR

“…31 Obviously, the inhibitor-bound species of caspase-9 is different than for caspase-8, and therefore a comparison of the mechanisms is difficult on the basis of the presently available data. Rearrangements upon inhibitor binding in the executioner caspase-3 are limited to side chains of three hydrophobic residues lining the S2 pocket and no loop stabilization was observed 33,34 in contrast to the more extensive changes in capase-8. To what extent the unbound structure of caspase-3 differs from the zymogen cannot be assessed, as no structural information on the proform is available.…”

“…We therefore speculate that the increased overall stability of caspase-8 upon addition of the inhibitor is due to rigidification of the loops around the active site and possibly may also involve loop 4 and 5, which in procaspase-8 undergo conformational exchange. 22 Structures for inhibitor-free caspases have also been determined for caspase-3 33,34 and -9. 31 Caspase-9 belongs to the initiator caspases, and the proenzyme is monomeric, indicating that the mechanism of action might be highly similar to the one of caspase-8.…”