Synthesis, enzymatic evaluation, and docking studies of fluorogenic caspase 8 tetrapeptide substrates

The synthesis, enzymatic evaluation, and molecular modeling studies of new fluorogenic tetrapeptide-based substrates selective for caspase 8, having the general structure Ac-IETD-AXX, are described. Various fluorescent reporter groups (AXX), i.e., 3- and 4-substituted coumarins and quinolin-2(1H)-ones were synthesized by von Pechmann condensation. They were subsequently coupled with the caspase-8-selective tetrapeptide Ac-IETD-OH under newly developed synthetic conditions to give the desired substrates in good yields and in high enantiomeric purity. Based on K(M) and V(max) values, the new compounds proved to be excellent substrates for recombinant human caspase 8. In contrast, the K(M) values for the same compounds as substrates for human caspase 3 were approximately 10-20-fold higher. Molecular modeling studies based on the X-ray crystal structures of both human caspases 3 and 8 revealed that there is sufficient room within both active sites to accommodate substrates with moderately bulky substituents in the 3- and 4-positions of the fluorogenic coumarins and quinolin-2(1H)-ones. Automated docking of the substrates into the active sites of both human caspases 3 and 8 with the program AutoDock 3 gave structures similar to the published crystallographic structures for the same tetrapeptide bound to caspase 8 in the form of an irreversible inhibitor. The calculated binding energies for the new substrates to either caspase 3 or 8 showed little difference between the substrates, consistent with the K(M) data. In addition, the calculated binding energies (DeltaG) to caspase 8 were considerably more negative than those to caspase 3, also consistent with the K(M) data. A possible molecular interaction that might explain the selectivity of the IETD tetrapeptide motif for caspase 8 over caspase 3 is discussed.