(650.11) Conformational Plasticity of initiator caspases

Poster Board Number: A121

Mithun Nag Karadi Giridhar (University of Texas at Arlington )

Caspases are an important family of enzymes that are key players in the Apoptotic pathway. In the extrinsic apoptotic pathway, Initiator Caspases: 8, 10 and cFLIP are monomers which need an activation platform to dimerize and subsequent cleavage to produce the active enzyme; Executioner caspases however exist as dimers under physiological conditions and have to be processed at specific regions for them to be activated.  We have studied the equilibrium unfolding of inactive (active site mutant) monomeric initiator caspase 8 by urea denaturation studies and obtained thermodynamic parameters.  Initiator caspases have a total free energy of 7kcal/mole whereas executioner caspases have a total free energy of 25.8kcal/mole, dimerization contributes to 18.8kcal/mole and the monomeric step contributes to 7kcal/mole which is comparable to the initiators. These results indicate a conservation of the monomeric fold in all caspases with dimers having mutations at sites that modulate networks to stabilize the dimeric fold. The free energy decreases as the pH decreases below pH 7 and rises above pH 8.5 which corresponds to titration of Histidine and Cysteine residues that are important for stabilizing the tertiary structure. Initiator caspases play an important role in a variety of other pathways like necroptosis, cell differentiation and motility which are less well understood compared to its role in apoptosis.  These studies are vital to understand allosteric regulation and the conformational plasticity of initiator caspases which could help us design drugs to divert these enzymes in desired pathways to effect the fate of a cell.

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