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Caspases have similar structure of active site. Exosite that could be utilized to improve activity has been found in caspase-7 (Boucher, Blais et al. 2012). Caspase-7 also has an inhibitory allosteric site that could bind with small molecule FICA, presenting a zymogen-like conformation (Hardy, Lam et al. 2004).
Although there is no evident exosite found in caspase-3, some allosteric sites, (most of which are located on the dimer interface,) has been studied by mutagenesis. Some of mutant residues can modulate the activity of caspase-3 or even procaspase-3. The procaspase-3 was detected only little activity because the orientation of ILA (prematured L2 loop) and ILB loop cannot form an active site pocket (Bose, Pop et al. 2003).
V266E is a mutation that improves caspase-3 activity dramatically. Even in the uncleavable procaspase-3 (D5A, D26A, D175A), V266E mutant zymogen is also pseudo-activated (60-fold activity). Interestingly, V266E does not change a lot conformation around active site in the active caspase-3. Based on the crystal structure, L2’ loop is partially disorder at 185’-180’. In the homology model of zymogen caspase-3, the catalytic C163 is buried in the dimer interface. E266, which is longer than V266, could push the ILB to the exposed position to make catalytic part is similar as active caspase-3. This active procaspase-3 cannot be inhibited by endogenous XIAP like normal cleaved caspase-3. So it provides us an option for apoptosis stimuli with intrinsic efficiency.
It was found recently that many other mutant residues on the dimer interface might play an important role on inhibition of caspase-3 through manipulating the hydrogen bond or remote talking across whole dimer. For instance, V266H inhibit caspase-3 activity totally, but the double mutation Y197C/V266H the activity is rescued. Structural evidence demonstrates that H266 clashes with Y197, which prevent R164 insert in the dimer interface, which is indispensable to form correct L2 conformation and catalytic loop bundle. Y197C mutation relief this clash.
