The bioinorganic chemistry of apoptosis: potential inhibitory zinc binding sites in caspase-3.

Zn(2+) inhibits the action of several of the caspases and thus may act as a regulator of apoptosis. Reversal of this inhibition is one possible approach for the development of apoptosis-based therapies. Few studies describe the molecular details of the Zn(2+)-caspase interaction, the understanding of which is essential for the success of any therapeutic strategies. Enzyme kinetics and biophysical studies have shown that the inhibition is of mixed type with prominent (ca. 60 % of inhibition) uncompetitive characteristics and an IC50 of 0.8 µM under the conditions used. Fluorescence-based techniques confirmed that, during inhibition in the sub-micromolar range, substrate binding remains unaffected. A new zinc binding site composed of the catalytic histidine and a nearby methionine residue, rather than the catalytic histidine and cysteine dyad, is proposed based on the experimental observations. DFT models were used to demonstrate that the proposed site could be the preferred inhibitory zinc binding site.