Regulating cysteine protease activity: essential role of protease inhibitors as guardians and regulators.

Cysteine proteases are widespread in nature. Their implication in numerous vital processes and pathologies make them highly attractive targets for drug design. The proper functioning and regulation of activity of cysteine proteases is a delicate balance of many factors, one of the most crucial being the protease inhibitors. In this review the basic principles of physiological protease inhibition by protein inhibitors are discussed with the focus on papain-like lysosomal cysteine proteases and the caspases, and their inhibitors.