Cysteine proteases of parasites: A remarkable diversity of function

Summary Papain family cysteine proteases function primarily intracellularly in higher eukaryotes, but are often extracellular proteases in protozoan helminths. The utility of this class of enzymes is reflected in the diversity of functions they perform in both parasite life cycles and the pathogenesis of parasitic diseases. Examples include secretion of proteases into the gut of parasitic worms for hemoglobin degradation, release from the surface of nematodes to degrade cuticular proteins during molting, and facilitating excystment of protozoa.