Expression of cysteine proteinases and cystatins in parasites and use of cysteine proteinase inhibitors in parasitic diseases. Part III: Protozoa (1)

Out of five classes of proteases (cysteine, serine, threonine, aspartate and glutamate), cysteine proteases (CPs) are responsible for hydrolysis of peptide bonds essential in several biological activities. In protozoa, as with helminths, not only do CPs play the major role in nutrients digestion, but they also have several functions for parasite survival such as differentiation of life cycle stages, immunomodulation of host immune response, and autophagy. Most well-characterized CPs in protozoa that were investigated in the last two decades belong to papain-family enzymes (clan CA, family C1). The present review highlights, in general, several aspects of CPs functions in protozoal survival and different strategies utilized in development of potent CPIs. The review also includes detailed data regarding T. gondii CPs, and their inhibitors wether exogenous (CPIs) or endogenous cystatins (CYSs).