[Significant proteases of flukes--molecular characteristic and their biological functions]

Proteases (peptide hydrolases) aside from known catabolic functions and protein processing play numerous roles in many tasks imposed by a parasitic life cycle; this includes parasite immunoevasion, excystment/encystment, tissue penetration and digestion of host tissue for nutrition. They have been identified in biological systems from viruses to vertebrates. Proteases are divided into four major groups on the basis of the catalytic mechanism used during the hydrolytic process - serine, aspartic, cysteine and metalloproteinases; other 'undefined' or cryptic proteases may also exist. There are a number of excellent reviews covering general parasite proteinases but by far the largest number of papers in the literature report on enzymes belonging to the papain superfamily of cysteine proteinases. It turns out, however, that although many different proteases have been characterized in parasitic helminths many of them have yet to be discovered.