The enigmatic asparaginyl endopeptidase of helminth parasites

Cell Press
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Journal Article
Trends In Parasitology, 2009, 25 (2), pp. 59 - 61
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The clan CD cysteine protease of Schistosoma mansoni (SmAE, also known as Sm32 or schistosome legumain) is an asparaginyl endopeptidase that cleaves C-terminal to asparaginyl (Asn) residues. The enzyme was considered to function in the hydrolytic degradation of host haemoglobin within the parasite gut 1 B. Götz and M.Q. Klinkert, Expression and partial characterization of a cathepsin B-like enzyme (Sm31) and a proposed `haemoglobinase (Sm32) from Schistosoma mansoni, Biochem. J. 290 (1993), pp. 801806. View Record in Scopus | Cited By in Scopus (26)[1]. However, Dalton and Brindley [2] proposed that the primary role of the enzyme was the trans-processing and activation of other schistosome proteases after noting that zymogens of clan AA aspartic proteases (cathepsin D) and clan CA cysteine proteases (cathepsins L, F, B1 and C) each possessed an asparaginyl-endopeptidase-cleavage site at the juncture between the prosegment and mature enzyme domain. Prosegment removal by trans-processing exposes the active site of the mature enzyme to enable entry of haemoglobin substrate. Cytochemistry revealed that schistosome asparaginyl endopeptidase is expressed in the gastrodermis surrounding the gut lumen, the same locality as the haemoglobin-degrading enzymes. Sajid et al. [3] provided support for this trans-processing hypothesis by showing that a recombinant S. mansoni asparaginyl endopeptidase could convert the zymogen of schistosome cathepsin B1 to a mature enzyme in vitro. Delcroix et al. [4] also showed that 20% of cathepsin B activity was lost when the expression of asparaginyl endopeptidase of three-week-old male and female schistosomes was knocked down using RNA interference (RNAi).
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