Sequence and structure similarities of cathepsin B from the parasite Schistosoma mansoni and human liver

An update on proteases and protease inhibitors from trematodes

Trematodes, a class of parasitic flatworms, are responsible for a variety of devastating diseases in humans and animals, with schistosomiasis and fascioliasis being prominent examples. Trematode proteolytic systems involved in the host-parasite interaction have emerged as key contributors to the success of trematodes in establishing and maintaining infections. This review concentrates on diverse proteases and protease inhibitors employed by trematodes and provides an update on recent advances in their molecular-level characterization, with a focus on function, structure, and therapeutic target potential.

Role of the tegument and gut in nutrient uptake by parasitic platyhelminths

The ease of procuring nutrient is probably the main selection pressure that drives and maintains the host–parasite relationship. The feeding activities of the ectoparasitic monogeneans exhibit similarities with the predatory turbellarians, with certain monopisthocotylean members feeding by means of a protrusible pharynx. These parasites degrade fish skin by secreting enzymes extracorporeally, but most of the digestion is carried out intracellularly in cells lining a well-differentiated gut. Some polyopisthocotylean monogeneans, however, living within the vascularized gill chamber, took advantage of the availability of a more highly nutritious, consistent, and renewable diet in the form of blood, and this represented a major step in the evolution of endoparasitism. Blood provides a rich source of carbohydrates for the production of energy and amino acids and fatty acids for the synthesis of parasite molecules and for egg production. The external surfaces of all parasitic flatworms depart from turbellarian character and are composed of a multifunctional syncytial tegument that is permeable to a variety of small organic solutes. Glucose and amino acid transporter molecules situated in the tegumental surface and basal membranes of trematodes and cestodes function in the uptake of these molecules and their distribution to the parasite tissues. Cestodes are bereft of any vestige of a gut, but their tegument has become elaborated into a highly efficient digestive–absorptive layer that competes with the vertebrate mucosa for nutrients. The patterns of energy metabolism in adult flatworm parasites are generally anaerobic and based on glycogen, with abbreviated metabolic pathways and the loss of biosynthetic capacities. In contrast to the tegument, the role of the gut is to digest host macromolecules and subsequently absorb the soluble products. However, the switch to blood as the major source of nutrient necessitated development of a means of overcoming the problems of blood clotting, attack by immune effector mechanisms, and the intracellular accumulations of haematin pigment. Digenean trematode, in contrast to monogeneans, digest blood extracellularly and their secretions include molecules capable of lysing erythrocytes and preventing blood clotting. Digestion of the ingested proteins is generally rapid, involving a range of cathepsin-like cysteine and aspartic proteases, which reduce the blood meal to absorbable peptides that are most likely further catabolized to amino acids by intracellular aminopeptidases. The parasites dispose of accumulated haematin by simply emptying the contents of their blind-ended gut.

SmCB2, a novel tegumental cathepsin B from adult Schistosoma mansoni

Papain-like cysteine endopeptidases have been recognized as potential targets for chemotherapy and serodiagnostic reagents in infections with the human parasitic helminth Schistosoma. A novel cathepsin B endopeptidase from adult S. mansoni has been isolated and characterized. The enzyme is termed SmCB2 to distinguish it from the first recorded schistosome cathepsin B, SmCB1, also known as Sm31. A rapid and convenient protocol involving anion exchange and affinity chromatography is described for the isolation of SmCB1 and SmCB2 from the same parasite starting material. SmCB2 has been functionally expressed in and purified from Pichia pastoris. Both native and recombinant SmCB2 migrate similarly (33 kDa) by SDS-PAGE. Both display strict acidic pH activity profiles and similar K(m) and k(cat) for dipeptidyl amidomethylcoumarin substrates. We conclude that the recombinant enzyme is properly folded. The S(2) subsite specificity of recombinant SmCB2 exhibits the preferences Phe>Leu>Val>>Arg. By immunoblotting with anti-SmCB2 IgG, a 33 kDa protein was identified in soluble extracts of male schistosomes. By immunohistochemistry, SmCB2 was localized in the tegumental tubercles and parenchyma of males with less product being visualized in the parenchyma of females. The enzyme may be lysosomal and function at the host parasite-interface.

Proteinases and associated genes of parasitic helminths

Many parasites have deployed proteinases to accomplish some of the tasks imposed by a parasitic life style, including tissue penetration, digestion of host tissue for nutrition and evasion of host immune responses. Information on proteinases from trematodes, cestodes and nematode parasites is reviewed, concentrating on those worms of major medical and economical importance. Their biochemical characterization is discussed, along with their putative biological roles and, where available, their associated genes. For example, proteinases expressed by the various stages of the schistosome life-cycle, in particular the well-characterized cercarial elastase which is involved in the penetration of the host skin and the variety of proteinases, such as cathepsin B (Sm31), cathepsin L1, cathepsin L2, cathepsin D, cathepsin C and legumain (Sm32), which are believed to be involved in the catabolism of host haemoglobin. The various endo- and exoproteinases of Fasciola hepatica, the causative agent of liver fluke disease, are reviewed, and recent reports of how these enzymes have been successfully employed in cocktail vaccines are discussed. The various proteinases of cestodes and of the diverse superfamilies of parasitic nematodes are detailed, with special attention being given to those parasites for which most is known, including species of Taenia, Echinococcus, Spirometra, Necator, Acylostoma and Haemonchus. By far the largest number of papers in the literature and entries to the sequence data bases dealing with proteinases of parasitic helminths report on enzymes belonging to the papain superfamily of cysteine proteinases. Accordingly, the final section of the review is devoted to a phylogenetic analysis of this superfamily using over 150 published sequences. This analysis shows that the papain superfamily can be divided into two major branches. Branch A contains the cathepin Bs, the cathepsin Cs and a novel family termed cathepsin Xs, while Branch B contains the cruzipains, cathepsin Ls, papain-like and aleurain/cathepsin H-like proteinases. The relationships of the helminth proteinases, and similar proteinases from protozoan parasites and other organisms, within these groups are discussed.

Proteolytic degradation of host hemoglobin by schistosomes

Schistosomes acquire amino acids for growth, development, and reproduction by catabolizing hemoglobin obtained from ingested host erythrocytes. While the biochemical pathway(s) involved has not been determined definitively, a number of proteases including schistosome legumain and cathepsin L-, D-, B- and C-like enzymes have been ascribed roles in the degradation of hemoglobin to diffusible peptides. Transcripts encoding these schistosome proteases, which appear to be expressed in the gastrodermis and cecum of the schistosome, have been reported. Because these enzymes are candidate targets at which to direct novel anti-schistosomal therapies, the comparative biochemistry of these and their counterpart mammalian proteases is now the focus of research in a number of laboratories. This paper reviews reports dating from 40 years ago to the present on how schistosomes digest host-derived hemoglobin, and interprets apparent anomalies in some earlier compared to later reports, the latter having benefited from the availability of PCR and gene cloning technologies. More specifically, the review concentrates on five proteolytic enzymes, and their associated genes, which have been ascribed key roles in the pathway of hemoglobin degradation.

Structural analysis of the catalytic site of AcCP-1, a cysteine proteinase secreted by the hookworm Ancylostoma caninum

Previously, we have reported that hookworms secrete cysteine proteinase activity that is capable of cleaving the cathepsin L-specific substrate Z-Phe-Arg-AMC. We have also reported the gene sequences of novel cathepsin B-like proteinases from hookworms, but have been unable to locate cathepsin L-like genes that could account for the presence of the cathepsin L activity in these parasites. Here we present an homology model for the secreted hookworm cysteine proteinase AcCP-1 based upon the crystal structure co-ordinates of human cathepsin B. The model predicts that substrate binding and specificity differs between AcCP-1 and cathepsin B, and demonstrates that AcCP-1 would preferentially cleave Phe-Arg over Arg-Arg. This thereby provides an explanation for our previous observations that the hookworm proteinase, while structurally cathepsin B-like, displays a cathepsin L-like substrate specificity.