Background: The protozoan Giardia duodenalis, which causes giardiasis, is an intestinal
parasite that commonly affects humans, mainly pre-school children. Although there are asymptomatic
cases, the main clinical features are chronic and acute diarrhea, nausea, abdominal pain, and malabsorption
syndrome. Little is currently known about the virulence of the parasite, but some cases of
chronic gastrointestinal alterations post-infection have been reported even when the infection was
asymptomatic, suggesting that the cathepsin L proteases of the parasite may be involved in the damage
at the level of the gastrointestinal mucosa.
Objective: The aim of this study was the in silico identification and characterization of extracellular
cathepsin L proteases in the proteome of G. duodenalis.
Methods: The NP_001903 sequence of cathepsin L protease from Homo sapiens was searched against
the Giardia duodenalis proteome. The subcellular localization of Giardia duodenalis cathepsin L
proteases was performed in the DeepLoc-1.0 server. The construction of a phylogenetic tree of the
extracellular proteins was carried out using the Molecular Evolutionary Genetics Analysis software
(MEGA X). The Robetta server was used for the construction of the three-dimensional models. The
search for possible inhibitors of the extracellular cathepsin L proteases of Giardia duodenalis was
performed by entering the three-dimensional structures in the FINDSITEcomb drug discovery tool.
Results: Based on the amino acid sequence of cathepsin L from Homo sapiens, 8 protein sequences
were identified that have in their modular structure the Pept_C1A domain characteristic of cathepsins
and two of these proteins (XP_001704423 and XP_001704424) are located extracellularly. Threedimensional
models were designed for both extracellular proteins and several inhibitory ligands with a
score greater than 0.9 were identified. In vitro studies are required to corroborate if these two extracellular
proteins play a role in the virulence of Giardia duodenalis and to discover ligands that may be
useful as therapeutic targets that interfere in the mechanism of pathogenesis generated by the parasite.
Conclusion: In silico analysis identified two proteins in the Giardia duodenalis protein repertoire whose
characteristics allowed them to be classified as cathepsin L proteases, which may be secreted into the
extracellular medium to act as virulence factors. Three-dimensional models of both proteins allowed the
identification of inhibitory ligands with a high score. The results suggest that administration of those
compounds might be used to block the endopeptidase activity of the extracellular cathepsin L proteases,
interfering with the mechanisms of pathogenesis of the protozoan parasite Giardia duodenalis.