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Trypanosoma cruzi, a protozoan parasite, is the etiologic agent of American
trypanosomiasis or Chagas' disease. Chagas' disease afflicts more than 24
million individuals in South and Central America producing a debilitating
life-long disease. It is the leading cause of heart failure in many Latin
American countries. Currently, there is no satisfactory treatment for this
parasitic infection. Cruzain (also known as cruzipain, gp 57/51), the major
cysteine protease present in T. cruzi, is critical for the development and
survival of the parasite within the host cells, making this enzyme a target for
potential trypanocidal drugs. Here we report the X-ray crystal structure of
cruzain complexed with the potent inhibitor Z-Phe-Ala-fluoromethyl ketone. The
structure was determined at 2.35 A (Rcryst = 0.15) by molecular replacement
using a modified papain as the search model. The refined structure is compared
to papain. Features which distinguish cruzain from papain are discussed since
they may aid in the design of specificity inhibitors. Fluorescence microscopy
shows that a biotinylated form of the bound inhibitor does not effectively reach
host proteases in their lysosomal compartment, but is selectively taken up by
the parasite. The inhibitor greatly reduces parasitemia in a cell culture
system, without adverse effects to mammalian cells. This biological selectivity
can be exploited, in conjunction with unique active site features revealed by
the crystal structure, to develop chemotherapy for Chagas' disease.
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