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Glutathione S-transferase (GST), an essential detoxification enzyme in parasitic
helminths, is a major vaccine target and an attractive drug target against
schistosomiasis and other helminthic diseases. Crystal structures of the 26 kDa
GST from the helminth Schistosoma japonica (SjGST) have been determined for the
unligated enzyme (resolution = 2.4 A, R-factor = 19.7%) and for the enzyme bound
to the leading antischistosomal drug praziquantel (resolution = 2.6 A, R-factor
= 21.2%). The protein, recombinantly expressed using the Pharamacia PGEX-3X
vector for production of GST fusion proteins, contains all 218 residues of SjGST
and an additional 13 residues at the C terminus. The structure of unligated
SjGST shows that the glutathione binding site pre-exists unchanged in the
ligand-free enzyme and is conserved between parasitic and the mammalian class mu
enzymes. At therapeutic concentrations the leading antischistosomal drug
praziquantel (PZQ) binds one drug per enzyme homodimer in the dimer interface
groove adjoining the two catalytic sites. This establishes a protein target for
PZQ, identifies the GST non-substrate ligand transport site, and implicates PZQ
in steric inhibition of SjGST catalytic and transport for large ligands. Thus,
increased expression or mutagenesis of SjGST by the parasite may confer
resistance to PZQ. Differences in the xenobiotic binding region between
parasitic and mammalian GSTs reveal a distinct substrate repertoire for SjGST
and, together with the newly identified PZQ binding site, provide the basis for
design of novel antischistosomal drugs. Due to the widespread use expression
systems based on SjGST fusions, the atomic structure of SjGST should also
provide an important tool for phasing fusion protein structures by molecular
replacement.
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