Within hours after the ingestion of a blood meal, the mosquito midgut epithelium synthesizes a chitinous sac, the peritrophic matrix. Plasmodium ookinetes traverse the peritrophic matrix while escaping the mosquito midgut.
Chitinases (EC 3.2.1.14) are critical for parasite invasion of the midgut: the presence of the
chitinase inhibitor,
allosamidin, in an infectious blood meal prevents oocyst development. A
chitinase gene, PgCHT1, recently has been identified in the
avian malaria parasite P. gallinaceum. We used the sequence of PgCHT1 to identify a P. falciparum
chitinase gene,
PfCHT1, in the P. falciparum genome database.
PfCHT1 differs from PgCHT1 in that the P. falciparum gene lacks
proenzyme and chitin-binding domains.
PfCHT1 was expressed as an active recombinant
enzyme in Escherichia coli.
PfCHT1 shares with PgCHT1 a substrate preference unique to Plasmodium
chitinases: the
enzymes cleave tri- and tetramers of GlcNAc from
penta- and hexameric oligomers and are unable to cleave smaller native
chitin oligosaccharides. The pH activity profile of
PfCHT1 and its IC(50) (40 nM) to
allosamidin are distinct from
endochitinase activities secreted by P. gallinaceum ookinetes. Homology modeling predicts that PgCHT1 has a novel pocket in the catalytic active site that
PfCHT1 lacks, which may explain the differential sensitivity of
PfCHT1 and PgCHT1 to
allosamidin.
PfCHT1 may be the ortholog of a second, as yet unidentified,
chitinase gene of P. gallinaceum. These results may allow us to develop novel strategies of blocking human
malaria transmission based on interfering with P. falciparum
chitinase.