Regulation of carbohydrate metabolism during Giardia encystment.
Giardia intestinalis trophozoites encyst when they are exposed to bile. During encystment, events related to the inducible synthesis of a novel N-acetyl-D-galactosamine (GalNAc) homopolymer, occur. Within the first 6 h of encystment, mRNA for glucosamine 6-P isomerase (GPI), the first inducible enzyme unique to this pathway appears, oxygen uptake rates double from non-encysting levels, and metronidazole (MTZ) inhibits oxygen uptake. Within 12 h, GPI and its activity are detectable and OU decreases 50% from non-encysting levels; glucose's stimulation and MTZ's inhibition of oxygen uptake cease. In contrast, aspartate uptake remained constant throughout the 40 h monitored. Two genes, gpi 1 and 2 encode for GPI, but only gpi1 is expressed during encystment. Glucosamine 6-P (GlcN6P), the synthetic product of GPI, activates UDP-N-acetylglucosamine (UDP-GlcNAc) pyrophosphorylase, a downstream enzyme, 3 to 5-fold in the direction of UDP-GlcNAc synthesis. UDP-GlcNAc is epimerized to UDP-GalNAc and UDP-GalNAc is polymerized by "cyst wall synthase" (beta 1 --> 3 GalNAc transferase) into a highly insoluble beta 1,3-linked homopolymer. This GalNAc polysaccharide, the major component of cyst wall filaments, forms, in conjunction with polypeptides, the outer cyst wall of Giardia.
Department of Biology, Northeastern University, Boston, Massachusetts 02115, USA. ejarroll@lynx.neu.edu
E L
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Jarroll
P T
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Macechko
P A
PA
Steimle
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D
Bulik
C D
CD
Karr
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H
van Keulen
T A
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Paget
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Gerwig
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Kamerling
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Vliegenthart
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Erlandsen
D000116:Acetylgalactosamine / Q000235:genetics / Q000378:metabolism*; D019747:Aldose-Ketose Isomerases / Q000235:genetics* / Q000378:metabolism; D000818:Animals; D002236:Carbohydrate Conformation; D002240:Carbohydrate Sequence; D015971:Gene Expression Regulation, Enzymologic; D017125:Genes, Protozoan; D016829:Giardia lamblia / Q000235:genetics / Q000254:growth & development* / Q000378:metabolism; D005873:Giardiasis / Q000469:parasitology; D006801:Humans; D008969:Molecular Sequence Data; D011134:Polysaccharides / Q000378:metabolism*
D011134:Polysaccharides; C020582:glucosamine-6-phosphate isomerase; D019747:Aldose-Ketose Isomerases; D000116:Acetylgalactosamine