Nematodes represent a diverse phylum of both free living and parasitic species. While the species Caenorhabditis elegans is a valuable model organism, parasitic nematodes or helminths pose a serious threat to human health. Indeed, helminths cause many
neglected tropical diseases that afflict humans. Nematode
glycoconjugates have been implicated in evasive
immunomodulation, a hallmark of
nematode infections. One
monosaccharide residue present in the
glycoconjugates of several human pathogens is galactofuranose (Galf). This five-membered ring isomer of
galactose has not been detected in mammals, making Galf metabolic
enzymes attractive therapeutic targets. The only known pathway for biosynthetic incorporation of Galf into
glycoconjugates depends upon generation of the glycosyl donor
UDP-Galf by the flavoenzyme
uridine 5'-diphosphate (
UDP) galactopyranose mutase (UGM or Glf). A putative UGM encoding gene (glf-1) was recently identified in C. elegans. We sought to assess the catalytic activity of the corresponding gene product (CeUGM). CeUGM catalyzes the isomerization of
UDP-Galf and
UDP-
galactopyranose (
UDP-Galp). In the presence of
enzyme, substrate, and a hydride source, a galactose-N5-FAD adduct was isolated, suggesting the CeUGM
flavin adenine dinucleotide (
FAD) cofactor serves as a nucleophile in covalent catalysis. Homology modeling and
protein variants indicate that CeUGM possesses an active site similar to that of prokaryotic
enzymes, despite the low sequence identity (∼15%) between eukaryotic and prokaryotic UGM
proteins. Even with the primary sequence differences, heterocyclic UGM inhibitors developed against prokaryotic
proteins also inhibit CeUGM activity. We postulate that inhibitors of CeUGM can serve as chemical probes of Galf in nematodes and as
anthelmintic leads. The available data suggest that CeUGM facilitates the biosynthetic incorporation of Galf into nematode
glycoconjugates through generation of the glycosyl donor
UDP-Galf.