The causative agent of human
African trypanosomiasis, Trypanosoma brucei, lacks de novo
purine biosynthesis and depends on
purine salvage from the host. The
purine salvage pathway is redundant and contains two routes to guanosine-5'-monophosphate (GMP) formation: conversion from xanthosine-5'-monophosphate (
XMP) by
GMP synthase (
GMPS) or direct salvage of
guanine by
hypoxanthine-guanine phosphoribosyltransferase (
HGPRT). We show recombinant T. brucei GMPS efficiently catalyzes GMP formation. Genetic knockout of
GMPS in bloodstream parasites led to depletion of
guanine nucleotide pools and was lethal. Growth of
gmps null cells was only rescued by supraphysiological
guanine concentrations (100 μM) or by expression of an extrachromosomal copy of
GMPS.
Hypoxanthine was a competitive inhibitor of
guanine rescue, consistent with a common uptake/metabolic conversion mechanism. In mice,
gmps null parasites were unable to establish an
infection demonstrating that
GMPS is essential for virulence and that plasma
guanine is insufficient to support parasite
purine requirements. These data validate
GMPS as a potential therapeutic target for treatment of human
African trypanosomiasis. The ability to strategically inhibit key metabolic
enzymes in the
purine pathway unexpectedly bypasses its functional redundancy by exploiting both the nature of pathway flux and the limited nutrient environment of the parasite's extracellular niche.