Abstract
The metabolic pathway of purine nucleotides in parasitic protozoa is a potent drug target for treatment of parasitemia. Guanosine 5’-monophosphate reductase (GMPR), which catalyzes the deamination of guanosine 5’-monophosphate (GMP) to inosine 5’-monophosphate (IMP), plays an important role in the interconversion of purine nucleotides to maintain the intracellular balance of their concentration. However, only a few studies on protozoan GMPR have been reported at present. Herein, we identified the GMPR in Trypanosoma brucei, a causative protozoan parasite of African trypanosomiasis, and found that the GMPR proteins were consistently localized to glycosomes in T. brucei bloodstream forms. We characterized its recombinant protein to investigate the enzymatic differences between GMPRs of T. brucei and its host animals. T. brucei GMPR was distinct in having an insertion of a tandem repeat of the cystathionine β-synthase (CBS) domain, which was absent in mammalian and bacterial GMPRs. The recombinant protein of T. brucei GMPR catalyzed the conversion of GMP to IMP in the presence of NADPH, and showed apparent affinities for both GMP and NADPH different from those of its mammalian counterparts. Interestingly, the addition of monovalent cations such as K+ and NH4+ to the enzymatic reaction increased the GMPR activity of T. brucei, whereas none of the mammalian GMPR’s was affected by these cations. The monophosphate form of the purine nucleoside analog ribavirin inhibited T. brucei GMPR activity, though mammalian GMPRs showed no or only a little inhibition by it. These results suggest that the mechanism of the GMPR reaction in T. brucei is distinct from that in the host organisms. Finally, we demonstrated the inhibitory effect of ribavirin on the proliferation of trypanosomes in a dose-dependent manner, suggesting the availability of ribavirin to develop a new therapeutic agent against African trypanosomiasis.
Highlights
Purine nucleotides are synthesized de novo from their precursors such as amino acids and ribose 5-phosphate, and are produced from purine bases and ribose 5-phosphate through a salvage pathway
The parasitic protozoa Trypanosoma brucei is the causative reagent of African trypanosomiasis, and is infective to various mammalian species
It is known that Guanosine 5’-monophosphate reductase (GMPR) shows high similarities in amino acid sequence and structure to inosine 5’-monophosphate dehydrogenase (IMPDH), the enzyme catalyzing the NAD+-dependent oxidation of IMP to xanthosine 5’-monophosphate (XMP); GMPR and IMPDH are generally distinguished by the cystathionine β-synthase (CBS) domain, which is well conserved in IMPDHs but absent in GMPRs [1]
Summary
Purine nucleotides are synthesized de novo from their precursors such as amino acids and ribose 5-phosphate, and are produced from purine bases and ribose 5-phosphate through a salvage pathway. GMPR has been identified in various species from bacteria to mammals including parasitic protozoa [2], and has been structurally characterized by X-ray crystallography, which indicated that GMPR belongs to the family of (β/α) barrel proteins known as TIM barrel proteins. It is known that GMPR shows high similarities in amino acid sequence and structure to inosine 5’-monophosphate dehydrogenase (IMPDH), the enzyme catalyzing the NAD+-dependent oxidation of IMP to xanthosine 5’-monophosphate (XMP); GMPR and IMPDH are generally distinguished by the cystathionine β-synthase (CBS) domain, which is well conserved in IMPDHs but absent in GMPRs [1]. Detailed studies on GMPRs have been performed only on human and bacterial enzymes, and so the GMPRs in other organisms including protozoa are still poorly defined
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
