The mycobacterial guaB1 gene encodes a guanosine 5'-monophosphate reductase with a cystathione-β-synthase domain

Purine metabolism plays a pivotal role in bacterial life cycle, however, regulation of the de novo and purine salvage pathways have not been extensively detailed in mycobacteria. By gene knockout, biochemical and structural analyses, we identified Mycobacterium smegmatis (Msm) and Mycobacterium tuberculosis (Mtb) guaB1 gene product as a novel type of guanosine 5'-monophosphate reductase (GMPR), which recycles guanosine monophosphate to inosine monophosphate within the purine salvage pathway and contains cystathione β-synthase (CBS) domains with atypical orientation in the octamer. CBS domains share a much larger interacting area with a conserved catalytic domain in comparison with the only known CBS containing protozoan GMPR and closely related inosine monophosphate dehydrogenase structures. Our results revealed essential effect of pH on allosteric regulation of Msm GMPR activity and oligomerization with adenine and guanosine nucleotides binding to CBS domains.Bioinformatic analysis indicated the presence of GMPRs containing CBS domains across the entire Actinobacteria phylum.

Inhibition of guanosine monophosphate synthetase (GMPS) blocks glutamine metabolism and prostate cancer growth in vitro and in vivo

Cancer cells increase their uptake of nutrients and metabolize them to provide the necessary building blocks for new cancer cells. Glutamine is a critical nutrient in cancer, however its contribution to purine metabolism in prostate cancer has not previously been determined. Guanosine monophosphate synthetase (GMPS) acts in the de novo purine biosynthesis pathway, utilizing a glutamine amide to synthesize the guanine nucleotide and replenish the purine pool in proliferative cancer cells. This study demonstrates that GMPS mRNA expression correlates with Gleason score in prostate cancer samples, while high GMPS expression was associated with decreased rates of overall and disease/progression-free survival. Pharmacological inhibition or knockdown of GMPS significantly decreased cell growth in both LNCaP and PC-3 prostate cancer cells. GMPS knockdown was rescued by addition of extracellular guanosine to the media, suggesting a direct effect on nucleotide synthesis. We utilized 15N-(amide)-glutamine and U-13C5-glutamine metabolomics to dissect the pathways involved, and intriguingly, despite similar growth inhibition by GMPS knockdown, we show unique metabolic effects across each cell line. PC-3 cells showed a build-up of purine precursors, as well as activation of purine salvage pathways highlighted by significant increases in guanine, adenosine, inosine and cytosine. Both cell lines exhibited increased levels of pyrimidines and prioritized TCA cycle in distinct ways to produce increased aspartate, another important purine precursor. Using a PC-3 xenograft mouse model, tumor growth was also significantly decreased after GMPS knockdown. These data further highlight the importance of glutamine metabolism for prostate cancer cell growth and provide support for GMPS as a new therapeutic target in prostate cancer.

The Role of Purinergic Signaling in Trichomonas vaginalis Infection

: Trichomoniasis, one of the most common non-viral sexually transmitted infections worldwide, is caused by the parasite Trichomonas vaginalis. The pathogen colonizes the human urogenital tract and the infection is associated with complications such as adverse pregnancy outcomes, cervical cancer, and an increase in HIV transmission. The mecha-nisms of pathogenicity are multifactorial, and controlling immune responses is essential for infection maintenance. Extra-cellular purine nucleotides are released by cells in physiological and pathological conditions, and they are hydrolyzed by enzymes called ecto-nucleotidases. The cellular effects of nucleotides and nucleosides occur via binding to purinoceptors, or throughthe uptake by nucleoside transporters. Altogether, enzymes, receptors and transporters constitute the purinergic signaling, a cellular network that regulates several effects in practically all systems including mammals, helminths, proto-zoa, bacteria, and fungi. In this context, this review updates the data on purinergic signaling involved in T. vaginalis biol-ogy and interaction with host cells, focusing on the characterization of ecto-nucleotidases and on purine salvage pathways. The implications of the final products, the nucleosides adenosine and guanosine, for human neutrophil response and vagi-nal epithelial cell damage reveal the purinergic signaling as a potential new mechanism for alternative drug targets.

In vitro effects of purine and pyrimidine analogues on Leishmania donovani and Leishmania infantum promastigotes and intracellular amastigotes

Inhibition of parasite metabolic pathways is a rationale for new chemotherapeutic strategies. The pyrimidine and purine salvage pathways are thus targets against

Purine salvage in the apicomplexan Sarcocystis neurona, and generation of hypoxanthine-xanthine-guanine phosphoribosyltransferase-deficient clones for positive-negative selection of transgenic parasites

SUMMARYSarcocystis neurona is an apicomplexan parasite that causes severe neurological disease in horses and marine mammals. The Apicomplexa are all obligate intracellular parasites that lack purine biosynthesis pathways and rely on the host cell for their purine requirements. Hypoxanthine-xanthine-guanine phosphoribosyltransferase (HXGPRT) and adenosine kinase (AK) are key enzymes that function in two complementary purine salvage pathways in apicomplexans. Bioinformatic searches of the S. neurona genome revealed genes encoding HXGPRT, AK and all of the major purine salvage enzymes except purine nucleoside phosphorylase. Wild-type S. neurona were able to grow in the presence of mycophenolic acid (MPA) but were inhibited by 6-thioxanthine (6-TX), suggesting that the pathways involving either HXGPRT or AK are functional in this parasite. Prior work with Toxoplasma gondii demonstrated the utility of HXGPRT as a positive-negative selection marker. To enable the use of HXGPRT in S. neurona, the SnHXGPRT gene sequence was determined and a gene-targeting plasmid was transfected into S. neurona. SnHXGPRT-deficient mutants were selected with 6-TX, and single-cell clones were obtained. These Sn∆HXG parasites were susceptible to MPA and could be complemented using the heterologous T. gondii HXGPRT gene. In summary, S. neurona possesses both purine salvage pathways described in apicomplexans, thus allowing the use of HXGPRT as a positive-negative drug selection marker in this parasite.

Inhibition and Structure of Toxoplasma gondii Purine Nucleoside Phosphorylase

ABSTRACT The intracellular pathogen Toxoplasma gondii is a purine auxotroph that relies on purine salvage for proliferation. We have optimized T. gondii purine nucleoside phosphorylase ( Tg PNP) stability and crystallized Tg PNP with phosphate and immucillin-H, a transition-state analogue that has high affinity for the enzyme. Immucillin-H bound to Tg PNP with a dissociation constant of 370 pM, the highest affinity of 11 immucillins selected to probe the catalytic site. The specificity for transition-state analogues indicated an early dissociative transition state for Tg PNP. Compared to Plasmodium falciparum PNP, large substituents surrounding the 5′-hydroxyl group of inhibitors demonstrate reduced capacity for Tg PNP inhibition. Catalytic discrimination against large 5′ groups is consistent with the inability of Tg PNP to catalyze the phosphorolysis of 5′-methylthioinosine to hypoxanthine. In contrast to mammalian PNP, the 2′-hydroxyl group is crucial for inhibitor binding in the catalytic site of Tg PNP. This first crystal structure of TgPNP describes the basis for discrimination against 5′-methylthioinosine and similarly 5′-hydroxy-substituted immucillins; structural differences reflect the unique adaptations of purine salvage pathways of Apicomplexa .

GuaA and GuaB Are Essential for Borrelia burgdorferi Survival in the Tick-Mouse Infection Cycle

ABSTRACT Pathogens lacking the enzymatic pathways for de novo purine biosynthesis are required to salvage purines and pyrimidines from the host environment for synthesis of DNA and RNA. Two key enzymes in purine salvage pathways are IMP dehydrogenase (GuaB) and GMP synthase (GuaA), encoded by the guaB and guaA genes, respectively. While these genes are typically found on the chromosome in most bacterial pathogens, the guaAB operon of B orrelia burgdorferi is present on plasmid cp26, which also harbors a number of genes critical for B. burgdorferi viability. Using molecular genetics and an experimental model of the tick-mouse infection cycle, we demonstrate that the enzymatic activities encoded by the guaAB operon are essential for B. burgdorferi mouse infectivity and provide a growth advantage to spirochetes in the tick. These data indicate that the GuaA and GuaB proteins are critical for the survival of B. burgdorferi in the infection cycle and highlight a potential difference in the requirements for purine salvage in the disparate mammalian and tick environments.

Purine Salvage Pathways among Borrelia Species

ABSTRACT Genome sequencing projects on two relapsing fever spirochetes, Borrelia hermsii and Borrelia turicatae, revealed differences in genes involved in purine metabolism and salvage compared to those in the Lyme disease spirochete Borrelia burgdorferi. The relapsing fever spirochetes contained six open reading frames that are absent from the B. burgdorferi genome. These genes included those for hypoxanthine-guanine phosphoribosyltransferase (hpt), adenylosuccinate synthase (purA), adenylosuccinate lyase (purB), auxiliary protein (nrdI), the ribonucleotide-diphosphate reductase alpha subunit (nrdE), and the ribonucleotide-diphosphate reductase beta subunit (nrdF). Southern blot assays with multiple Borrelia species and isolates confirmed the presence of these genes in the relapsing fever group of spirochetes but not in B. burgdorferi and related species. TaqMan real-time reverse transcription-PCR demonstrated that the chromosomal genes (hpt, purA, and purB) were transcribed in vitro and in mice. Phosphoribosyltransferase assays revealed that, in general, B. hermsii exhibited significantly higher activity than did the B. burgdorferi cell lysate, and enzymatic activity was observed with adenine, hypoxanthine, and guanine as substrates. B. burgdorferi showed low but detectable phosphoribosyltransferase activity with hypoxanthine even though the genome lacks a discernible ortholog to the hpt gene in the relapsing fever spirochetes. B. hermsii incorporated radiolabeled hypoxanthine into RNA and DNA to a much greater extent than did B. burgdorferi. This complete pathway for purine salvage in the relapsing fever spirochetes may contribute, in part, to these spirochetes achieving high cell densities in blood.