Human tuberculosis is a chronic infectious disease affecting millions of lives. Because of emerging resistance to current medications, new therapeutic drugs are needed. One potential new target is hypoxanthine-guanine phosphoribosyltransferase (MtHGPRT), a key enzyme of the purine salvage pathway. Here, newly synthesized acyclic nucleoside phosphonates (ANPs) have been shown to be competitive inhibitors of MtHGPRT with Ki values as low as 0.69 μM. Prodrugs of these compounds arrest the growth of a virulent strain of M. tuberculosis with MIC50 values as low as 4.5 μM and possess low cytotoxicity in mammalian cells (CC50 values as high as >300 μM). In addition, the first crystal structures of MtHGPRT (2.03-2.76 Å resolution) have been determined, three of these in complex with novel ANPs and one with GMP and pyrophosphate. These data provide a solid foundation for the further development of ANPs as selective inhibitors of MtHGPRT and as antituberculosis agents.
Escherichia coli (Ec) cells possess two purine salvage enzymes: xanthine-guanine phosphoribosyltransferase (XGPRT) and hypoxanthine phosphoribosyltransferase (HPRT). EcXGPRT shares a common structural feature with other members of this family, a flexible loop that closes over the active site during catalysis. The replacement of six of these amino acids by alanine has no effect on the Km for the two substrates. However, the Ki for the nucleoside monophosphate increases by 27-fold, and the kcat is reduced by ∼200-fold. Nucleoside phosphonates (NP) are good inhibitors of EcXGPRT and EcHPRT, with Ki values as low as 10 nM. In the absence of the flexible loop, these values increase by 5- to 30-fold, indicating the importance of the loop for high-affinity inhibition. Crystal structures of two NPs in complex with EcXGPRT explain the tight binding. Prodrugs of NPs with low Ki values for EcXGPRT or EcHPRT exhibit IC50 values between 5 and 23 μM against Mycobacterium tuberculosis in cell-based assays, suggesting that these compounds are therapeutic leads against pathogenic bacteria.
Drug resistant uropathogenic E. coli associated with urinary tract infections (UTIs) are a serious and debilitating health threat. Therefore new drug targets to treat this disease need to be explored. One possible approach is to block the synthesis of the nucleoside monophosphates required for DNA/RNA production in these pathogens. In E. coli, the purine salvage pathway has two 6‐oxopurine phosphoribosyltransferases (PRTs), xanthine‐guanine PRT (EcXGPRT) and hypoxanthine PRT (EcHPRT). Here, we investigate acyclic nucleoside phosphonates (ANPs) as inhibitors of EcHPRT and have determined six crystal structures of EcHPRT in complex with ANPs. These data reveal the binding modes of these compounds and can assist in future rational structure‐based design efforts. It is suggested that a combination of inhibitors that block de novo and salvage pathways is a plausible approach to developing new antibiotics for E. coli UTIs. In addition, we provide details of a novel approach to accelerating the crystallization of this enzyme that may be of general applicability for rational drug discovery.
Therapeutic treatment of tuberculosis (TB) is becoming increasingly problematic due to the emergence of drug resistant Mycobacterium tuberculosis (Mt). Thus, new targets for anti-TB drug discovery need to be identified to combat and eradicate this disease. One such target is hypoxanthine-guanine phosphoribosyltransferase (HGPRT) which synthesises the 6-oxopurine nucleoside monophosphates essential for DNA/RNA production. [3R,4R]-4-Hypoxanthin-9-yl-3-((S)-2-hydroxy-2-phosphonoethyl)oxy-1-N-(phosphonopropionyl)pyrrolidine and [3R,4R]-4-guanin-9-yl-3-((S)-2-hydroxy-2-phosphonoethyl)oxy-1-N-(phosphonopropionyl)pyrrolidine (compound 6) are the most potent inhibitors of MtHGPRT yet discovered having K values of 60 nM. The crystal structure of the MtHGPRT.6 complex was obtained and compared with that of human HGPRT in complex with the same inhibitor. These structures provide explanations for the 60-fold difference in the inhibition constants between these two enzymes and a foundation for the design of next generation inhibitors. In addition, crystal structures of MtHGPRT in complex with two pyrrolidine nucleoside phosphosphonate inhibitors plus pyrophosphate provide insights into the final stage of the catalytic reaction. As the first step in ascertaining if such compounds have the potential to be developed as anti-TB therapeutics, the tetra-(ethyl L-phenylalanine) tetraamide prodrug of 6 was tested in cell based assays. This compound arrested the growth of virulent Mt not only in its replicating phase (IC of 14 μΜ) but also in its latent phase (IC of 29 μΜ). Furthermore, it arrested the growth of Mt in infected macrophages (MIC of 85 μΜ) and has a low cytotoxicity in mammalian cells (CC of 132 ± 20 μM). These inhibitors are therefore viewed as forerunners of new anti-TB chemotherapeutics.
Helicobacter pylori (Hp) is a human pathogen that lives in the gastric mucosa of approximately 50% of the world's population causing gastritis, peptic ulcers and gastric cancer. An increase in resistance to current drugs has sparked the search for new Hp drug targets and therapeutics. One target is the disruption of nucleic acid production which can be achieved by impeding the synthesis of the 6oxopurine nucleoside monophosphates, precursors of DNA and RNA. These metabolites are synthesised by Hp xanthine-guanine-hypoxanthine phosphoribosyltransferase (XGHPRT). Here, nucleoside phosphonates have been evaluated which inhibit the activity of this enzyme with Ki values as low as 200 nM. Prodrugs of these compounds arrest the growth of Hp at a concentration of 50 µM in cell based assays. The kinetic properties of HpXGHPRT have been determined together with its X-ray crystal structure in the absence and presence of 9-[(N-3-phosphonopropyl)-aminomethyl-9-deazahypoxanthine, providing a basis for new antibiotic development.
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