Study of Uridine 5'-Diphosphate (UDP)-Galactopyranose Mutase Using UDP-5-Fluorogalactopyranose as a Probe: Incubation Results and Mechanistic Implications.
ABSTRACT: Uridine 5'-diphosphate-5-fluorogalactopyranose (UDP-5F-Galp, 7) was synthesized, and its effect on UDP-Galp mutase (UGM) was investigated. UGM facilitated the hydrolysis of 7 to yield UDP and 5-oxogalactose (24), but no 11 was detected. (19)F NMR and trapping experiments demonstrated that the reaction involves the initial formation of a substrate-cofactor adduct followed by decomposition of the resulting C5 gem-fluorohydrin to generate a 5-oxo intermediate (10). The results support the current mechanistic proposal for UGM and suggest new directions for designing mechanism-based inhibitors.
Project description:Galactofuranose (Galf) residues are present in cell wall glycoconjugates of numerous pathogenic microbes. Uridine 5'-diphosphate (UDP) Galf, the biosynthetic precursor of Galf-containing glycoconjugates, is produced from UDP-galactopyranose (UDP-Galp) by the flavoenzyme UDP-galactopyranose mutase (UGM). The gene encoding UGM (glf) is essential for the viability of pathogens, including Mycobacterium tuberculosis, and this finding underscores the need to understand how UGM functions. Considerable effort has been devoted to elucidating the catalytic mechanism of UGM, but progress has been hindered by a lack of structural data for an enzyme-substrate complex. Such data could reveal not only substrate binding interactions but how UGM can act preferentially on two very different substrates, UDP-Galp and UDP-Galf, yet avoid other structurally related UDP sugars present in the cell. Herein, we describe the first structure of a UGM-ligand complex, which provides insight into the catalytic mechanism and molecular basis for substrate selectivity. The structure of UGM from Klebsiella pneumoniae bound to the substrate analog UDP-glucose (UDP-Glc) was solved by X-ray crystallographic methods and refined to 2.5 A resolution. The ligand is proximal to the cofactor, a finding that is consistent with a proposed mechanism in which the reduced flavin engages in covalent catalysis. Despite this proximity, the glucose ring of the substrate analog is positioned such that it disfavors covalent catalysis. This orientation is consistent with data indicating that UDP-Glc is not a substrate for UGM. The relative binding orientations of UDP-Galp and UDP-Glc were compared using saturation transfer difference NMR. The results indicate that the uridine moiety occupies a similar location in both ligand complexes, and this relevant binding mode is defined by our structural data. In contrast, the orientations of the glucose and galactose sugar moieties differ. To understand the consequences of these differences, we derived a model for the productive UGM-substrate complex that highlights interactions that can contribute to catalysis and substrate discrimination.
Project description:Aspergillus fumigatus is a human pathogen responsible for deadly infections in immune-compromised patients. A potential strategy for treating A. fumigatus infections is by targeting the biosynthesis of cell wall components, such as galactofuranase, which is absent in humans. Galactofuranose biosynthesis is initiated by the flavoenzyme UDP-galactopyranose mutase (UGM), which converts UDP-galactopyranose (UDP-Galp) to UDP-galactofuranose (UDP-Galf). UGM requires the reduced form of the flavin for activity, which is obtained by reacting with NADPH. We aimed to identify inhibitors of UGM by screening a kinase inhibitor library using ThermoFAD, a flavin fluorescence thermal shift assay. The screening assay identified flavopiridol as a compound that increased the melting temperature of A. fumigatus UGM. Further characterization showed that flavopiridol is a non-competitive inhibitor of UGM and docking studies suggest that it binds in the active site. This compound does not inhibit the prokaryotic UGM from Mycobacteria tuberculosis.
Project description:Aspergillus fumigatus is an opportunistic human pathogen responsible for deadly, invasive infections in immunocompromised patients. The A. fumigatus cell wall is a complex network of polysaccharides among them galactofuran, which is absent in humans. UDP-galactopyranose mutase (UGM) catalyzes the conversion of UDP-galactofuranose (UDP-Galf) to UDP-galactopyranose (UDP-Galp) and is an important virulence factor. UGM is a flavin-dependent enzyme that requires the reduced flavin for activity; flavin reduction is achieved by reaction with NADPH. The aim of this work was to discover inhibitors of UGM by targeting the NADPH binding site using an ADP-TAMRA probe in a high-throughput screening assay. The flavonoids (2S)-hesperetin and (2S)-naringenin were validated as competitive inhibitors of UGM against NADPH with Ki values of 6?µM and 74?µM, respectively. To gain insight into the active chemical substituents involved in the inhibition of UGM, several derivatives of these inhibitors were studied. The results show that the hydroxyl groups of (2S)-hesperetin are important for inhibition, in particular the phenyl-chroman moiety. Congo red susceptibility assay and growth temperature effects showed that these compounds affected cell wall biosynthesis in A. fumigatus. This work is the first report of inhibition studies on UGM from eukaryotic human pathogens.
Project description:UDP-galactopyranose mutase (UGM) requires reduced FAD (FAD(red)) to catalyze the reversible interconversion of UDP-galactopyranose (UDP-Galp) and UDP-galactofuranose (UDP-Galf). Recent structural and mechanistic studies of UGM have provided evidence for the existence of an FAD-Galf/p adduct as an intermediate in the catalytic cycle. These findings are consistent with Lewis acid/base chemistry involving nucleophilic attack by N5 of FAD(red) at C1 of UDP-Galf/p. In this study, we employed a variety of FAD analogues to characterize the role of FAD(red) in the UGM catalytic cycle using positional isotope exchange (PIX) and linear free energy relationship studies. PIX studies indicated that UGM reconstituted with 5-deaza-FAD(red) is unable to catalyze PIX of the bridging C1-OP(?) oxygen of UDP-Galp, suggesting a direct role for the FAD(red) N5 atom in this process. In addition, analysis of kinetic linear free energy relationships of k(cat) versus the nucleophilicity of N5 of FAD(red) gave a slope of ? = -2.4 ± 0.4. Together, these findings are most consistent with a chemical mechanism for UGM involving an S(N)2-type displacement of UDP from UDP-Galf/p by N5 of FAD(red).
Project description:UDP-galactopyranose mutase (UGM) is the key enzyme involved in the biosynthesis of Galf. UDP-Galp and UDP-Galf are two natural substrates of UGM. A protocol that combines the use of STD-NMR spectroscopy, molecular modeling, and CORCEMA-ST calculations was applied to the investigation of the binding of UDP-Galf and its C3-fluorinated analogue to UGM from Klebsiella pneumoniae. UDP-Galf and UDP-[3-F]Galf were bound to UGM in a manner similar to that of UDP-Galp. The interconversions of UDP-Galf and UDP-[3-F]Galf to their galactopyranose counterparts were catalyzed by the reduced (active) UGM with different catalytic efficiencies, as observed by NMR spectroscopy. The binding affinities of UDP-Galf and UDP-[3-F]Galf were also compared with those of UDP-Galp and UDP by competition STD-NMR experiments. When UGM was in the oxidized (inactive) state, the binding affinities of UDP-Galf, UDP-Galp, and UDP-[3-F]Galf were of similar magnitudes and were lower than that of UDP. However, when UGM was in the reduced state, UDP-Galp had higher binding affinity compared with UDP. Molecular dynamics (MD) simulations indicated that the "open" mobile loop in UGM "closes" upon binding of the substrates. Combined MD simulations and STD-NMR experiments were used to create models of UGM with UDP-Galf and UDP-[3-F]Galf as bound ligands. Calculated values of saturation-transfer effects with CORCEMA-ST (complete relaxation and conformational exchange matrix analysis of saturation transfer) were compared to the experimental STD effects and permitted differentiation between two main conformational families of the bound ligands. Taken together, these results are used to rationalize the different rates of catalytic turnover of UDP-Galf and UDP-[3-F]Galf and shed light on the mechanism of action of UGM.
Project description:UDP-galactopyranose mutase (Glf or UGM) catalyzes the formation of uridine 5'-diphosphate-?-d-galactofuranose (UDP-Galf) from UDP-galactopyranose (UDP-Galp). The enzyme is required for the production of Galf-containing glycans. UGM is absent in mammals, but members of the Corynebacterineae suborder require UGM for cell envelope biosynthesis. The need for UGM in some pathogens has prompted the search for inhibitors that could serve as antibiotic leads. Optimizing inhibitor potency, however, has been challenging. The UGM from Klebsiella pneumoniae (KpUGM), which is not required for viability, is more effectively impeded by small-molecule inhibitors than are essential UGMs from species such as Mycobacterium tuberculosis or Corynebacterium diphtheriae. Why KpUGM is more susceptible to inhibition than other orthologs is not clear. One potential source of difference is UGM ortholog conformation. We previously determined a structure of CdUGM bound to a triazolothiadiazine inhibitor in the open form, but it was unclear whether the small-molecule inhibitor bound this form or to the closed form. By varying the terminal tag (CdUGM-His6 and GSG-CdUGM), we crystallized CdUGM to capture the enzyme in different conformations. These structures reveal a pocket in the active site that can be exploited to augment inhibitor affinity. Moreover, they suggest the inhibitor binds the open form of most prokaryotic UGMs but can bind the closed form of KpUGM. This model and the structures suggest strategies for optimizing inhibitor potency by exploiting UGM conformational flexibility.
Project description: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.
Project description:[reaction: see text] UDP-D-galactofuranose (2), which is essential for both cell growth and virulence in many pathogenic microorganisms, is converted from UDP-D-galactopyranose (UDP-Galp, 1) by the flavin adenine dinucleotide (FAD)-dependent enzyme UDP-galactopyranose mutase (UGM). Here, we report the synthesis of UDP-GalOH (13) and show it as an inhibitor for UGM with a binding affinity similar to that of 1. These results are more consistent with a mechanism involving an oxocarbenium ion intermediate in UGM catalysis.
Project description:UDP-galactopyranose mutase (UGM) is a flavin-containing enzyme that catalyzes the reversible conversion of UDP-galactopyranose (UDP-Galp) to UDP-galactofuranose (UDP-Galf). As in prokaryotic UGMs, the flavin needs to be reduced for the enzyme to be active. Here we present the first eukaryotic UGM structures from Aspergillus fumigatus (AfUGM). The structures are of UGM alone, with the substrate UDP-Galp and with the inhibitor UDP. Additionally, we report the structures of AfUGM bound to substrate with oxidized and reduced flavin. These structures provide insight into substrate recognition and structural changes observed upon substrate binding involving the mobile loops and the critical arginine residues Arg-182 and Arg-327. Comparison with prokaryotic UGM reveals that despite low sequence identity with known prokaryotic UGMs the overall fold is largely conserved. Structural differences between prokaryotic UGM and AfUGM result from inserts in AfUGM. A notable difference from prokaryotic UGMs is that AfUGM contains a third flexible loop (loop III) above the si-face of the isoalloxazine ring that changes position depending on the redox state of the flavin cofactor. This loop flipping has not been observed in prokaryotic UGMs. In addition we have determined the crystals structures and steady-state kinetic constants of the reaction catalyzed by mutants R182K, R327K, R182A, and R327A. These results support our hypothesis that Arg-182 and Arg-327 play important roles in stabilizing the position of the diphosphates of the nucleotide sugar and help to facilitate the positioning of the galactose moiety for catalysis.
Project description:Can accumulation of a normally transient metabolite affect fungal biology? UDP-4-keto-6-deoxyglucose (UDP-KDG) represents an intermediate stage in conversion of UDP-glucose to UDP-rhamnose. Normally, UDP-KDG is not detected in living cells, because it is quickly converted to UDP-rhamnose by the enzyme UDP-4-keto-6-deoxyglucose-3,5-epimerase/-4-reductase (ER). We previously found that deletion of the er gene in Botrytis cinerea resulted in accumulation of UDP-KDG to levels that were toxic to the fungus due to destabilization of the cell wall. Here we show that these negative effects are at least partly due to inhibition by UDP-KDG of the enzyme UDP-galactopyranose mutase (UGM), which reversibly converts UDP-galactopyranose (UDP-Galp) to UDP-galactofuranose (UDP-Galf). An enzymatic activity assay showed that UDP-KDG inhibits the B. cinerea UGM enzyme with a Ki of 221.9 µM. Deletion of the ugm gene resulted in strains with weakened cell walls and phenotypes that were similar to those of the er deletion strain, which accumulates UDP-KDG. Galf residue levels were completely abolished in the ?ugm strain and reduced in the ?er strain, while overexpression of the ugm gene in the background of a ?er strain restored Galf levels and alleviated the phenotypes. Collectively, our results show that the antifungal activity of UDP-KDG is due to inhibition of UGM and possibly other nucleotide sugar-modifying enzymes and that the rhamnose metabolic pathway serves as a shunt that prevents accumulation of UDP-KDG to toxic levels. These findings, together with the fact that there is no Galf in mammals, support the possibility of developing UDP-KDG or its derivatives as antifungal drugs.IMPORTANCE Nucleotide sugars are donors for the sugars in fungal wall polymers. We showed that production of the minor sugar rhamnose is used primarily to neutralize the toxic intermediate compound UDP-KDG. This surprising finding highlights a completely new role for minor sugars and other secondary metabolites with undetermined function. Furthermore, the toxic potential of predicted transition metabolites that never accumulate in cells under natural conditions are highlighted. We demonstrate that UDP-KDG inhibits the UDP-galactopyranose mutase enzyme, thereby affecting production of Galf, which is one of the components of cell wall glycans. Given the structural similarity, UDP-KDG likely inhibits additional nucleotide sugar-utilizing enzymes, a hypothesis that is also supported by our findings. Our results suggest that UDP-KDG could serve as a template to develop antifungal drugs.