UDP-glucosamine as a substrate for dolichyl monophosphate glucosamine synthesis.
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ABSTRACT:
SUBMITTER: McDowell W
PROVIDER: S-EPMC1153095 | biostudies-other | 1986 Feb
REPOSITORIES: biostudies-other
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Project description:Uridine 5'-diphosphate-glucuronic acid (UDP-GlcA) and UDP-galacturonic acid (UDP-GalA), the unique carboxylic acid-formed sugar nucleotides, are key precursors involved in the biosynthesis of numerous cell components. Limited availability of those components has been hindering the development of efficient ways towards facile synthesis of bioactive glycans such as glycosaminoglycans. In current study, we biochemically characterized two UDP-sugar pyrophosphorylases from Arabidopsis thaliana (AtUSP) and Bifidobacterium infantis ATCC15697 (BiUSP), and compared their activities towards a panel of sugar-1-phosphates and derivatives. Both enzymes showed significant pyrophosphorylation activities towards GlcA-1-phosphate, and AtUSP also exhibited comparable activity towards GalA-1-phosphate. By combining with monosaccharide-1-phosphate kinases, we have developed an efficient and facile one-pot three-enzyme approach to quickly obtain hundreds milligrams of UDP-GlcA and UDP-GalA.
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Project description:A glucose acceptor was isolated from soya beans by extraction with chloroform/methanol (2:1, v/v), followed by DEAE-cellulose column chromatography of the extract. This acceptor could not be distinguished from liver dolichyl monophosphate by t.l.c. It could replace dolichyl monophosphate as a mannose acceptor with a liver enzyme and its glucosylated derivative could replace dolichyl monophosphate glucose as a glucose donor in the same system. These results, together with those already reported [Pont Lezica, Brett, Romero Martinez & Dankert (1975) Biochem, Biophys. Res. Commun. 66, 980-987], indicate that the acceptor from soya bean is a dolichyl monophosphate. Gel filtration of its glucosylated derivative on Sephadex G-75 in the presence of sodium deoxycholate indicated that the acceptor contained 17 or 18 isoprene units. An enzyme preparation from pea seedlings was shown to use endogenous acceptors to form lipid phosphate sugars containing mannose and N-acetylglucosamine from GDP-mannose and UDP-N-acetylglucosamine. Chromatographic and degradative techniques indicated that the compounds formed were lipid monophosphate mannose, lipid pyrophosphate N-acetylglucosamine, lipid pyrophosphate chitobiose and a series of lipid pyrophosphate oligosaccharides containing both mannose and N-acetylglucosamine. None of these compounds was degraded by catalytic hydrogenation, and so the lipid moiety in each case was probably an alpha-saturated polyprenol. The endogenous acceptors for mannose and N-acetylglucosamine in peas may therefore be dolichyl monophosphate, as has been found in mammalian systems.
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Project description:The presence of xylose and galactose residues in the structure of trichomonad lipoglycans was indicated by previous studies and the modification of any glycoconjugate with either monosaccharide requires the respective presence of the nucleotide sugars, UDP-xylose and UDP-galactose. Biosynthesis of UDP-xylose de novo is mediated by UDP-xylose synthase (UXS; UDP-glucuronic acid decarboxylase), which converts UDP-glucuronic acid to UDP-xylose, whereas UDP-galactose can be generated from UDP-glucose by UDP-galactose epimerases (GalE). Trichomonas vaginalis cDNAs, encoding proteins with homology to these enzymes from other eukaryotes, were isolated. The recombinant T. vaginalis UDP-xylose synthase and UDP-galactose epimerase were expressed in Escherichia coli and tested via high pressure liquid chromatography to demonstrate their enzymatic activities. Thereby, in this first report on enzymes involved in glycoconjugate biosynthesis in this organism, we demonstrate the existence of xylose and galactose synthesising pathways in T. vaginalis.
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Project description:UDP-glucuronosyltransferase (UGT) isozymes catalyze detoxification of numerous chemical toxins present in our daily diet and environment by conjugation to glucuronic acid. The special properties and enzymatic mechanism(s) that enable endoplasmic reticulum-bound UGT isozymes to convert innumerable structurally diverse lipophiles to excretable glucuronides are unknown. Inhibition of cellular UGT1A7 and UGT1A10 activities and of [33P]orthophosphate incorporation into immunoprecipitable proteins after exposure to curcumin or calphostin-C indicated that the isozymes are phosphorylated. Furthermore, inhibition of UGT phosphorylation and activity by treatment with PKCepsilon-specific inhibitor peptide supported PKC involvement. Co-immunoprecipitation, colocalization by means of immunofluorescence, and cross-linking studies of PKCepsilon and UGT1A7His revealed that the proteins reside within 11.4 angstroms of each other. Moreover, mutation of three PKC sites in each UGT isozyme demonstrated that T73A/G and T202A/G caused null activity, whereas S432G-UGT1A7 caused a major shift of its pH-8.5 optimum to 6.4 with new substrate selections, including 17beta-estradiol. S432G-UGT1A10 exhibited a minor pH shift without substrate alterations. PKCepsilon involvement was confirmed by the demonstration that PKCepsilon overexpression enhanced activity of UGT1A7 but not of its S432 mutant and the conversion of 17beta-[14C]estradiol by S432G-UGT1A7 but not by UGT1A7. Consistent with these observations, treatment of UGT1A7-transfected cells with PKCepsilon-specific inhibitor peptide or general PKC inhibitors increased 17beta-estradiol catalysis between 5- and 11-fold, with parallel decreases in phosphoserine-432. Here, we report a mechanism involving PKC-mediated phosphorylation of UGT such that phosphoserine/threonine regulates substrate specificity in response to chemical exposures, which possibly confers survival benefit.
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Project description:N-linked glycosylation of proteins is the most common post-translational modification of proteins. The enzyme UDP-N-acetylglucosamine-dolichyl-phosphate N-acetylglucosaminephosphotransferase (DPAGT1) catalyses the first step of N-glycosylation, and DPAGT1 knockout is embryonic lethal in mice. In this study, we identified the sole orthologue (algn-7) of the human DPAGT1 in the nematode C. elegans. The gene activity was disrupted by RNAi and deletion mutagenesis, which resulted in larval lethality, defects in oogenesis and oocyte-to-embryo transition. Endomitotic oocytes, abnormal fusion of pronuclei, abnormal AB cell rotation, disruption of permeation barriers of eggs, and abnormal expression of chitin and chitin synthase in oocytes and eggs were the typical phenotypes observed. The results indicate that N-glycosylation is indispensable for these processes. We further screened an N-glycosylated protein database of C. elegans, and identified 456 germline-expressed genes coding N-glycosylated proteins. By examining RNAi phenotypes, we identified five germline-expressed genes showing similar phenotypes to the algn-7 (RNAi) animals. They were ribo-1, stt-3, ptc-1, ptc-2, and vha-19. We identified known congenital disorders of glycosylation (CDG) genes (ribo-1 and stt-3) and a recently found CDG gene (vha-19). The results show that phenotype analyses using the nematode could be a powerful tool to detect new CDG candidate genes and their associated gene networks.
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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.
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Project description:Glutamine amidotransferases catalyze the amination of a wide range of molecules using the amide nitrogen of glutamine. The family provides numerous examples for study of multi-active-site regulation and interdomain communication in proteins. Guanosine 5'-monophosphate synthetase (GMPS) is one of three glutamine amidotransferases in de novo purine biosynthesis and is responsible for the last step in the guanosine branch of the pathway, the amination of xanthosine 5'-monophosphate (XMP). In several amidotransferases, the intramolecular path of ammonia from glutamine to substrate is understood; however, the crystal structure of GMPS only hinted at the details of such transfer. Rapid kinetics studies provide insight into the mechanism of the substrate-induced changes in this complex enzyme. Rapid mixing of GMPS with substrates also manifests absorbance changes that report on the kinetics of formation of a reactive intermediate as well as steps in the process of rapid transfer of ammonia to this intermediate. Isolation and use of the adenylylated nucleotide intermediate allowed the study of the amido transfer reaction distinct from the ATP-dependent reaction. Changes in intrinsic tryptophan fluorescence upon mixing of enzyme with XMP suggest a conformational change upon substrate binding, likely the ordering of a highly conserved loop in addition to global domain motions. In the GMPS reaction, all forward rates before product release appear to be faster than steady-state turnover, implying that release is likely rate-limiting. These studies establish the functional role of a substrate-induced conformational change in the GMPS catalytic cycle and provide a kinetic context for the formation of an ammonia channel linking the distinct active sites.
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Project description:Glucosamine-6-phosphate (GlcN-6-P) synthase catalyses the first and practically irreversible step in hexosamine metabolism. The final product of this pathway, uridine 5' diphospho N-acetyl-D-glucosamine (UDP-GlcNAc), is an essential substrate for assembly of bacterial and fungal cell walls. Moreover, the enzyme is involved in phenomenon of hexosamine induced insulin resistance in type II diabetes, which makes it a potential target for antifungal, antibacterial and antidiabetic therapy. The crystal structure of the isomerase domain of GlcN-6-P synthase from human pathogenic fungus Candida albicans, in complex with UDP-GlcNAc has been solved recently but it has not revealed the molecular mechanism of inhibition taking place under UDP-GlcNAc influence, the unique feature of the eukaryotic enzyme. UDP-GlcNAc is a physiological inhibitor of GlcN-6-P synthase, binding about 1 nm away from the active site of the enzyme. In the present work, comparative molecular dynamics simulations of the free and UDP-GlcNAc-bounded structures of GlcN-6-P synthase have been performed. The aim was to complete static X-ray structural data and detect possible changes in the dynamics of the two structures. Results of the simulation studies demonstrated higher mobility of the free structure when compared to the liganded one. Several amino acid residues were identified, flexibility of which is strongly affected upon UDP-GlcNAc binding. Importantly, the most fixed residues are those related to the inhibitor binding process and to the catalytic reaction. The obtained results constitute an important step toward understanding of mechanism of GlcN-6-P synthase inhibition by UDP-GlcNAc molecule.
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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.
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Project description:Orotidine 5'-monophosphate decarboxylase (ODCase) accelerates the decarboxylation of orotidine 5'-monophosphate (OMP) to uridine 5'-monophosphate (UMP) by 17 orders of magnitude. Eight new crystal structures with ligand analogues combined with computational analyses of the enzyme's short-lived intermediates and the intrinsic electronic energies to distort the substrate and other ligands improve our understanding of the still controversially discussed reaction mechanism. In their respective complexes, 6-methyl-UMP displays significant distortion of its methyl substituent bond, 6-amino-UMP shows the competition between the K72 and C6 substituents for a position close to D70, and the methyl and ethyl esters of OMP both induce rotation of the carboxylate group substituent out of the plane of the pyrimidine ring. Molecular dynamics and quantum mechanics/molecular mechanics computations of the enzyme-substrate complex also show the bond between the carboxylate group and the pyrimidine ring to be distorted, with the distortion contributing a 10-15% decrease of the ??G(?) value. These results are consistent with ODCase using both substrate distortion and transition-state stabilization, primarily exerted by K72, in its catalysis of the OMP decarboxylation reaction.
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