Kinetic properties of <Emphasis Type="Italic">Mycobacterium tuberculosis</Emphasis> bifunctional GlmU

The UDP-N-acetylglucosamine (UDP-GlcNAc) is present as one of the glycosyl donors for disaccharide linker (d-N-GlcNAc-l-rhamnose) and the precursor of peptidoglycan in mycobacteria. The bifunctional enzyme GlmU involves in the last two sequential steps of UDP-GlcNAc synthetic pathway. Glucosamine-1-phosphate acetyltransferase catalyzes the formation of N-acetylglucosamine-1-phosphate (GlcNAc-1-P) from glucosamine-1-phosphate (GlcN-1-P) and acetyl coenzyme A (Acetyl CoA), and N-acetylglucosamine-1-phosphate uridyltransferase catalyzes the synthesis of UDP-GlcNAc from GlcNAc-1-P and UTP. The previous studies demonstrating the essentiality of GlmU to mycobacterial survival supported GlmU as a novel and potential target for TB drugs. In this work, two accurate and simple colorimetric assays based on 96-well microtiter plate were developed to measure the kinetic properties of bifunctional GlmU including initial velocity, optimal temperature, optimal pH, the effect of Mg²⁺, and the kinetic parameters. Both of the colorimetric assays for bifunctional GlmU enzyme activities and the kinetic properties will facilitate high-throughput screening of GlmU inhibitors.     

Copurification of glucosamine-1-phosphate acetyltransferase and N-acetylglucosamine-1-phosphate uridyltransferase activities of Escherichia coli: characterization of the glmU gene product as a bifunctional enzyme catalyzing two subsequent steps in the pathway for UDP-N-acetylglucosamine synthesis.

The glmU gene product of Escherichia coli was recently identified as the N-acetylglucosamine-1-phosphate uridyltransferase activity which catalyzes the formation of UDP-N-acetylglucosamine, an essential precursor for cell wall peptidoglycan and lipopolysaccharide biosyntheses (D. Mengin-Lecreulx and J. van Heijenoort, J. Bacteriol. 175:6150-6157, 1993). Evidence that the purified GlmU protein is in fact a bifunctional enzyme which also catalyzes acetylation of glucosamine-1-phosphate, the preceding step in the same pathway, is now provided. Kinetic parameters of both reactions were investigated, indicating in particular that the acetyltransferase activity of the enzyme is fivefold higher than its uridyltransferase activity. In contrast to the uridyltransferase activity, which is quite stable and insensitive to thiol reagents, the acetyltransferase activity was rapidly lost when the enzyme was stored in the absence of reducing thiols or acetyl coenzyme A or was treated with thiol-alkylating agents, suggesting the presence of at least one essential cysteine residue in or near the active site. The acetyltransferase activity is greatly inhibited by its reaction product N-acetylglucosamine-1-phosphate and, interestingly, also by UDP-N-acetylmuramic acid, which is one of the first precursors specific for the peptidoglycan pathway. The detection in crude cell extracts of a phosphoglucosamine mutase activity finally confirms that the route from glucosamine-6-phosphate to UDP-N-acetylglucosamine occurs via glucosamine-1-phosphate in bacteria.     

Self-association studies of the bifunctional N-acetylglucosamine-1-phosphate uridyltransferase from Escherichia coli

The N-acetylglucosamine-1-phosphate uridyltransferase (GlmU) is a key bifunctional enzyme in the biosynthesis of UDP-GlcNAc, a precursor in the synthesis of cell wall peptidoglycan. Crystal structures of the enzyme from different bacterial strains showed that the polypeptide forms a trimer through a unique parallel left-handed beta helix domain. Here, we show that the GlmU enzyme from Escherichia coli forms a hexamer in solution. Sedimentation equilibrium analytical ultracentrifugation demonstrated that the enzyme is in a trimer/hexamer equilibrium. Small-angle X-ray scattering studies were performed to determine the structure of the hexameric assembly and showed that two trimers assemble through their N-terminal domains. The interaction is mediated by a loop that undergoes a large conformational change in the uridyl transferase reaction, a feature that may affect the enzymatic activity of GlmU.     

Characterization of substrate binding and catalysis in the potential antibacterial target N-acetylglucosamine-1-phosphate uridyltransferase (GlmU)

N-Acetylglucosamine-1-phosphate uridyltransferase (GlmU) catalyzes the first step in peptidoglycan biosynthesis in both Gram-positive and Gram-negative bacteria. The products of the GlmU reaction are essential for bacterial survival, making this enzyme an attractive target for antibiotic drug discovery. A series of Haemophilus influenzae GlmU (hiGlmU) structures were determined by X-ray crystallography in order to provide structural and functional insights into GlmU activity and inhibition. The information derived from these structures was combined with biochemical characterization of the K25A, Q76A, D105A, Y103A, V223A, and E224A hiGlmU mutants in order to map these active-site residues to catalytic activity of the enzyme and refine the mechanistic model of the GlmU uridyltransferase reaction. These studies suggest that GlmU activity follows a sequential substrate-binding order that begins with UTP binding noncovalently to the GlmU enzyme. The uridyltransferase active site then remains in an open apo-like conformation until N-acetylglucosamine-1-phosphate (GlcNAc-1-P) binds and induces a conformational change at the GlcNAc-binding subsite. Following the binding of GlcNAc-1-P to the UTP-charged uridyltransferase active site, the non-esterified oxygen of GlcNAc-1-P performs a nucleophilic attack on the alpha-phosphate group of UTP. The new data strongly suggest that the mechanism of phosphotransfer in the uridyltransferase reaction in GlmU is primarily through an associative mechanism with a pentavalent phosphate intermediate and an inversion of stereochemistry. Finally, the structural and biochemical characterization of the uridyltransferase active site and catalytic mechanism described herein provides a basis for the structure-guided design of novel antibacterial agents targeting GlmU activity.     

Dissection of the bifunctional Escherichia coli N-acetylglucosamine-1-phosphate uridyltransferase enzyme into autonomously functional domains and evidence that trimerization is absolutely required for glucosamine-1-phosphate acetyltransferase activity and cell growth

The bifunctional N-acetylglucosamine-1-phosphate uridyltransferase (GlmU) enzyme catalyzes both the acetylation of glucosamine 1-phosphate and the uridylation of N-acetylglucosamine 1-phosphate, two subsequent steps in the pathway for UDP-N-acetylglucosamine synthesis in bacteria. In our previous work describing its initial characterization in Escherichia coli, we proposed that the 456-amino acid (50.1 kDa) protein might possess separate uridyltransferase (N-terminal) and acetyltransferase (C-terminal) domains. In the present study, we confirm this hypothesis by expression of the two independently folding and functional domains. A fragment containing the N-terminal 331 amino acids (Tr331, 37.1 kDa) has uridyltransferase activity only, with steady-state kinetic parameters similar to the full-length protein. Further deletion of 80 amino acid residues at the C terminus results in a 250-amino acid fragment (28.6 kDa) still exhibiting significant uridyltransferase activity. Conversely, a fragment containing the 233 C-terminal amino acids (24.7 kDa) exhibits acetyltransferase activity exclusively. None of these individual domains could complement a chromosomal glmU mutation, indicating that each of the two activities is essential for cell viability. Analysis of truncated GlmU proteins by gel filtration further localizes regions of the protein involved in its trimeric organization. Interestingly, overproduction of the truncated Tr331 protein in a wild-type strain results in a rapid depletion of endogenous acetyltransferase activity, an arrest of peptidoglycan synthesis and cell lysis. It is shown that the acetyltransferase activity of the full-length protein is abolished once trapped within heterotrimers formed in presence of the truncated protein, suggesting that this enzyme activity absolutely requires a trimeric organization and that the catalytic site involves regions of contact between adjacent monomers. Data are discussed in connection with the recently obtained crystal structure of the truncated Tr331 protein.     

1-Deoxy-D-xylulose 5-phosphate reductoisomerase (IspC) from Mycobacterium tuberculosis: towards understanding mycobacterial resistance to fosmidomycin

1-Deoxy-d-xylulose 5-phosphate reductoisomerase (IspC) catalyzes the first committed step in the mevalonate-independent isopentenyl diphosphate biosynthetic pathway and is a potential drug target in some pathogenic bacteria. The antibiotic fosmidomycin has been shown to inhibit IspC in a number of organisms and is active against most gram-negative bacteria but not gram positives, including Mycobacterium tuberculosis, even though the mevalonate-independent pathway is the sole isopentenyl diphosphate biosynthetic pathway in this organism. Therefore, the enzymatic properties of recombinant IspC from M. tuberculosis were characterized. Rv2870c from M. tuberculosis converts 1-deoxy-d-xylulose 5-phosphate to 2-C-methyl-d-erythritol 4-phosphate in the presence of NADPH. The enzymatic activity is dependent on the presence of Mg(2+) ions and exhibits optimal activity between pH 7.5 and 7.9; the K(m) for 1-deoxyxylulose 5-phosphate was calculated to be 47.1 microM, and the K(m) for NADPH was 29.7 microM. The specificity constant of Rv2780c in the forward direction is 1.5 x 10(6) M(-1) min(-1), and the reaction is inhibited by fosmidomycin, with a 50% inhibitory concentration of 310 nM. In addition, Rv2870c complements an inactivated chromosomal copy of IspC in Salmonella enterica, and the complemented strain is sensitive to fosmidomycin. Thus, M. tuberculosis resistance to fosmidomycin is not due to intrinsic properties of Rv2870c, and the enzyme appears to be a valid drug target in this pathogen.     

Identification of M. tuberculosis Rv3441c and M. smegmatis MSMEG_1556 and Essentiality of M. smegmatis MSMEG_1556

The normal growth of mycobacteria attributes to the integrity of cell wall core which consists of peptidoglycan (PG), arabinogalactan (AG) and mycolic acids. N-acetyl glucosamine (GlcNAc) is an essential component in both PG and AG of mycobacterial cell wall. The biosynthetic pathway for UDP-N-acetylglucosamine (UDP-GlcNAc), as a sugar donor of GlcNAc, is different in prokaryotes and eukaryotes. The conversion of glucosamine-6-phosphate to glucosamine-1-phosphate, which is catalyzed by phosphoglucosamine mutase (GlmM), is unique to prokaryotes. Bioinformatic analysis showed that Msm MSMEG_1556 and Mtb Rv3441c are homologous to Ec GlmM. In this study, soluble Msm MSMEG_1556 protein and Mtb Rv3441c protein were expressed in E. coli BL21(DE3) and their phosphoglucosamine mutase activity were detected. In order to further investigate the essentiality of MSMEG_1556 for the growth of M. smegmatis, we generated a conditional MSMEG_1556 knockout mutant, which harbored thermo-sensitive rescue plasmid carrying Mtb Rv3441c. As the rescue plasmid was unable to complement MSMEG_1556 deficiency at 42°C, MSMEG_1556 knockout mutant did not grow. The dramatic morphological changes of MSMEG_1556 knockout mutant after temperature shift from 30°C to 42°C have been observed by scanning electron microscope. These results demonstrated that MSMEG_1556 is essential for growth of M. smegmatis. This study provided evidence that GlmM enzyme could be as a potential target for developing anti-tuberculosis drugs.     

Structure of a small-molecule inhibitor complexed with GlmU from Haemophilus influenzae reveals an allosteric binding site

N-Acetylglucosamine-1-phosphate uridyltransferase (GlmU) is an essential enzyme in aminosugars metabolism and an attractive target for antibiotic drug discovery. GlmU catalyzes the formation of uridine-diphospho-N-acetylglucosamine (UDP-GlcNAc), an important precursor in the peptidoglycan and lipopolisaccharide biosynthesis in both Gram-negative and Gram-positive bacteria. Here we disclose a 1.9 A resolution crystal structure of a synthetic small-molecule inhibitor of GlmU from Haemophilus influenzae (hiGlmU). The compound was identified through a high-throughput screening (HTS) configured to detect inhibitors that target the uridyltransferase active site of hiGlmU. The original HTS hit exhibited a modest micromolar potency (IC(50) approximately 18 microM in a racemic mixture) against hiGlmU and no activity against Staphylococcus aureus GlmU (saGlmU). The determined crystal structure indicated that the inhibitor occupies an allosteric site adjacent to the GlcNAc-1-P substrate-binding region. Analysis of the mechanistic model of the uridyltransferase reaction suggests that the binding of this allosteric inhibitor prevents structural rearrangements that are required for the enzymatic reaction, thus providing a basis for structure-guided design of a new class of mechanism-based inhibitors of GlmU.     

Identification,cloning, purification,and enzymatic characterization of Mycobacterium tuberculosis 1-deoxy-D-xylulose 5-phosphate synthase

The enzyme encoded by Rv2682c in Mycobacterium tuberculosis is a functional 1-deoxy-D-xylulose 5-phosphate synthase (DXS), suggesting that the pathogen utilizes the mevalonate-independent pathway for isopentenyl diphosphate and subsequent polyprenyl phosphate synthesis. These key precursors are vital in the biosynthesis of many essential aspects of the mycobacterial cell wall. Rv2682c encodes the conserved DRAG sequence that has been proposed as a signature motif for DXSs and also all 13 conserved amino acid residues thought to be important to the function of transketolase enzymes. Recombinant Rv2682c is capable of utilizing glyceraldehyde 3-phosphate and erythrose 4-phosphate as well as D- and L-glyceraldehyde as aldose substrates. The enzyme has K(m) values of 40 microM, 6.1 microM, 5.6 mM, and 4.5 mM for pyruvate, D-glyceraldehyde 3-phosphate, D-glyceraldehyde, and L-glyceradehyde, respectively. Rv2682c has an absolute requirement for divalent cation and thiamin diphosphate as cofactors. The K(d) (thiamin diphosphate )for the native M. tuberculosis DXS activity partially purified from M. tuberculosis cytosol is 1 microM in the presence of Mg(2+).     

Design and synthesis of novel cell wall inhibitors of Mycobacterium tuberculosis GlmM and GlmU

GlmM and GlmU are key enzymes in the biosynthesis of UDP-N-acetyl-d-glucosamine (UDP-GlcNAc), an essential precursor of peptidoglycan and the rhamnose–GlcNAc linker region in the mycobacterial cell wall. These enzymes are involved in the conversion of two important precursors of UDP-GlcNAc, glucosamine-6-phosphate (GlcN-6-P) and glucosamine-1-phosphate (GlcN-1-P). GlmM converts GlcN-6-P to GlcN-1-P, GlmU is a bifunctional enzyme, whereby GlmU converts GlcN-1-P to GlcNAc-1-P and then catalyzes the formation of UDP-GlcNAc from GlcNAc-1-P and uridine triphosphate. In the present study, methyl 2-amino-2-deoxyl-α-d-glucopyranoside 6-phosphate (1α), methyl 2-amino-2-deoxyl-β-d-glucopyranoside 6-phosphate (1β), two analogs of GlcN-6-P, were synthesized as GlmM inhibitors; 2-azido-2-deoxy-α-d-glucopyranosyl phosphate (2) and 2-amino-2,3-dideoxy-3-fluoro-α-d-glucopyranosyl phosphate (3), analogs of GlcN-1-P, were synthesized firstly as GlmU inhibitors. Compounds 1α, 1β, 2, and 3 as possible inhibitors of mycobacterial GlmM and GlmU are reported herein. Compound 3 showed promising inhibitory activities against GlmU, whereas 1α, 1β and 2 were inactive against GlmM and GlmU even at high concentrations.     

Inactivation of the mycobacterial rhamnosyltransferase, which is needed for the formation of the arabinogalactan-peptidoglycan linker, leads to irreversible loss of viability

Temperature-sensitive mutant 2-20/32 of Mycobacterium smegmatis mc(2)155 was isolated and genetically complemented with a Mycobacterium tuberculosis H37Rv DNA fragment that contained a single open reading frame. This open reading frame is designated Rv3265c in the M. tuberculosis H37Rv genome. Rv3265c shows homology to the Escherichia coli gene wbbL, which encodes a dTDP-Rha:alpha-D-GlcNAc-pyrophosphate polyprenol, alpha-3-L-rhamnosyltransferase. In E. coli this enzyme is involved in O-antigen synthesis, but in mycobacteria it is required for the rhamnosyl-containing linker unit responsible for the attachment of the cell wall polymer mycolyl-arabinogalactan to the peptidoglycan. The M. tuberculosis wbbL homologue, encoded by Rv3265c, was shown to be capable of restoring an E. coli K12 strain containing an insertionally inactivated wbbL to O-antigen positive. Likewise, the E. coli wbbL gene allowed 2-20/32 to grow at higher non-permissive temperatures. The rhamnosyltransferase activity of M. tuberculosis WbbL was demonstrated in 2-20/32 as was the loss of this transferase activity in 2-20/32 at elevated temperatures. The wbbL of the temperature-sensitive mutant contained a single-base change that converted what was a proline in mc(2)155 to a serine residue. Exposure of 2-20/32 to higher non-permissive temperatures resulted in bacteria that could not be recovered at the lower permissive temperatures.     

N-Acetyl-1-D-myo-inosityl-2-amino-2-deoxy-alpha-D-glucopyranoside deacetylase (MshB) is a key enzyme in mycothiol biosynthesis

Mycothiol is a novel thiol produced only by actinomycetes and is the major low-molecular-weight thiol in mycobacteria. Mycothiol was previously shown to be synthesized from 1-D-myo-inosityl-2-amino-2-deoxy-alpha-D-glucopyranoside by ligation with cysteine followed by acetylation. A novel mycothiol-dependent detoxification enzyme, mycothiol conjugate amidase, was recently identified in Mycobacterium smegmatis and shown to have a homolog, Rv1082, in Mycobacterium tuberculosis. In the present study we found that a protein encoded by the M. tuberculosis open reading frame Rv1170, a homolog of Rv1082, possesses weak mycothiol conjugate amidase activity but shows substantial deacetylation activity with 1-D-myo-inosityl-2-acetamido-2-deoxy-alpha-D-glucopyranoside (GlcNAc-Ins), a hypothetical mycothiol biosynthetic precursor. The availability of this protein enabled us to develop an assay for GlcNAc-Ins, which was used to demonstrate that GlcNAc-Ins is present in M. smegmatis at a level about twice that of mycothiol. It was shown that GlcNAc-Ins is absent in mycothiol-deficient mutant strain 49 of M. smegmatis and that this strain can concentrate GlcNAc-Ins from the medium and convert it to mycothiol. This demonstrates that GlcNAc-Ins is a key intermediate in the pathway of mycothiol biosynthesis. Assignment of Rv1170 as the gene coding the deacetylase in the M. tuberculosis genome represents the first identification of a gene of the mycothiol biosynthesis pathway. The presence of a large cellular pool of substrate for this enzyme suggests that it may be important in regulating mycothiol biosynthesis.     

Identification of the glmU gene encoding N-acetylglucosamine-1-phosphate uridyltransferase in Escherichia coli.

The physiological properties of the EcoURF-1 open reading frame, which precedes the glmS gene at 84 min on the Escherichia coli chromosome (J. E. Walker, N. J. Gay, M. Saraste, and A. N. Eberle, Biochem. J. 224:799-815, 1984), were investigated. A thermosensitive conditional mutant in which the synthesis of the gene product was impaired at 43 degrees C was constructed. The inactivation of the gene in exponentially growing cells rapidly inhibited peptidoglycan synthesis. As a result, various alterations of cell shape were observed, and cell lysis finally occurred when the peptidoglycan content was 37% lower than that of normally growing cells. Analysis of the pools of peptidoglycan precursors revealed a large accumulation of N-acetylglucosamine-1-phosphate and the concomitant depletion of the pools of the seven peptidoglycan nucleotide precursors located downstream in the pathway, a result indicating that the mutational block was in the step leading from N-acetylglucosamine-1-phosphate and UTP to the formation of UDP-N-acetylglucosamine. In vitro assays showed that the overexpression of this gene in E. coli cells, directed by appropriate plasmids, led to a high overproduction (from 25- to 410-fold) of N-acetylglucosamine-1-phosphate uridyltransferase activity. This allowed us to purify this enzyme to homogeneity in only two chromatographic steps. The gene for this enzyme, which is essential for peptidoglycan and lipopolysaccharide biosyntheses, was designated glmU.     

Crystal structure of Streptococcus pneumoniae N-acetylglucosamine-1-phosphate uridyltransferase bound to acetyl-coenzyme A reveals a novel active site architecture

The bifunctional bacterial enzyme N-acetyl-glucosamine-1-phosphate uridyltransferase (GlmU) catalyzes the two-step formation of UDP-GlcNAc, a fundamental precursor in bacterial cell wall biosynthesis. With the emergence of new resistance mechanisms against beta-lactam and glycopeptide antibiotics, the biosynthetic pathway of UDP-GlcNAc represents an attractive target for drug design of new antibacterial agents. The crystal structures of Streptococcus pneumoniae GlmU in unbound form, in complex with acetyl-coenzyme A (AcCoA) and in complex with both AcCoA and the end product UDP-GlcNAc, have been determined and refined to 2.3, 2.5, and 1.75 A, respectively. The S. pneumoniae GlmU molecule is organized in two separate domains connected via a long alpha-helical linker and associates as a trimer, with the 50-A-long left-handed beta-helix (LbetaH) C-terminal domains packed against each other in a parallel fashion and the C-terminal region extended far away from the LbetaH core and exchanged with the beta-helix from a neighboring subunit in the trimer. AcCoA binding induces the formation of a long and narrow tunnel, enclosed between two adjacent LbetaH domains and the interchanged C-terminal region of the third subunit, giving rise to an original active site architecture at the junction of three subunits.     

Crystal structures of Streptococcus pneumoniae N-acetylglucosamine-1-phosphate uridyltransferase, GlmU, in apo form at 2.33 A resolution and in complex with UDP-N-acetylglucosamine and Mg(2+) at 1.96 A resolution

N-Acetylglucosamine-1-phosphate uridyltransferase (GlmU) is an essential bacterial enzyme with both an acetyltransferase and a uridyltransferase activity which have been mapped to the C-terminal and N-terminal domains, respectively. GlmU performs the last two steps in the synthesis of UDP-N-acetylglucosamine (UDP-GlcNAc), which is an essential precursor in both the peptidoglycan and the lipopolysaccharide metabolic pathways. GlmU is therefore an attractive target for potential antibiotics. Knowledge of its three-dimensional structure would provide a basis for rational drug design. We have determined the crystal structures of Streptococcus pneumoniae GlmU (SpGlmU) in apo form at 2.33 A resolution, and in complex with UDP-N-acetyl glucosamine and the essential co-factor Mg(2+) at 1.96 A resolution. The protein structure consists of an N-terminal domain with an alpha/beta-fold, containing the uridyltransferase active site, and a C-terminal domain with a long left-handed beta-sheet helix (LbetaH) domain. An insertion loop containing the highly conserved sequence motif Asn-Tyr-Asp-Gly protrudes from the left-handed beta-sheet helix domain. In the crystal, S. pneumoniae GlmU forms exact trimers, mainly through contacts between left-handed beta-sheet helix domains. UDP-N-acetylglucosamine and Mg(2+) are bound at the uridyltransferase active site, which is in a closed form. We propose a uridyltransferase mechanism in which the activation energy of the double negatively charged phosphorane transition state is lowered by charge compensation of Mg(2+) and the side-chain of Lys22.     

Effect of Phosphoglucosamine Mutase on Biofilm Formation and Antimicrobial Susceptibilities in M. smegmatis glmM Gene Knockdown Strain

UDP-N-acetylglucosamine (UDP-GlcNAc) is a direct glycosyl donor of linker unit (L-Rhamnose-D-GlcNAc) and an essential precursor of peptidoglycan in mycobacteria. Phosphoglucosamine mutase (GlmM) is involved in the formation of glucosamine-1-phosphate from glucosamine-6-phosphate, the second step in UDP-GlcNAc biosynthetic pathway. We have demonstrated that GlmM protein is essential for the growth of M. smegmatis. To facilitate the analysis of the GlmM protein function in mycobacteria, a tetracycline inducible M. smegmatis glmM gene knockdown strain was constructed by using an antisense RNA technology. After induction with 20 ng/ml tetracycline, the expression of GlmM protein in glmM gene knockdown strain was significantly decreased, resulting in a decline of cell growth. The morphological changes of glmM gene knockdown strain induced with 20 ng/ml tetracycline have been observed by scanning electron microscope and transmission electron microscope. Furthermore, insufficient GlmM protein reduced the biofilm formation and increased the sensitivity to isoniazid and ethambutol in M. smegmatis, indicating that GlmM protein had effect on the biofilm formation and the senstivity to some anti-tuberculosis drugs targeting the cell wall. These results provide a new insight on GlmM functions in mycobacteria, suggesting that GlmM could be a potential target for development of new anti-tuberculosis drug.     

Crystal structure of the bifunctional N-acetylglucosamine 1-phosphate uridyltransferase from Escherichia coli: a paradigm for the related pyrophosphorylase superfamily.

N-acetylglucosamine 1-phosphate uridyltransferase (GlmU) is a cytoplasmic bifunctional enzyme involved in the biosynthesis of the nucleotide-activated UDP-GlcNAc, which is an essential precursor for the biosynthetic pathways of peptidoglycan and other components in bacteria. The crystal structure of a truncated form of GlmU has been solved at 2.25 A resolution using the multiwavelength anomalous dispersion technique and its function tested with mutagenesis studies. The molecule is composed of two distinct domains connected by a long alpha-helical arm: (i) an N-terminal domain which resembles the dinucleotide-binding Rossmann fold; and (ii) a C-terminal domain which adopts a left-handed parallel beta-helix structure (LbetaH) as found in homologous bacterial acetyltransferases. Three GlmU molecules assemble into a trimeric arrangement with tightly packed parallel LbetaH domains, the long alpha-helical linkers being seated on top of the arrangement and the N-terminal domains projected away from the 3-fold axis. In addition, the 2.3 A resolution structure of the GlmU-UDP-GlcNAc complex reveals the structural bases required for the uridyltransferase activity. These structures exemplify a three-dimensional template for the development of new antibacterial agents and for studying other members of the large family of XDP-sugar bacterial pyrophosphorylases.     

Kinetic Modelling of GlmU Reactions - Prioritization of Reaction for Therapeutic Application

Mycobacterium tuberculosis(Mtu), a successful pathogen, has developed resistance against the existing anti-tubercular drugs necessitating discovery of drugs with novel action. Enzymes involved in peptidoglycan biosynthesis are attractive targets for antibacterial drug discovery. The bifunctional enzyme mycobacterial GlmU (Glucosamine 1-phosphate N-acetyltransferase/ N-acetylglucosamine-1-phosphate uridyltransferase) has been a target enzyme for drug discovery. Its C- and N- terminal domains catalyze acetyltransferase (rxn-1) and uridyltransferase (rxn-2) activities respectively and the final product is involved in peptidoglycan synthesis. However, the bifunctional nature of GlmU poses difficulty in deciding which function to be intervened for therapeutic advantage. Genetic analysis showed this as an essential gene but it is still unclear whether any one or both of the activities are critical for cell survival. Often enzymatic activity with suitable high-throughput assay is chosen for random screening, which may not be the appropriate biological function inhibited for maximal effect. Prediction of rate-limiting function by dynamic network analysis of reactions could be an option to identify the appropriate function. With a view to provide insights into biochemical assays with appropriate activity for inhibitor screening, kinetic modelling studies on GlmU were undertaken. Kinetic model of Mtu GlmU-catalyzed reactions was built based on the available kinetic data on Mtu and deduction from Escherichia coli data. Several model variants were constructed including coupled/decoupled, varying metabolite concentrations and presence/absence of product inhibitions. This study demonstrates that in coupled model at low metabolite concentrations, inhibition of either of the GlmU reactions cause significant decrement in the overall GlmU rate. However at higher metabolite concentrations, rxn-2 showed higher decrement. Moreover, with available intracellular concentration of the metabolites and in vivo variant of model, uncompetitive inhibition of rxn-2 caused highest decrement. Thus, at physiologically relevant metabolite concentrations, targeting uridyltranferase activity of Mtu GlmU would be a better choice for therapeutic intervention.     

Identification of amino acids involved in catalytic process of M. tuberculosis GlmU acetyltransferase

M. tuberculosis GlmU is a bifunctional enzyme with acetyltransferase activity in C-terminus and uridyltransferase activity in N-terminus, and it is involved in the biosynthesis of glycosyl donor UDP-N-acetylglucosamine (UDP-GlcNAc). The crystal structure of M. tuberculosis GlmU clearly determines the active site and catalytic mechanism of GlmU uridyltransferase domain but not succeed in GlmU acetyltransferase domain. Sequence comparison analysis revealed highly conserved amino acid residues in the C-terminus between M. tuberculosis GlmU and GlmU enzymes from other bacteria. To find the essential amino acids related to M. tuberculosis GlmU acetyltransferase activity, we substituted 10 conserved amino acids in the acetyltransferase domain of M. tuberculosis GlmU by site-directed mutagenesis. All the mutant GlmU proteins were largely expressed in soluble and purified by affinity chromatography. Enzyme assays showed that K362A, H374A, Y398A and W460A mutants abolished more than 90?% activity of M. tuberculosis GlmU acetyltransferase and totally lost the affinity with two substrates, suggesting the potential substrate-binding functions. However, K403A, S416A, N456A and E458A mutants exhibited decreased GlmU acetyltransferase activity and lower kinetic parameters, probably responsible for substrate releasing by conformation shifting.