Kinetic measurements and mechanism determination of Stf0 sulfotransferase using mass spectrometry

Mycobacterial carbohydrate sulfotransferase Stf0 catalyzes the sulfuryl group transfer from 3'-phosphoadenosine-5'-phosphosulfate (PAPS) to trehalose. The sulfation of trehalose is required for the biosynthesis of sulfolipid-1, the most abundant sulfated metabolite found in Mycobacterium tuberculosis. In this paper, an efficient enzyme kinetics assay for Stf0 using electrospray ionization (ESI) mass spectrometry is presented. The kinetic constants of Stf0 were measured, and the catalytic mechanism of the sulfuryl group transfer reaction was investigated in initial rate kinetics and product inhibition experiments. In addition, Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry was employed to detect the noncovalent complexes, the Stf0-PAPS and Stf0-trehalose binary complexes, and a Stf0-3'-phosphoadenosine 5'-phosphate-trehalose ternary complex. The results from our study strongly suggest a rapid equilibrium random sequential Bi-Bi mechanism for Stf0 with formation of a ternary complex intermediate. In this mechanism, PAPS and trehalose bind and their products are released in random fashion. To our knowledge, this is the first detailed mechanistic data reported for Stf0, which further demonstrates the power of mass spectrometry in elucidating the reaction pathway and catalytic mechanism of promising enzymatic systems.     

Sulfolipid-1 biosynthesis restricts Mycobacterium tuberculosis growth in human macrophages

Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis, is a highly evolved human pathogen characterized by its formidable cell wall. Many unique lipids and glycolipids from the Mtb cell wall are thought to be virulence factors that mediate host-pathogen interactions. An intriguing example is Sulfolipid-1 (SL-1), a sulfated glycolipid that has been implicated in Mtb pathogenesis, although no direct role for SL-1 in virulence has been established. Previously, we described the biochemical activity of the sulfotransferase Stf0 that initiates SL-1 biosynthesis. Here we show that a stf0-deletion mutant exhibits augmented survival in human but not murine macrophages, suggesting that SL-1 negatively regulates the intracellular growth of Mtb in a species-specific manner. Furthermore, we demonstrate that SL-1 plays a role in mediating the susceptibility of Mtb to a human cationic antimicrobial peptide in vitro, despite being dispensable for maintaining overall cell envelope integrity. Thus, we hypothesize that the species-specific phenotype of the stf0 mutant is reflective of differences in antimycobacterial effector mechanisms of macrophages.     

The OtsAB pathway is essential for trehalose biosynthesis in Mycobacterium tuberculosis

The disaccharide trehalose is the major free sugar in the cytoplasm of mycobacteria; it is a constituent of cell wall glycolipids, and it plays a role in mycolic acid transport during cell wall biogenesis. The pleiotropic role of trehalose in the biology of Mycobacterium tuberculosis and its absence from mammalian cells suggests that its biosynthesis may provide a useful target for novel drugs. However, there are three potential pathways for trehalose biosynthesis in M. tuberculosis, and the aim of the present study was to introduce mutations into each of the pathways to determine whether or not they are functionally redundant. The results show that the OtsAB pathway, which generates trehalose from glucose and glucose-6-phosphate, is the dominant pathway required for M. tuberculosis growth in laboratory culture and for virulence in a mouse model. Of the two otsB homologues annotated in the genome sequence of M. tuberculosis, only OtsB2 (Rv3372) has a functional role in the pathway. OtsB2, trehalose-6-phosphate phosphatase, is strictly essential for growth and provides a tractable target for high throughput screening. Inactivation of the TreYZ pathway, which can generate trehalose from alpha-1,4-linked glucose polymers, had no effect on the growth of M. tuberculosis in vitro or in mice. Deletion of the treS gene altered the late stages of pathogenesis of M. tuberculosis in mice, significantly increasing the time to death in a chronic infection model. Because the TreS enzyme catalyzes the interconversion of trehalose and maltose, the mouse phenotype could reflect either a requirement for synthesis of additional trehalose or, conversely, a requirement for breakdown of stored trehalose to liberate free glucose.     

Trehalose is required for growth of Mycobacterium smegmatis

Mycobacteria contain high levels of the disaccharide trehalose in free form as well as within various immunologically relevant glycolipids such as cord factor and sulfolipid-1. By contrast, most bacteria use trehalose solely as a general osmoprotectant or thermoprotectant. Mycobacterium tuberculosis and Mycobacterium smegmatis possess three pathways for the synthesis of trehalose. Most bacteria possess only one trehalose biosynthesis pathway and do not elaborate the disaccharide into more complex metabolites, suggesting a distinct role for trehalose in mycobacteria. We disabled key enzymes required for each of the three pathways in M. smegmatis by allelic replacement. The resulting trehalose biosynthesis mutant was unable to proliferate and enter stationary phase unless supplemented with trehalose. At elevated temperatures, however, the mutant was unable to proliferate even in the presence of trehalose. Genetic complementation experiments showed that each of the three pathways was able to recover the mutant in the absence of trehalose, even at elevated temperatures. From a panel of trehalose analogs, only those with the native alpha,alpha-(1,1) anomeric stereochemistry rescued the mutant, whereas alternate stereoisomers and general osmo- and thermoprotectants were inactive. These findings suggest a dual role for trehalose as both a thermoprotectant and a precursor of critical cell wall metabolites.     

Elucidation and chemical modulation of sulfolipid-1 biosynthesis in Mycobacterium tuberculosis

Mycobacterium tuberculosis possesses unique cell-surface lipids that have been implicated in virulence. One of the most abundant is sulfolipid-1 (SL-1), a tetraacyl-sulfotrehalose glycolipid. Although the early steps in SL-1 biosynthesis are known, the machinery underlying the final acylation reactions is not understood. We provide genetic and biochemical evidence for the activities of two proteins, Chp1 and Sap (corresponding to gene loci rv3822 and rv3821), that complete this pathway. The membrane-associated acyltransferase Chp1 accepts a synthetic diacyl sulfolipid and transfers an acyl group regioselectively from one donor substrate molecule to a second acceptor molecule in two successive reactions to yield a tetraacylated product. Chp1 is fully active in vitro, but in M. tuberculosis, its function is potentiated by the previously identified sulfolipid transporter MmpL8. We also show that the integral membrane protein Sap and MmpL8 are both essential for sulfolipid transport. Finally, the lipase inhibitor tetrahydrolipstatin disrupts Chp1 activity in M. tuberculosis, suggesting an avenue for perturbing SL-1 biosynthesis in vivo. These data complete the SL-1 biosynthetic pathway and corroborate a model in which lipid biosynthesis and transmembrane transport are coupled at the membrane-cytosol interface through the activity of multiple proteins, possibly as a macromolecular complex.     

5'-adenosinephosphosulfate lies at a metabolic branch point in mycobacteria

Bacterial sulfate assimilation pathways provide for activation of inorganic sulfur for the biosynthesis of cysteine and methionine, through either adenosine 5'-phosphosulfate (APS) or 3'-phosphoadenosine 5'-phosphosulfate (PAPS) as intermediates. PAPS is also the substrate for sulfotransferases that produce sulfolipids, putative virulence factors, in Mycobacterium tuberculosis such as SL-1. In this report, genetic complementation using Escherichia coli mutant strains deficient in APS kinase and PAPS reductase was used to define the M. tuberculosis and Mycobacterium smegmatis CysH enzymes as APS reductases. Consequently, the sulfate assimilation pathway of M. tuberculosis proceeds from sulfate through APS, which is acted on by APS reductase in the first committed step toward cysteine and methionine. Thus, M. tuberculosis most likely produces PAPS for the sole use of this organism's sulfotransferases. Deletion of CysH from M. smegmatis afforded a cysteine and methionine auxotroph consistent with a metabolic branch point centered on APS. In addition, we have redefined the substrate specificity of the B. subtilis CysH, formerly designated a PAPS reductase, as an APS reductase, based on its ability to complement a mutant E. coli strain deficient in APS kinase. Together, these studies show that two conserved sequence motifs, CCXXRKXXPL and SXGCXXCT, found in the C termini of all APS reductases, but not in PAPS reductases, may be used to predict the substrate specificity of these enzymes. A functional domain of the M. tuberculosis CysC protein was cloned and expressed in E. coli, confirming the ability of this organism to make PAPS. The expression of recombinant M. tuberculosis APS kinase provides a means for the discovery of inhibitors of this enzyme and thus of the biosynthesis of SL-1.     

Trehalose synthase of Mycobacterium smegmatis: purification, cloning, expression, and properties of the enzyme.

Trehalose synthase (TreS) catalyzes the reversible interconversion of trehalose (glucosyl-alpha,alpha-1,1-glucose) and maltose (glucosyl-alpha1-4-glucose). TreS was purified from the cytosol of Mycobacterium smegmatis to give a single protein band on SDS gels with a molecular mass of approximately 68 kDa. However, active enzyme exhibited a molecular mass of approximately 390 kDa by gel filtration suggesting that TreS is a hexamer of six identical subunits. Based on amino acid compositions of several peptides, the treS gene was identified in the M. smegmatis genome sequence, and was cloned and expressed in active form in Escherichia coli. The recombinant protein was synthesized with a (His)(6) tag at the amino terminus. The interconversion of trehalose and maltose by the purified TreS was studied at various concentrations of maltose or trehalose. At a maltose concentration of 0.5 mm, an equilibrium mixture containing equal amounts of trehalose and maltose (42-45% of each) was reached during an incubation of about 6 h, whereas at 2 mm maltose, it took about 22 h to reach the same equilibrium. However, when trehalose was the substrate at either 0.5 or 2 mm, only about 30% of the trehalose was converted to maltose in >or= 12 h, indicating that maltose is the preferred substrate. These incubations also produced up to 8-10% free glucose. The K(m) for maltose was approximately 10 mm, whereas for trehalose it was approximately 90 mm. While beta,beta-trehalose, isomaltose (alpha1,6-glucose disaccharide), kojibiose (alpha1,2) or cellobiose (beta1,4) were not substrates for TreS, nigerose (alpha1,3-glucose disaccharide) and alpha,beta-trehalose were utilized at 20 and 15%, respectively, as compared to maltose. The enzyme has a pH optimum of about 7 and is inhibited in a competitive manner by Tris buffer. [(3)H]Trehalose is converted to [(3)H]maltose even in the presence of a 100-fold or more excess of unlabeled maltose, and [(14)C]maltose produces [(14)C]trehalose in excess unlabeled trehalose, suggesting the possibility of separate binding sites for maltose and trehalose. The catalytic mechanism may involve scission of the incoming disaccharide and transfer of a glucose to an enzyme-bound glucose, as [(3)H]glucose incubated with TreS and either unlabeled maltose or trehalose results in formation of [(3)H]disaccharide. TreS also catalyzes production of a glucosamine disaccharide from maltose and glucosamine, suggesting that this enzyme may be valuable in carbohydrate synthetic chemistry.     

A novel trehalase from Mycobacterium smegmatis - purification, properties, requirements

Trehalose is a nonreducing disaccharide of glucose (alpha,alpha-1,1-glucosyl-glucose) that is essential for growth and survival of mycobacteria. These organisms have three different biosynthetic pathways to produce trehalose, and mutants devoid of all three pathways require exogenous trehalose in the medium in order to grow. Mycobacterium smegmatis and Mycobacterium tuberculosis also have a trehalase that may be important in controlling the levels of intracellular trehalose. In this study, we report on the purification and characterization of the trehalase from M. smegmatis, and its comparison to the trehalase from M. tuberculosis. Although these two enzymes have over 85% identity throughout their amino acid sequences, and both show an absolute requirement for inorganic phosphate for activity, the enzyme from M. smegmatis also requires Mg(2+) for activity, whereas the M. tuberculosis trehalase does not require Mg(2+). The requirement for phosphate is unusual among glycosyl hydrolases, but we could find no evidence for a phosphorolytic cleavage, or for any phosphorylated intermediates in the reaction. However, as inorganic phosphate appears to bind to, and also to greatly increase the heat stability of, the trehalase, the function of the phosphate may involve stabilizing the protein conformation and/or initiating protein aggregation. Sodium arsenate was able to substitute to some extent for the sodium phosphate requirement, whereas inorganic pyrophosphate and polyphosphates were inhibitory. The purified trehalase showed a single 71 kDa band on SDS gels, but active enzyme eluted in the void volume of a Sephracryl S-300 column, suggesting a molecular mass of about 1500 kDa or a multimer of 20 or more subunits. The trehalase is highly specific for alpha,alpha-trehalose and did not hydrolyze alpha,beta-trelalose or beta,beta-trehalose, trehalose dimycolate, or any other alpha-glucoside or beta-glucoside. Attempts to obtain a trehalase-negative mutant of M. smegmatis have been unsuccessful, although deletions of other trehalose metabolic enzymes have yielded viable mutants. This suggests that trehalase is an essential enzyme for these organisms. The enzyme has a pH optimum of 7.1, and is active in various buffers, as long as inorganic phosphate and Mg(2+) are present. Glucose was the only product produced by the trehalase in the presence of either phosphate or arsenate.     

Activation of human neutrophils by Mycobacterium tuberculosis-derived sulfolipid-1.

The principal sulfatide of a group of acidic lipids from virulent Mycobacterium tuberculosis, sulfolipid-1 (SL-1), stimulates neutrophil superoxide (O2-) generation and, at lower concentrations, primes neutrophil response to several other metabolic agonists including FMLP, and PMA. These responses to SL-1 were examined in relation to diacylglycerol (DAG) generation, Ca2+ availability and activation of guanine nucleotide binding proteins to clarify the signal transduction pathways involved. Pertussis toxin inhibited the ability of SL-1 to both stimulate neutrophils directly and to prime neutrophils for subsequent responses induced by PMA, suggesting a role for one or more guanine nucleotide regulating proteins in both responses. SL-1 induced a rise in neutrophil DAG levels. DAG generation was inhibited by pretreatment of cells with pertussis toxin. Depletion of extracellular Ca2+ ablated O2- release induced by stimulatory levels of SL-1 but did not inhibit the priming effect induced by substimulatory concentrations of the lipid. Investigation of the activation of the neutrophil NADPH oxidase in a cell-free system revealed that the SL-1-priming effect was associated with translocation of the soluble cytosolic factors required for activation of the enzyme. Cytosolic factor translocation was not observed in pertussis toxin pretreated cells. Our results provide evidence for the role of a guanine nucleotide binding protein in both priming and direct activation of neutrophils by SL-1. This G protein regulates both SL-1-induced DAG generation and cytosolic cofactor translocation involved in neutrophil activation and priming. The multiplicity of effects of SL-1 on signal transduction pathways leading to phagocyte activation and priming may exert a profound influence on the pathogenicity of M. tuberculosis.     

Molecular characterization of the Aspergillus nidulans treA gene encoding an acid trehalase required for growth on trehalose

Aspergillus nidulans conidiospores contain high levels of the non-reducing disaccharide trehalose. We show that upon induction of conidiospore germination, the trehalose pool is rapidly degraded and a glycerol pool is transiently accumulated. A trehalase with an acidic pH optimum was purified from conidiospores. Characterization of the treA gene encoding this trehalase shows that it is homologous to Saccharomyces cerevisiae vacuolar acid trehalase, the product of the ATH1 gene, and to two related proteins of unknown function identified in Mycobacterium tuberculosis and Mycobacterium leprae . A. nidulans mutants that lack acid trehalase activity were constructed by gene replacement at the treA locus. Analysis of these mutants suggests that the treA gene product is localized in the conidiospore wall, is required for growth on trehalose as a carbon source, and is not involved in the mobilization of the intracellular pool of trehalose. Therefore, it is proposed that a cytoplasmic regulatory trehalase is controlling this latter process.     

The role of MmpL8 in sulfatide biogenesis and virulence of Mycobacterium tuberculosis

To study the role of MmpL8-mediated lipid transport in sulfatide biogenesis, we insertionally inactivated the mmpL8 gene in Mycobacterium tuberculosis. Characterization of this strain showed that the synthesis of mature sulfolipid SL-1 was interrupted and that a more polar sulfated molecule, termed SL-N, accumulated within the cell. Purification of SL-N and structural analysis identified this molecule as a family of 2,3-diacyl-alpha,alpha'-D-trehalose-2'-sulfates. This structure suggests that transport and biogenesis of SL-1 are coupled and that the final step in sulfatide biosynthesis may be the extra-cellular esterification of two trehalose 6-positions with hydroxyphthioceranic acids. To assess the effect of the loss of this anionic surface lipid on virulence, we infected mice via aerosol with the MmpL8 mutant and found that, although initial replication rates and containment levels were identical, compared with the wild type, a significant attenuation of the MmpL8 mutant strain in time-to-death was observed. Early in infection, differential expression of cytokines and cytokine receptors revealed that the mutant strain less efficiently suppresses key indicators of a Th1-type immune response, suggesting an immunomodulatory role for sulfatides in the pathogenesis of tuberculosis.     

Uptake of unnatural trehalose analogs as a reporter for Mycobacterium tuberculosis

The detection of tuberculosis currently relies upon insensitive and unspecific techniques; newer diagnostics would ideally co-opt specific bacterial processes to provide real-time readouts. The trehalose mycolyltransesterase enzymes (antigens 85A, 85B and 85C (Ag85A, Ag85B, Ag85C)) serve as essential mediators of cell envelope function and biogenesis in Mycobacterium tuberculosis. Through the construction of a systematically varied sugar library, we show here that Ag85 enzymes have exceptionally broad substrate specificity. This allowed exogenously added synthetic probes to be specifically incorporated into M. tuberculosis growing in vitro and within macrophages. Even bulky substituents, such as a fluorescein-containing trehalose probe (FITC-trehalose), were incorporated by growing bacilli, thereby producing fluorescent bacteria; microscopy revealed selective labeling of poles and membrane. Addition of FITC-trehalose to M. tuberculosis-infected macrophages allowed selective, sensitive detection of M. tuberculosis within infected mammalian macrophages. These studies suggest that analogs of trehalose may prove useful as probes of function and for other imaging modalities.     

The Mycobacterium tuberculosis Ag85A is a novel diacylglycerol acyltransferase involved in lipid body formation

Mycobacterium tuberculosis accumulates large amounts of triacylglycerol (TAG) which acts as storage compounds for energy and carbon. The mycobacterial triacylglycerols stored in the form of intracellular lipid droplets are essential for long-term survival of M. tuberculosis during a dormant state. We report here that when the M. tuberculosis mycolytransferase Ag85A is overexpressed in Mycobacterium smegmatis mc(2)155, cell morphology was changed and the cells became grossly enlarged. A massive formation of lipid bodies and a change in lipid pattern was observed simultaneously. We suspected a possible role of Ag85A in the acyl lipid metabolism and discovered that the enzyme possesses acyl-CoA:diacylglycerol acyltransferase (DGAT) activity in addition to its well-known function as mycolyltransferase. Ag85A mediates the transesterification of diacylglycerol using long-chain acyl-CoA as acyl donors. The K(m) and K(cat) values for palmitoleoyl-coenzyme A were 390 μM and 55.54 min(-1) respectively. A docking model suggests that palmitoleoyl-coenzyme A and 1,2-dipalmitin occupy the same active site as trehalose 6,6'-dimycolate and trehalose 6'-monomycolate. The site-directed Ser126Ala mutation of the active site proved that this residue is involved in the catalytic activity of this enzyme. Although not proven conclusively for dormant stage of M. tuberculosis, our novel finding about the synthesis of TAGs by Ag85A strongly suggests that Ag85A may play a significant role in the formation of lipid storage bodies and thus also in the establishment and maintenance of a persistent tuberculosis infection.     

Oxidative stress response and its role in sensitivity to isoniazid in mycobacteria: characterization and inducibility of ahpC by peroxides in Mycobacterium smegmatis and lack of expression in M. aurum and M. tuberculosis.

Mycobacterium tuberculosis is a natural mutant with inactivated oxidative stress regulatory gene oxyR. This characteristic has been linked to the exquisite sensitivity of M. tuberculosis to isonicotinic acid hydrazide (INH). In the majority of mycobacteria tested, including M. tuberculosis, oxyR is divergently transcribed from ahpC, a gene encoding a homolog of the subunit of alkyl hydroperoxide reductase that carries out substrate peroxide reduction. Here we compared ahpC expression in Mycobacterium smegmatis, a mycobacterium less sensitive to INH, with that in two highly INH sensitive species, M. tuberculosis and Mycobacterium aurum. The ahpC gene of M. smegmatis was cloned and characterized, and the 5' ends of ahpC mRNA were mapped by S1 nuclease protection analysis. M. smegmatis AhpC and eight other polypeptides were inducible by exposure to H2O2 or organic peroxides, as determined by metabolic labeling and Western blot (immunoblot) analysis. In contrast, M. aurum displayed differential induction of only one 18-kDa polypeptide when exposed to organic peroxides. AhpC could not be detected in this organism by immunological means. AhpC was also below detection levels in M. tuberculosis H37Rv. These observations are consistent with the interpretation that ahpC expression and INH sensitivity are inversely correlated in the mycobacterial species tested. In further support of this conclusion, the presence of plasmid-borne ahpC reduced M. smegmatis susceptibility to INH. Interestingly, mutations in the intergenic region between oxyR and ahpC were identified and increased ahpC expression observed in deltakatG M. tuberculosis and Mycobacterium bovis INH(r) strains. We propose that mutations activating ahpC expression may contribute to the emergence of INH(r) strains.     

Synthesis of mono- and dideoxygenated alpha,alpha-trehalose analogs

In this work, we describe the synthesis and NMR characterization of four mono- and four dideoxygenated analogs of alpha,alpha-D-trehalose. The symmetrical (2,2'-, 3,3'-, 4,4'- and 6,6'-) dideoxy analogs were obtained via selective protection and subsequent radical deoxygenation of the desired hydroxyl group set. The unsymmetrical (2'-, 3'-, 4'- and 6'-) monodeoxy analogs were synthesized by desymmetrization of alpha,alpha-trehalose and subsequent deoxygenation under radical conditions. Complete assignment of all (1)H and (13)C resonances in the spectra of these deoxytrehaloses was achieved through the extensive use of 2D [(1)H,(1)H] and [(1)H,(13)C] correlation NMR experiments. The synthesis of these trehalose analogs sets the stage for future biochemical and NMR-based studies to probe the substrate interactions of trehalose with the recently identified mycobacterial sulfotransferase Stf0.     

Osmotic adaptation of Thermus thermophilus RQ-1: lesson from a mutant deficient in synthesis of trehalose

Strains of Thermus thermophilus accumulate primarily trehalose and smaller amounts of mannosylglycerate in response to salt stress in yeast extract-containing media (O. C. Nunes, C. M. Manaia, M. S. da Costa, and H. Santos, Appl. Environ. Microbiol. 61:2351-2357, 1995). A 2.4-kbp DNA fragment from T. thermophilus strain RQ-1 carrying otsA (encoding trehalose-phosphate synthase [TPS]), otsB (encoding trehalose-phosphate phosphatase [TPP]), and a short sequence of the 5' end of treS (trehalose synthase [TreS]) was cloned from a gene library. The sequences of the three genes (including treS) were amplified by PCR and sequenced, revealing that the genes were structurally linked. To understand the role of trehalose during salt stress in T. thermophilus RQ-1, we constructed a mutant, designated RQ-1M6, in which TPS (otsA) and TPP (otsB) genes were disrupted by gene replacement. Mutant RQ-1M6 accumulated trehalose and mannosylglycerate in a medium containing yeast extract and NaCl. However, growth in a defined medium (without yeast extract, known to contain trehalose) containing NaCl led to the accumulation of mannosylglycerate but not trehalose. The deletion of otsA and otsB reduced the ability to grow in defined salt-containing medium, with the maximum salinity being 5% NaCl for RQ-1 and 3% NaCl for RQ-1M6. The lower salt tolerance observed in the mutant was relieved by the addition of trehalose to the growth media. In contrast to trehalose, the addition of glycine betaine, mannosylglycerate, maltose, and glucose to the growth medium did not allow the mutant to grow at higher salinities. The results presented here provide crucial evidence for the importance of the TPS/TPP pathway for the synthesis and accumulation of trehalose and the decisive contribution of this disaccharide to osmotic adaptation in T. thermophilus RQ-1.     

Sulfolipid accumulation in <Emphasis Type="Italic">Mycobacterium tuberculosis</Emphasis> disrupted in the <Emphasis Type="Italic">mce2</Emphasis> operon

Mycobacterium tuberculosis, the causative agent of tuberculosis, has a lipid-rich cell wall that serves as an effective barrier against drugs and toxic host cell products, which may contribute to the organism’s persistence in a host. M. tuberculosis contains four homologous operons called nice (mce1–4) that encode putative ABC transporters involved in lipid importation across the cell wall. Here, we analyzed the lipid composition of M. tuberculosis disrupted in the mce2 operon. High resolution mass spectrometric and thin layer chromatographic analyses of the mutant’s cell wall lipid extracts showed accumulation of SL-1 and SL₁₂₇₈ molecules. Radiographic quantitative analysis and densitometry revealed 2.9, 3.9 and 9.8-fold greater amount of [³⁵S] SL-1 in the mce2 operon mutant compared to the wild type M. tuberculosis during the early/mid logarithmic, late logarithmic and stationary phase of growth in liquid broth, respectively. The amount of [³⁵S] SL₁₂₇₈ in the mutant also increased progressively over the same growth phases. The expression of the mce2 operon genes in the wild type strain progressively increased from the logarithmic to the stationary phase of bacterial growth in vitro, which inversely correlated with the proportion of radiolabel incorporation into SL-1 and SL₁₂₇₈ at these phases. Since the mce2 operon is regulated in wild type M. tuberculosis, its cell wall may undergo changes in SL-1 and SL₁₂₇₈ contents during a natural course of infection and this may serve as an important adaptive strategy for M. tuberculosis to maintain persistence in a host.     

Analysis and modeling of mycolyl-transferases in the CMN group

Mycolyl-transferases are a family of proteins that are specifically present in the CMN (Corynebacterium, Mycobacterium and Nocardia) genera and are responsible for the synthesis of cell wall components. We modeled the three-dimensional structures of mycolyl-transfersases from Corynebacterium and Nocardia using homology modeling methods based on the crystal structures of mycolyl-transferases from M. tuberculosis. Comparison of the models revealed significant differences in their substrate binding site. Some mycolyl-transferases identified by the following Gene Ids: Nfa25110, Nfa45560, Nfa7210, Nfa38260, Nfa32420, Nfa23770, Nfa43800, Nfa30260, Dip0365, Ncgl0987, Ce1488, Ncgl0885, Ce0984, Ncgl2101, Ncgl0336, Ce0356 are associated with a relatively larger substrate binding site and amino acid residue mutations (D40N, R43D/G, S236N/A) are likely to affect binding to trehalose.     

Characterization of sulfolipids of Mycobacterium tuberculosis H37Rv by multiple-stage linear ion-trap high-resolution mass spectrometry with electrospray ionization reveals that the family of sulfolipid II predominates

Mycobacterium tuberculosis, the causative agent of tuberculosis, is unique among bacterial pathogens in that it contains a wide array of complex lipids and lipoglycans on its cell wall. Among them, the sulfated glycolipid, termed the sulfolipid, is thought to mediate specific host-pathogen interactions during infection. Sulfolipids (SLs), including sulfolipid I (SL-I) and sulfolipid II (SL-II), are 2,3,6,6'-tetraacyltrehalose 2'-sulfates. SL-I was identified as a family of homologous 2-palmitoyl(stearoyl)-3-phthioceranoyl-6,6'-bis(hydroxyphthioceranoy1)trehalose 2'-sulfates and was believed to be the principal sulfolipid of M. tuberculosis strain H37Rv. We cultured and extracted sulfolipids using various conditions, including those originally described, and employed high-resolution multiple-stage linear ion-trap mass spectrometry with electrospray ionization to characterize the structure of the principal SL. We revealed that SL-II, a family of homologous 2-stearoyl(palmitoyl)-3,6,6'-tris(hydroxyphthioceranoy1)trehalose 2'-sulfates, rather than SL-I is the principal sulfolipid class. We identified a great number of isomers resulting from permutation of the various hydroxyphthioceranoyl substituents at positions 6 and 6' of the trehalose backbone for each of the SL-II species in the entire family. We redefined the structure of this important lipid family that was misassigned using the traditional methods 40 years ago.     

Expression, purification and properties of shikimate dehydrogenase from Mycobacterium tuberculosis

Tuberculosis, caused by Mycobacterium tuberculosis, continues to be one of the main diseases to mankind. It is urgent to discover novel drug targets for appropriate antimicrobial agents against this human pathogen. The shikimate pathway is considered as an attractive target for the discovery of novel antibiotics for its essentiality in bacteria and absence in mammalian cells. The Mycobacterium tuberculosis aroE-encoded shikimate dehydrogenase was cloned, expressed and purified. Sequence alignment analysis shows that shikimate dehydrogenase of Mycobacterium tuberculosis exhibit the pattern of G-X-(N/S)-V-(T/S)-X-PX-K, which is highly conserved within the shikimate dehydrogenase family. The recombinant shikimate dehydrogenase spectrum determined by CD spectroscopy showed that the percentages for alpha-helix, beta-sheet, beta-turn, and random coil were 29.2 %, 9.3 %, 32.7 %, and 28.8 %, respectively. The enzymatic characterization demonstrates that it appears to be fully active at pH from 9.0 to 12, and temperature 63(o)C. The apparent Michaelis constant for shikimic acid and NADP(+) were calculated to be about 29.5 microM and 63 microM. The recombinant shikimate dehydrogenase catalyzes the substrate in the presence of NADP(+) with an enzyme turnover number of 399 s(-1). Zymological studies suggest that the cloned shikimate dehydrogenase from M. tuberculosis has a pretty activity, and the work should help in the discovery of enzyme inhibitors and further of possible antimicrobial agents against Mycobacterium tuberculosis.