Novel structures of N-linked high-mannose type oligosaccharides containing alpha-D-galactofuranosyl linkages in Aspergillus niger alpha-D-glucosidase

Biochemical characterization of a glycoside hydrolase family 43 β-D-galactofuranosidase from the fungus Aspergillus niger

β-D-Galactofuranose (Galf) and its polysaccharides are found in bacteria, fungi and protozoa but do not occur in mammalian tissues, and thus represent a specific target for anti-pathogenic drugs. Understanding the enzymatic degradation of these polysaccharides is therefore of great interest, but the identity of fungal enzymes with exclusively galactofuranosidase activity has so far remained elusive. Here we describe the identification and characterization of a galactofuranosidase from the industrially important fungus Aspergillus niger. Analysis of glycoside hydrolase family 43 subfamily 34 (GH43_34) members via conserved unique peptide patterns and phylogeny, revealed the occurrence of distinct clusters and, by comparison with specificities of characterized bacterial members, suggested a basis for prediction of enzyme specificity. Using this rationale, in tandem with molecular docking, we identified a putative β-D-galactofuranosidase from A. niger which was recombinantly produced in Escherichia coli. The Galf-specific hydrolase, encoded by xynD demonstrates maximum activity at pH 5, 25 °C towards 4-nitrophenyl-β-galactofuranoside (pNP-β-Galf), with a K_m of 17.9 ± 1.9 mM and V_max of 70.6 ± 5.3 µM min^-1. The characterization of this first fungal GH43 galactofuranosidase offers further molecular insight into the degradation of Galf-containing structures.

Galactofuranose antigens, a target for diagnosis of fungal infections in humans

The use of biomarkers for the detection of fungal infections is of interest to complement histopathological and culture methods. Since the production of antibodies in immunocompromised patients is scarce, detection of a specific antigen could be effective for early diagnosis. D-Galactofuranose (Galf) is the antigenic epitope in glycoconjugates of several pathogenic fungi. Since Galf is not biosynthesized by mammals, it is an attractive candidate for diagnosis of infection. A monoclonal antibody that recognizes Galf is commercialized for detection of aspergillosis. The linkage of Galf in the natural glycans and the chemical structures of the synthesized Galf-containing oligosaccharides are described in this paper. The oligosaccharides could be used for the synthesis of artificial carbohydrate-based antigens, not enough exploited for diagnosis.

D-Galactofuranose (Galf) is the unit in polysaccharides and glycoconjugates of several pathogenic fungi that is recognized by the immune system. Since Galf is not synthesized by mammals, it is an attractive candidate for diagnosis of infection. Since the production of antibodies in immunocompromised patients is scarce, detection of a specific antigen could be effective for early diagnosis. An antibody that recognizes Galf is commercialized for the detection of aspergillosis. Chemically synthesized Galf-containing oligosaccharides, reviewed in this paper, could therefore be used for the synthesis of artificial carbohydrate-based antigens and in diagnosis.

Mapping N-linked glycosylation of carbohydrate-active enzymes in the secretome of Aspergillus nidulans grown on lignocellulose

BACKGROUND

The genus Aspergillus includes microorganisms that naturally degrade lignocellulosic biomass, secreting large amounts of carbohydrate-active enzymes (CAZymes) that characterize their saprophyte lifestyle. Aspergillus has the capacity to perform post-translational modifications (PTM), which provides an additional advantage for the use of these organisms as a host for the production of heterologous proteins. In this study, the N-linked glycosylation of CAZymes identified in the secretome of Aspergillus nidulans grown on lignocellulose was mapped.

RESULTS

Aspergillus nidulans was grown in glucose, xylan and pretreated sugarcane bagasse (SCB) for 96 h, after which glycoproteomics and glycomics were carried out on the extracellular proteins (secretome). A total of 265 proteins were identified, with 153, 210 and 182 proteins in the glucose, xylan and SCB substrates, respectively. CAZymes corresponded to more than 50 % of the total secretome in xylan and SCB. A total of 182 N-glycosylation sites were identified, of which 121 were detected in 67 CAZymes. A prevalence of the N-glyc sequon N-X-T (72.2 %) was observed in N-glyc sites compared with N-X-S (27.8 %). The amino acids flanking the validated N-glyc sites were mainly composed of hydrophobic and polar uncharged amino acids. Selected proteins were evaluated for conservation of the N-glyc sites in Aspergilli homologous proteins, but a pattern of conservation was not observed. A global analysis of N-glycans released from the proteins secreted by A. nidulans was also performed. While the proportion of N-glycans with Hex5 to Hex9 was similar in the xylan condition, a prevalence of Hex5 was observed in the SCB and glucose conditions.

CONCLUSIONS

The most common and frequent N-glycosylated motifs, an overview of the N-glycosylation of the CAZymes and the number of mannoses found in N-glycans were analyzed. There are many bottlenecks in protein production by filamentous fungi, such as folding, transport by vesicles and secretion, but N-glycosylation in the correct context is a fundamental event for defining the high levels of secretion of target proteins. A comprehensive analysis of the protein glycosylation processes in A. nidulans will assist with a better understanding of glycoprotein structures, profiles, activities and functions. This knowledge can help in the optimization of heterologous expression and protein secretion in the fungal host.

Glycosylation variants of a β-glucosidase secreted by a Taiwanese fungus, Chaetomella raphigera, exhibit variant-specific catalytic and biochemical properties

Cellulosic biomass is an abundant and promising energy source. To make cellulosic biofuels competitive against conventional fuels, conversion of rigid plant materials into sugars must become efficient and cost-effective. During cellulose degradation, cellulolytic enzymes generate cellobiose (β-(1→4)-glucose dimer) molecules, which in turn inhibit such enzymes by negative feedback. β-Glucosidases (BGLs) cleave cellobiose into glucose monomers, assisting overall cellulolytic activities. Therefore, BGLs are essential for efficient conversion of cellulosic biomass into biofuels, and it is important to characterize newly isolated BGLs for useful traits. Here, we report our discovery that the indigenous Taiwanese fungus Chaetomella raphigera strain D2 produces two molecular weight variants of a single BGL, D2-BGL (shortened to “D2”), which differ in O-glycosylation. The more extensively O-glycosylated form of native D2 (nD2L) has increased activity toward the natural substrate, cellobiose, compared to the less O-glycosylated form (nD2S). nD2L is more stable at 60°C, in acidic pH, and in the presence of the ionic detergent sodium dodecyl sulfate than nD2S. Furthermore, unlike nD2S, nD2L does not display substrate inhibition by an artificial substrate p-nitrophenyl glucopyranoside (pNPG), and the glucose feedback inhibition kinetics of nD2L is competitive (while it is non-competitive for nD2S), suggesting that these two glycovariants of D2 bind substrates differently. Interestingly, D2 produced in a heterologous system, Pichia pastoris, closely mimics properties of nD2S. Our studies suggest that O-glycosylation of D2 is important in determining its catalytic and biochemical properties.

GfsA encodes a novel galactofuranosyltransferase involved in biosynthesis of galactofuranose antigen of O-glycan in Aspergillus nidulans and Aspergillus fumigatus

The cells walls of filamentous fungi in the genus Aspergillus have galactofuranose (Galf)-containing polysaccharides and glycoconjugates, including O-glycans, N-glycans, fungal-type galactomannan and glycosylinositolphosphoceramide, which are important for cell wall integrity. Here, we attempted to identify galactofuranosyltransferases that couple Galf monomers onto other wall components in Aspergillus nidulans. Using reverse-genetic and biochemical approaches, we identified that the AN8677 gene encoded a galactofuranosyltransferase, which we called GfsA, involved in Galf antigen biosynthesis. Disruption of gfsA reduced binding of β-Galf-specific antibody EB-A2 to O-glycosylated WscA protein and galactomannoproteins. The results of an in-vitro Galf antigen synthase assay revealed that GfsA has β1,5- or β1,6-galactofuranosyltransferase activity for O-glycans in glycoproteins, uses UDP-d-Galf as a sugar donor, and requires a divalent manganese cation for activity. GfsA was found to be localized at the Golgi apparatus based on cellular fractionation experiments. ΔgfsA cells exhibited an abnormal morphology characterized by poor hyphal extension, hyphal curvature and limited formation of conidia. Several gfsA orthologues were identified in members of the Pezizomycotina subphylum of Ascomycota, including the human pathogen Aspergillus fumigatus. To our knowledge, this is the first characterization of a fungal β-galactofuranosyltransferase, which was shown to be involved in Galf antigen biosynthesis of O-glycans in the Golgi.

Approaching the secrets of N-glycosylation in Aspergillus fumigatus: characterization of the AfOch1 protein

The mannosyltransferase Och1 is the key enzyme for synthesis of elaborated protein N-glycans in yeast. In filamentous fungi genes implicated in outer chain formation are present, but their function is unclear. In this study we have analyzed the Och1 protein of Aspergillus fumigatus. We provide first evidence that poly-mannosylated N-glycans exist in A. fumigatus and that their synthesis requires AfOch1 activity. This implies that AfOch1 plays a similar role as S. cerevisiae ScOch1 in the initiation of an N-glycan outer chain. A Δafoch1 mutant showed normal growth under standard and various stress conditions including elevated temperature, cell wall and oxidative stress. However, sporulation of this mutant was dramatically reduced in the presence of high calcium concentrations, suggesting that certain proteins engaged in sporulation require N-glycan outer chains to be fully functional. A characteristic feature of AfOch1 and Och1 homologues from other filamentous fungi is a signal peptide that clearly distinguishes them from their yeast counterparts. However, this difference does not appear to have consequences for its localization in the Golgi. Replacing the signal peptide of AfOch1 by a membrane anchor had no impact on its ability to complement the sporulation defect of the Δafoch1 strain. The mutant triggered a normal cytokine response in infected murine macrophages, arguing against a role of outer chains as relevant Aspergillus pathogen associated molecular patterns. Infection experiments provided no evidence for attenuation in virulence; in fact, according to our data the Δafoch1 mutant may even be slightly more virulent than the control strains.

Chemoenzymatic synthesis, inhibition studies, and X-ray crystallographic analysis of the phosphono analog of UDP-Galp as an inhibitor and mechanistic probe for UDP-galactopyranose mutase

UDP (uridine diphosphate) galactopyranose mutase (UGM) is involved in the cell wall biosynthesis of many pathogenic microorganisms. UGM catalyzes the reversible conversion of UDP-α-D-galactopyranose into UDP-α-D-galactofuranose, with the latter being the precursor of galactofuranose (Galf) residues in cell walls. Glycoconjugates of Galf are essential components in the cell wall of various pathogenic bacteria, including Mycobacterium tuberculosis, the causative agent of tuberculosis. The absence of Galf in humans and its bacterial requirement make UGM a potential target for developing novel antibacterial agents. In this article, we report the synthesis, inhibitory activity, and X-ray crystallographic studies of UDP-phosphono-galactopyranose, a nonhydrolyzable C-glycosidic phosphonate. This is the first report on the synthesis of a phosphonate analog of UDP-α-D-galactopyranose by a chemoenzymatic phosphoryl coupling method. The phosphonate was evaluated against three bacterial UGMs and showed only moderate inhibition. We determined the crystal structure of the phosphonate analog bound to Deinococcus radiodurans UGM at 2.6 Å resolution. The phosphonate analog is bound in a novel conformation not observed in UGM-substrate complex structures or in other enzyme-sugar nucleotide phosphonate complexes. This complex structure provides a structural basis for the observed micromolar inhibition towards UGM. Steric clashes, loss of electrostatic stabilization between an active-site arginine (Arg305) and the phosphonate analog, and a 180° flip of the hexose moiety account for the differences in the binding orientations of the isosteric phosphonate analog and the physiological substrate. This provides new insight into the ability of a sugar-nucleotide-binding enzyme to orient a substrate analog in an unexpected geometry and should be taken into consideration in designing such enzyme inhibitors.

Production, purification, and characterization of human alpha1 proteinase inhibitor from Aspergillus niger

Human alpha one proteinase inhibitor (alpha1-PI) was cloned and expressed in Aspergillus niger, filamentious fungus that can grow in defined media and can perform glycosylation. Submerged culture conditions were established using starch as carbon source, 30% dissolved oxygen concentration, pH 7.0 and 28 degrees C. Eight milligrams per liter of active alpha1-PI were secreted to the growth media in about 40 h. Controlling the protein proteolysis was found to be an important factor in the production. The effects of various carbon sources, pH and temperature on the production and stability of the protein were tested and the product was purified and characterized. Two molecular weights variants of the recombinant alpha1-PI were produced by the fungus; the difference is attributed to the glycosylated part of the molecule. The two glycoproteins were treated with PNGAse F and the released glycans were analyzed by HPAEC, MALDI/TOF-MS, NSI-MS(n), and GC-MS. The MALDI and NSI- full MS spectra of permethylated N-glycans revealed that the N-glycans of both variants contain a series of high-mannose type glycans with 5-20 hexose units. Monosaccharide analysis showed that these were composed of N-acetylglucos-amine, mannose, and galactose. Linkage analysis revealed that the galactosyl component was in the furanoic conformation, which was attaching in a terminal non-reducing position. The Galactofuranose-containing high-mannnose type N-glycans are typical structures, which recently have been found as part of several glycoproteins produced by Aspergillus niger.

Contribution of galactofuranose to the virulence of the opportunistic pathogen Aspergillus fumigatus

The filamentous fungus Aspergillus fumigatus is responsible for a lethal disease called invasive aspergillosis that affects immunocompromised patients. This disease, like other human fungal diseases, is generally treated by compounds targeting the primary fungal cell membrane sterol. Recently, glucan synthesis inhibitors were added to the limited antifungal arsenal and encouraged the search for novel targets in cell wall biosynthesis. Although galactomannan is a major component of the A. fumigatus cell wall and extracellular matrix, the biosynthesis and role of galactomannan are currently unknown. By a targeted gene deletion approach, we demonstrate that UDP-galactopyranose mutase, a key enzyme of galactofuranose metabolism, controls the biosynthesis of galactomannan and galactofuranose containing glycoconjugates. The glfA deletion mutant generated in this study is devoid of galactofuranose and displays attenuated virulence in a low-dose mouse model of invasive aspergillosis that likely reflects the impaired growth of the mutant at mammalian body temperature. Furthermore, the absence of galactofuranose results in a thinner cell wall that correlates with an increased susceptibility to several antifungal agents. The UDP-galactopyranose mutase thus appears to be an appealing adjunct therapeutic target in combination with other drugs against A. fumigatus. Its absence from mammalian cells indeed offers a considerable advantage to achieve therapeutic selectivity.

N-glycan modification in Aspergillus species

The production by filamentous fungi of therapeutic glycoproteins intended for use in mammals is held back by the inherent difference in protein N-glycosylation and by the inability of the fungal cell to modify proteins with mammalian glycosylation structures. Here, we report protein N-glycan engineering in two Aspergillus species. We functionally expressed in the fungal hosts heterologous chimeric fusion proteins containing different localization peptides and catalytic domains. This strategy allowed the isolation of a strain with a functional alpha-1,2-mannosidase producing increased amounts of N-glycans of the Man5GlcNAc2 type. This strain was further engineered by the introduction of a functional GlcNAc transferase I construct yielding GlcNAcMan5GlcNac2 N-glycans. Additionally, we deleted algC genes coding for an enzyme involved in an early step of the fungal glycosylation pathway yielding Man3GlcNAc2 N-glycans. This modification of fungal glycosylation is a step toward the ability to produce humanized complex N-glycans on therapeutic proteins in filamentous fungi.

Protein O-glycosylation in fungi: diverse structures and multiple functions

Protein glycosylation is essential for eukaryotic cells from yeasts to humans. When compared to N-glycosylation, O-glycosylation is variable in sugar components and the mode of linkages connecting the sugars. In fungi, secretory proteins are commonly mannosylated by protein O-mannosyltransferase (PMT) in the endoplasmic reticulum, and subsequently glycosylated by several glycosyltransferases in the Golgi apparatus to form glycoproteins with diverse O-glycan structures. Protein O-glycosylation has roles in modulating the function of secretory proteins by enhancing the stability and solubility of the proteins, by affording protection from protease degradation, and by acting as a sorting determinant in yeasts. In filamentous fungi, protein O-glycosylation contributes to proper maintenance of fungal morphology, hyphal development, and differentiation. This review describes recent studies of the structure and function of protein O-glycosylation in industrially and medically important fungi.

Chemical probes of UDP-galactopyranose mutase

Many pathogenic prokaryotes and eukaryotes possess the machinery required to assemble galactofuranose (Galf)-containing glycoconjugates; these glycoconjugates can be critical for virulence or viability. Accordingly, compounds that block Galf incorporation may serve as therapeutic leads or as probes of the function of Galf-containing glycoconjugates. The enzyme UDP-galactopyranose mutase (UGM) is the only known generator of UDP-galactofuranose, the precursor to Galf residues. We previously employed a high-throughput fluorescence polarization assay to investigate the Klebsiella pneumoniae UGM. We demonstrate the generality of this assay by extending it to UGM from Mycobacterium tuberculosis. To identify factors influencing binding, we synthesized a directed library containing a 5-arylidene-2-thioxo-4-thiazolidinone core, a structure possessing features common to ligands for both homologs. Our studies offer a blueprint for identifying inhibitors of the growing family of UGM homologs and provide insight into UGM inhibition.

Synthesis of α-d-Galf-(1→2)-d-galactitol and α-d-Galf-(1→2)[β-d-Galf-(1→3)]-d-galactitol, oligosaccharide derivatives from Bacteroides cellulosolvens glycoproteins

The synthesis of alpha-D-galactofuranosyl-(1-->2)-D-galactitol, which has been isolated by reductive beta-elimination from glycoproteins of Bacteroides cellulosolvens and Clostridium thermocellum, is described. The approach of selective glycosylation of an aldono-1,4-lactone by the trichloroacetimidate method was employed. The synthesis of alpha-D-Gal f-(1-->2)[beta-D-Gal f-(1-->3)]-D-Galol, that contains Gal f units in both anomeric configurations, is also reported. These are the first synthetic oligosaccharides with alpha-D-Gal f, previously found in natural products.

Galactomannoproteins of Aspergillus fumigatus

Galactofuranose-containing molecules have been repeatedly shown to be important antigens among human fungal pathogens, including Aspergillus fumigatus. Immunogenic galactofuran determinants have been poorly characterized chemically, however. We reported here the characterization of two glycoproteins of A. fumigatus with an N-glycan containing galactofuranose. These proteins are a phospholipase C and a phytase. Chemical characterization of the N-glycan indicates that it is a mixture of Hex(5-13)HexNAc(2) oligosaccharides, the major molecular species corresponding to Hex(6-8)HexNAc(2). The N-glycan contained one galactofuranose unit that was in a terminal nonreducing position attached to the 2 position of Man. This single terminal nonreducing galactofuranose is essential for the immunoreactivity of the N-glycans assessed either with a monoclonal antibody that recognizes a tetra-beta-1,5-galactofuran chain of galactomannan or with Aspergillus-infected patient sera.

Identification of the catalytic nucleophile of the Family 31 alpha-glucosidase from Aspergillus niger via trapping of a 5-fluoroglycosyl-enzyme intermediate

The mechanism-based reagent 5-fluoro-alpha-d-glucopyranosyl fluoride (5F alpha GlcF) was used to trap a glycosyl-enzyme intermediate and identify the catalytic nucleophile at the active site of Aspergillus niger alpha-glucosidase (Family 31). Incubation of the enzyme with 5F alpha GlcF, followed by peptic proteolysis and comparative liquid chromatography/MS mapping allowed the isolation of a labelled peptide. Fragmentation analysis of this peptide by tandem MS yielded the sequence WYDMSE, with the label located on the aspartic acid residue (D). Comparison with the known protein sequence identified the labelled amino acid as Asp-224 of the P2 subunit.

Carboxyl group of residue Asp647 as possible proton donor in catalytic reaction of alpha-glucosidase from Schizosaccharomyces pombe

cDNA encoding Schizosaccharomyces pombe alpha-glucosidase was cloned from a library constructed from mRNA of the fission yeast, and expressed in Saccharomyces cerevisiae. The cDNA, 4176 bp in length, included a single ORF composed of 2910 bp encoding a polypeptide of 969 amino-acid residues with M(r) 106 138. The deduced amino-acid sequence showed a high homology to those of alpha-glucosidases from molds, plants and mammals. Therefore, the enzyme was categorized into the alpha-glucosidase family II. By site-directed mutagenesis, Asp481, Glu484 and Asp647 residues were confirmed to be essential in the catalytic reaction. The carboxyl group (-COOH) of the Asp647 residue was for the first time shown to be the most likely proton donor acting as the acid catalyst in the alpha-glucosidase of family II. Studies with the chemical modifier conduritol B epoxide suggested that the carboxylate group (-COO-) of the Asp481 residue was the catalytic nucleophile, although the role of the Glu484 residue remains obscure.

Glucoamylase overexpression and secretion in Aspergillus niger: analysis of glycosylation

We have studied the effects of overexpression and secretion of a homologous model glycoprotein, glucoamylase (GAM-1), on glycosylation in a single gene copy wild-type parent and multiple gene copy transformants of Aspergillus niger. In batch culture the B36 strain, which possess 80 additional copies of the GAM glaA gene, secreted about 5-8-fold more protein and GAM-1 than the parent strain (N402). A comparison of the glycosylation of GAM-1 secreted by the parent strain with that secreted by the multiple copy and hyper-secreting B36 strain showed that both the N-linked and O-linked glycan composition was very similar. Short oligomannose N-linked glycans were found (Man(7-8)GlcNAc(2)). O-Linked glycans were comprised of short (1-3 residues) oligosaccharide chains of mannose and galactose. Evidence is presented that this galactose is present in the novel galactofuranose conformation. This glycan composition of GAM-1 differed from that of a commercially available (A. niger) GAM source. Microsomes prepared from the mycelium showed a 2-3-fold co-ordinated increase in the activity of the dolichol phosphate:glycosyltransferases. Similar results were obtained from strains B1 (20 copies of glaA) and N402 when grown at a low dilution rate in a chemostat, although both the levels of GAM secretion and the activities of the dolichol phosphate:glycosyltransferases were lower than found in batch culture. These data suggest that A. niger is capable of secreting large amounts of a single glycoprotein combined with higher activity levels of the dolichol phosphate:glycosyltransferases without an increase in the heterogeneity of the glycan structures. Thus, from a biotechnological viewpoint, protein glycosylation may not be a bottleneck to enhanced glycoprotein production using A. niger.

Filamentous fungi as production organisms for glycoproteins of bio-medical interest

Filamentous fungi are commonly used in the fermentation industry for large scale production of glycoproteins. Several of these proteins can be produced in concentrations up to 20-40 g per litre. The production of heterologous glycoproteins is at least one or two orders of magnitude lower but research is in progress to increase the production levels. In the past years the structure of protein-linked carbohydrates of a number of fungal proteins has been elucidated, showing the presence of oligo-mannosidic and high-mannose chains, sometimes with typical fungal modifications. A start has been made to engineer the glycosylation pathway in filamentous fungi to obtain strains that show a more mammalian-like type of glycosylation. This mini review aims to cover the current knowledge of glycosylation in filamentous fungi, and to show the possibilities to produce glycoproteins with these organisms with a more mammalian-like type of glycosylation for research purposes or pharmaceutical applications.

The molecular biology of secreted enzyme production by fungi

Enzymes from filamentous fungi are already widely exploited, but new applications for known enzymes and new enzymic activities continue to be found. In addition, enzymes from less amenable non-fungal sources require heterologous production and fungi are being used as the production hosts. In each case there is a need to improve production and to ensure quality of product. While conventional, mutagenesis-based, strain improvement methods will continue to be applied to enzyme production from filamentous fungi the application of recombinant DNA techniques is beginning to reveal important information on the molecular basis of fungal enzyme production and this knowledge is now being applied both in the laboratory and commercially. We review the current state of knowledge on the molecular basis of enzyme production by filamentous fungi. We focus on transcriptional and post-transcriptional regulation of protein production, the transit of proteins through the secretory pathway and the structure of the proteins produced including glycosylation.

Structural studies of the O-antigenic polysaccharide from Escherichia coli O167

The structure of the O-antigenic polysaccharide from Escherichia coli O167:H5 has been investigated. Sugar and methylation analyses, fast-atom-bombardment mass spectrometry and 1H- and 13C-NMR spectroscopy were the main methods used. The structure of the repeating unit of the polysaccharide was found to be: [formula in text]. Oligosaccharide derivatives of the polysaccharide were obtained by HF solvolysis and by a Smith degradation. Furthermore, base treatment of the polysaccharide led to a degraded polymeric material. For the methylated polysaccharide the amide linkage between alanine and the galacturonic acid residue was reductively cleaved with LiBD4 in ethanol, to give, among other things, a 3-O-methyl galactose derivative.

Structural characterization of N-linked oligosaccharides from cellobiohydrolase I secreted by the filamentous fungus Trichoderma reesei RUTC 30

We have characterized the primary structures of the predominant N-linked oligosaccharides on cellobiohydrolase I from the filamentous fungus Trichoderma reesei RUTC30. Different enzymatic and chromatographic techniques were used to analyze six oligosaccharides. The combined data showed that the fungal carbohydrates have a core structure that is identical to the mammalian N-linked core. In the bulk of the N-glycans, the alpha-1,3 arm is extended with two mannoses and a glucose, suggesting incomplete processing of the oligosaccharides in the endoplasmic reticulum. The alpha-1,6 arm shows a remarkable heterogeneity: in addition to alpha-1,2-Man and alpha-1,6-Man, the presence of a terminal mannose alpha-1,6-phosphodiester was observed. This latter substituent has not been characterized before on mannosidase-processed N-glycan and its function and synthesis pathway are entirely unknown. The predominant N-glycans on cellobiohydrolase I can be represented as follows: GlcMan8GlcNAc2, GlcMan7GlcNAc2, Man7GlcNAc2, ManPGlcMan7GlcNAc2, GlcMan5GlcNAc2 and Man5GlcNAc2.

UDP-galactofuranose precursor required for formation of the lipopolysaccharide O antigen of Klebsiella pneumoniae serotype O1 is synthesized by the product of the rfbDKPO1 gene

The O-side-chain polysaccharide in the lipopolysaccharide of Klebsiella pneumoniae O1 is based on a backbone structure of repeat units of [-->3)-beta-D-Galf-(1-->3)-alpha-D-Galp-(1-->]; this structure is termed D-galactan I. The rfb (O-antigen biosynthesis) gene cluster directs the synthesis of D-galactan I and consists of six genes termed rfbA-FKPO1. In this paper we show that rfbDKPO1 encodes a UDP-galactopyranose mutase (NAD(P)H-requiring) (EC 5.4.99. 9), which forms uridine 5'-(trihydrogen diphosphate) P'-alpha-D-galactofuranosyl ester (UDP-Galf), the biosynthetic precursor of galactofuranosyl residues. The deduced amino acid sequence of rfbDKPO1 shows 85% and 37.5% identity to the rfbDKPO8 gene of K. pneumoniae serotype O8 and the glf gene of Escherichia coli, respectively. The molecular mass of the purified RfbDKPO1 enzyme is 45 kDa as determined by SDS-polyacrylamide gel electrophoresis, while gel filtration revealed a molecular mass of 92 kDa, suggesting a dimeric structure for the native protein. The rfbDKPO1 gene product interconverts uridine 5'-(trihydrogen diphosphate) P'-alpha-D-galactopyranosyl ester (UDP-Galp) and UDP-Galf. Unlike Glf, RfbDKPO1 showed a requirement for NADH or NADPH, which could not be replaced by NAD or NADP. RfbDKPO1 was used to synthesize milligram quantities of UDP-Galf, allowing this compound to be purified and fully characterized in an intact form for the first time. The structure of UDP-Galf was proven by NMR spectroscopy.

Structure of an exocellular polysaccharide of Lactobacillus helveticus TN-4, a spontaneous mutant strain of Lactobacillus helveticus TY1-2

Lactobacillus helveticus strain TN-4, a spontaneous mutant strain of Lactobacillus helveticus TY1-2, produced an exocellular polysaccharide from reconstituted skim milk. On the basis of the results of methylation analysis, enzymatic digestion, mild Smith degradation, mild acid hydrolysis, acetolysis, and 1D and 2D 1H-NMR spectroscopy, it was concluded that the polysaccharide has a D-galactofuranose containing hexasaccharide repeating unit with the following structure: [formula see text]