Characterization of the bga1-encoded glycoside hydrolase family 35 beta-galactosidase of Hypocrea jecorina with galacto-beta-D-galactanase activity

The archaeal highly thermostable GH35 family β-galactosidase DaβGal has a unique seven domain protein fold

The most extensively studied β-d-galactosidases (EC3.2.1.23) belonging to four glycoside hydrolase (GH) families 1, 2, 35, and 42 are widely distributed among Bacteria, Archaea and Eukaryotes. Here, we report a novel GH35 family β-galactosidase from the hyperthermophilic Thermoprotei archaeon Desulfurococcus amylolyticus (DaβGal). Unlike fungal monomeric six-domain β-galactosidases, the DaβGal enzyme is a dimer; it has an extra jelly roll domain D7 and three composite domains (D4, D5, and D6) that are formed by the distantly located polypeptide chain regions. The enzyme possesses a high specificity for β-d-galactopyranosides, and its distinguishing feature is the ability to cleave pNP-β-d-fucopyranoside. DaβGal efficiently catalyzes the hydrolysis of lactose at high temperatures, remains stable and active at 65 °С, and retains activity at 95 °С with a half-life time value equal to 73 min. These properties make archaeal DaβGal a more attractive candidate for biotechnology than the widely used fungal β-galactosidases.

The Glycoside Hydrolase Family 35 β-galactosidase from Trichoderma reesei debranches xyloglucan oligosaccharides from tamarind and jatobá

Trichoderma reesei (anamorph Hypocrea jecorina) produces an extracellular beta-galactosidase from Glycoside Hydrolase Family 35 (TrBga1). Hydrolysis of xyloglucan oligosaccharides (XGOs) by TrBga1 has been studied by hydrolysis profile analysis of both tamarind (Tamarindus indica) and jatobá (Hymenaea courbaril) seed storage xyloglucans using PACE and MALDI-ToF-MS for separation, quantification and identification of the hydrolysis products. The TrBga1 substrate preference for galactosylated oligosaccharides from both the XXXG- and XXXXG-series of jatobá xyloglucan showed that the doubly galactosylated oligosaccharides were the first to be hydrolyzed. Furthermore, the TrBga1 showed more efficient hydrolysis against non-reducing end dexylosylated oligosaccharides (GLXG/GXLG and GLLG). This preference may play a key role in xyloglucan degradation, since galactosyl removal alleviates steric hindrance for other enzymes in the xyloglucanolytic complex resulting in complete xyloglucan mobilization. Indeed, mixtures of TrBga1 with the α-xylosidase from Escherichia coli (YicI), which shows a preference towards non-galactosylated xyloglucan oligosaccharides, reveals efficient depolymerization when either enzyme is applied first. This understanding of the synergistic depolymerization contributes to the knowledge of plant cell wall structure, and reveals possible evolutionary mechanisms directing the preferences of debranching enzymes acting on xyloglucan oligosaccharides.

The Role of Arabinogalactan Type II Degradation in Plant-Microbe Interactions

Arabinogalactans (AGs) are structural polysaccharides of the plant cell wall. A small proportion of the AGs are associated with hemicellulose and pectin. Furthermore, AGs are associated with proteins forming the so-called arabinogalactan proteins (AGPs), which can be found in the plant cell wall or attached through a glycosylphosphatidylinositol (GPI) anchor to the plasma membrane. AGPs are a family of highly glycosylated proteins grouped with cell wall proteins rich in hydroxyproline. These glycoproteins have important and diverse functions in plants, such as growth, cellular differentiation, signaling, and microbe-plant interactions, and several reports suggest that carbohydrate components are crucial for AGP functions. In beneficial plant-microbe interactions, AGPs attract symbiotic species of fungi or bacteria, promote the development of infectious structures and the colonization of root tips, and furthermore, these interactions can activate plant defense mechanisms. On the other hand, plants secrete and accumulate AGPs at infection sites, creating cross-links with pectin. As part of the plant cell wall degradation machinery, beneficial and pathogenic fungi and bacteria can produce the enzymes necessary for the complete depolymerization of AGs including endo-β-(1,3), β-(1,4) and β-(1,6)-galactanases, β-(1,3/1,6) galactanases, α-L-arabinofuranosidases, β-L-arabinopyranosidases, and β-D-glucuronidases. These hydrolytic enzymes are secreted during plant-pathogen interactions and could have implications for the function of AGPs. It has been proposed that AGPs could prevent infection by pathogenic microorganisms because their degradation products generated by hydrolytic enzymes of pathogens function as damage-associated molecular patterns (DAMPs) eliciting the plant defense response. In this review, we describe the structure and function of AGs and AGPs as components of the plant cell wall. Additionally, we describe the set of enzymes secreted by microorganisms to degrade AGs from AGPs and its possible implication for plant-microbe interactions.

Hemicellulases from Penicillium and Talaromyces for lignocellulosic biomass valorization: A review

The term hemicellulose groups different polysaccharides with heterogeneous structures, mannans, xyloglucans, mixed-linkage β-glucans and xylans, which differ in their backbone and branches, and in the type and distribution of glycosidic linkages. The enzymatic degradation of these complex polymers requires the concerted action of multiple hemicellulases and auxiliary enzymes. Most commercial enzymes are produced by Trichoderma and Aspergillus species, but recent studies have disclosed Penicillium and Talaromyces as promising sources of hemicellulases. In this review, we summarize the current knowledge on the hemicellulolytic system of these genera, and the role of hemicellulases in the disruption and synthesis of glycosidic bonds. In both cases, the enzymes from Penicillium and Talaromyces represent an interesting alternative for valorization of lignocellulosic biomass in the current framework of circular economy.

Cold-Active β-Galactosidases: Insight into Cold Adaption Mechanisms and Biotechnological Exploitation

β-galactosidases (EC 3.2.1.23) catalyze the hydrolysis of β-galactosidic bonds in oligosaccharides and, under certain conditions, transfer a sugar moiety from a glycosyl donor to an acceptor. Cold-active β-galactosidases are identified in microorganisms endemic to permanently low-temperature environments. While mesophilic β-galactosidases are broadly studied and employed for biotechnological purposes, the cold-active enzymes are still scarcely explored, although they may prove very useful in biotechnological processes at low temperature. This review covers several issues related to cold-active β-galactosidases, including their classification, structure and molecular mechanisms of cold adaptation. Moreover, their applications are discussed, focusing on the production of lactose-free dairy products as well as on the valorization of cheese whey and the synthesis of glycosyl building blocks for the food, cosmetic and pharmaceutical industries.

The transcription factor ACE3 controls cellulase activities and lactose metabolism via two additional regulators in the fungus Trichoderma reesei

Fungi of the genus Trichoderma are a rich source of enzymes, such as cellulases and hemicellulases, that can degrade lignocellulosic biomass and are therefore of interest for biotechnological approaches seeking to optimize biofuel production. The essential transcription factor ACE3 is involved in cellulase production in Trichoderma reesei; however, the mechanism by which ACE3 regulates cellulase activities is unknown. Here, we discovered that the nominal ace3 sequence in the T. reesei genome available through the Joint Genome Institute is erroneously annotated. Moreover, we identified the complete ace3 sequence, the ACE3 Zn(II)2Cys6 domain, and the ACE3 DNA-binding sites containing a 5'-CGGAN(T/A)3-3' consensus. We found that in addition to its essential role in cellulase production, ace3 is required for lactose assimilation and metabolism in T. reesei Transcriptional profiling with RNA-Seq revealed that ace3 deletion down-regulates not only the bulk of the major cellulase, hemicellulase, and related transcription factor genes, but also reduces the expression of lactose metabolism-related genes. Additionally, we demonstrate that ACE3 binds the promoters of many cellulase genes, the cellulose response transporter gene crt1, and transcription factor-encoding genes, including xyr1 We also observed that XYR1 dimerizes to facilitate cellulase production and that ACE3 interacts with XYR1. Together, these findings uncover how two essential transcriptional activators mediate cellulase gene expression in T. reesei On the basis of these observations, we propose a model of how the interactions between ACE3, Crt1, and XYR1 control cellulase expression and lactose metabolism in T. reesei.

Intrinsic dynamic behavior of enzyme:substrate complexes govern the catalytic action of β-galactosidases across clan GH-A

The conformational itineraries taken by carbohydrate residues in the catalytic subsite of retaining glycoside hydrolases (GHs), harness the link between substrate conformation and reactivity. GHs' active sites may be described as a combination of subsites dedicated to the binding of individual sugar residues and to catalysis. The three-dimensional structure of GH:carbohydrate complexes has demonstrated that carbohydrate ring conformation changes in an ordered manner during catalysis. Here we demonstrate in silico that a link exists between subsite binding dynamics and substrate specificity for β-galactosidases from clan GH-A families GH1, GH2, GH35, GH42 and GH59. Different oligosaccharides were docked in the active site of reference β-galactosidase structures using Vina-Carb. Subsequent molecular dynamics (MD) simulations revealed that these enzymes favor a high degree of flexibility and ring distortion of the substrate the lytic subsite -1. Although the β-galactosidase families examined are structurally and mechanistically related, distinct patterns of ring distortion were unveiled for the different families. For β-galactosidases, three different family-dependent reaction itineraries (1S3 → 4H3‡ → 4C1, 1,4B → 4H3/ 4E‡ → 4C1, and 1S5 → 4E/ 4H5‡ → 4C1) were identified, all compatible with the antiperiplanar lone pair hypothesis (ALPH) for the hydrolysis of β-glycosides. This comparative study reveals the fuzzy character of the changes in carbohydrate ring geometry prior to carbohydrate hydrolysis.

Genome-Wide Analysis of β-Galactosidases in Xanthomonas campestris pv. campestris 8004

Bacterial β-galactosidase is involved in lactose metabolism and acts as a prevalent reporter enzyme used in studying the activities of prokaryotic and eukaryotic promoters. Xanthomonas campestris pv. campestris (Xcc) is the pathogen of black rot disease in crucifers. β-Galactosidase activity can be detected in Xcc culture, which makes Escherichia coli LacZ unable to be used as a reporter enzyme in Xcc. To systemically understand the β-galactosidase in Xcc and construct a β-galactosidase -deficient strain for promoter activity analysis using LacZ as a reporter, we here analyzed the putative β-galactosidases in Xcc 8004. As glycosyl hydrolase (GH) family 2 (GH2) and 35 (GH35) family enzymes were reported to have beta-galactosidase activities, we studied all of them encoded by Xcc 8004. When expressed in E. coli, only two of the enzymes, XC1214 and XC2985, were found to have β-galactosidase activity. When deleted from the Xcc 8004 genome, only the XC1214 mutant had no β-galactosidase activity, and other GH2 and GH35 gene deletions resulted in no significant reduction in β-galactosidase activity. Therefore, XC1214 is the main β-galactosidase in Xcc 8004. Notably, we have constructed a β-galactosidase-free strain that can be employed in gene traps using LacZ as a reporter in Xcc. The results reported herein should facilitate the development of high-capacity screening assays that utilize the LacZ reporter system in Xcc.

Microchip capillary gel electrophoresis combined with lectin affinity enrichment employing magnetic beads for glycoprotein analysis

Due to the constant search for reliable methods to investigate glycoproteins in complex biological samples, an alternative approach combining affinity enrichment with rapid and sensitive analysis on-a-chip is presented. Glycoproteins were specifically captured by lectin-coated magnetic beads, eluted by competitive sugars, and investigated with microchip capillary gel electrophoresis (MCGE), i.e., CGE-on-a-chip. We compared our results to sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) data, which turned out to be in very good agreement. While SDS-PAGE offers the possibility of subsequent mass spectrometric analysis of captured and separated analytes, MCGE scores with time savings, higher throughput, and lower sample consumption as well as quality control (QC) and process analytical technology (PAT) applicability. Due to these advantages, a lectin-based glycoprotein capture protocol can easily be optimized. In our case, two different types of magnetic beads were tested and compared regarding lectin binding. The selectivity of our strategy was demonstrated with a set of model glycoproteins, as well as with human serum and serum depleted from high-abundance proteins. The specificity of the capturing method was investigated revealing to a certain degree an unspecific binding between each sample and the beads themselves, which has to be considered for any specific enrichment and data interpretation. In addition, two glycoproteins from Trichoderma atroviride, a fungus with mycoparasitic activity and only barely studied glycoproteome, were enriched by means of a lectin and so identified for the first time.

Graphical abstract

Heterologous gene expression in filamentous fungi

Filamentous fungi are critical to production of many commercial enzymes and organic compounds. Fungal-based systems have several advantages over bacterial-based systems for protein production because high-level secretion of enzymes is a common trait of their decomposer lifestyle. Furthermore, in the large-scale production of recombinant proteins of eukaryotic origin, the filamentous fungi become the vehicle of choice due to critical processes shared in gene expression with other eukaryotic organisms. The complexity and relative dearth of understanding of the physiology of filamentous fungi, compared to bacteria, have hindered rapid development of these organisms as highly efficient factories for the production of heterologous proteins. In this review, we highlight several of the known benefits and challenges in using filamentous fungi (particularly Aspergillus spp., Trichoderma reesei, and Neurospora crassa) for the production of proteins, especially heterologous, nonfungal enzymes. We review various techniques commonly employed in recombinant protein production in the filamentous fungi, including transformation methods, selection of gene regulatory elements such as promoters, protein secretion factors such as the signal peptide, and optimization of coding sequence. We provide insights into current models of host genomic defenses such as repeat-induced point mutation and quelling. Furthermore, we examine the regulatory effects of transcript sequences, including introns and untranslated regions, pre-mRNA (messenger RNA) processing, transcript transport, and mRNA stability. We anticipate that this review will become a resource for researchers who aim at advancing the use of these fascinating organisms as protein production factories, for both academic and industrial purposes, and also for scientists with general interest in the biology of the filamentous fungi.

Metabolism of D-galactose is dispensable for the induction of the beta-galactosidase (bgaD) and lactose permease (lacpA) genes in Aspergillus nidulans

In this study, we analyze the expression of the Aspergillus nidulans bgaD-lacpA gene couple (encoding an intracellular beta-galactosidase and a lactose permease) in the presence of D-galactose. This monosaccharide can be catabolized via alternative, independent pathways in this model organism. The inductive capabilities of intermediates of the two alternative routes of D-galactose utilization were addressed in loss-of-function mutants defective in a defined step in one of the two pathways. In a galactokinase (galE9) mutant, the cluster is strongly induced by D-galactose, suggesting that formation of Leloir pathway intermediates is not required. The expression profiles of bgaD and lacpA were similar in wild type, L-arabinitol dehydrogenase (araA1), and hexokinase (hxkA1) negative backgrounds, indicating that intermediates of the oxido-reductive pathway downstream of galactitol are not necessary either. Furthermore, bgaD-lacpA transcription was not induced in any of the tested strains when galactitol was provided as the growth substrate. An hxkA1/galE9 double mutant cannot grow on d-galactose at all, but still produced bgaD and lacpA transcripts upon transfer to d-galactose. We therefore concluded that the physiological inducer of the bgaD-lacpA gene cluster upon growth on D-galactose is the nonmetabolized sugar itself.

Extra- and intracellular lactose catabolism in Penicillium chrysogenum: phylogenetic and expression analysis of the putative permease and hydrolase genes

Penicillium chrysogenum is used as an industrial producer of penicillin. We investigated its catabolism of lactose, an abundant component of whey used in penicillin fermentation, comparing the type strain NRRL 1951 with the high producing strain AS-P-78. Both strains grew similarly on lactose as the sole carbon source under batch conditions, exhibiting almost identical time profiles of sugar depletion. In silico analysis of the genome sequences revealed that P. chrysogenum features at least five putative β-galactosidase (bGal)-encoding genes at the annotated loci Pc22g14540, Pc12g11750, Pc16g12750, Pc14g01510 and Pc06g00600. The first two proteins appear to be orthologs of two Aspergillus nidulans family 2 intracellular glycosyl hydrolases expressed on lactose. The latter three P. chrysogenum proteins appear to be distinct paralogs of the extracellular bGal from A. niger, LacA, a family 35 glycosyl hydrolase. The P. chrysogenum genome also specifies two putative lactose transporter genes at the annotated loci Pc16g06850 and Pc13g08630. These are orthologs of paralogs of the gene encoding the high-affinity lactose permease (lacpA) in A. nidulans for which P. chrysogenum appears to lack the ortholog. Transcript analysis of Pc22g14540 showed that it was expressed exclusively on lactose, whereas Pc12g11750 was weakly expressed on all carbon sources tested, including D-glucose. Pc16g12750 was co-expressed with the two putative intracellular bGal genes on lactose and also responded on L-arabinose. The Pc13g08630 transcript was formed exclusively on lactose. The data strongly suggest that P. chrysogenum exhibits a dual assimilation strategy for lactose, simultaneously employing extracellular and intracellular hydrolysis, without any correlation to the penicillin-producing potential of the studied strains.

Peculiarities and applications of galactanolytic enzymes that act on type I and II arabinogalactans

Arabinogalactans (AGs) are branched galactans to which arabinose residues are bound as side chains and are widely distributed in plant cell walls. They can be grouped into two types based on the structures of their backbones. Type I AGs have β-1,4-galactan backbones and are often covalently linked to the rhamnogalacturonan-I region of pectins. Type II AGs have β-1,3-galactan backbones and are often covalently linked to proteins. The main enzymes involved in the degradation of AGs are endo-β-galactanases, exo-β-galactanases, and β-galactosidases, although other enzymes such as α-L-arabinofuranosidases, β-L-arabinopyranosidases, and β-D-glucuronidases are required to remove the side chains for efficient degradation of the polysaccharides. Galactanolytic enzymes have a wide variety of potential uses, including the bioconversion of AGs to fermentable sugars for production of commodity chemicals like ethanol, biobleaching of cellulose pulp, modulation of pectin properties, improving animal feed, and determining the chemical structure of AGs. This review summarizes our current knowledge about the biochemical properties and potential applications of AG-degrading enzymes.

Characterization of a glycoside hydrolase family 1 β-galactosidase from hot spring metagenome with transglycosylation activity

A novel, thermostable, alkalophilic β-D-galactosidase (Mbgl) was isolated from a metagenome of geothermal springs in northern Himalayan region of India. Mbgl was 447 amino acids in size and had conserved catalytic residues E170 and E358, indicating that it belonged to family 1 of glycosyl hydrolases showing maximum homology (89 %) with uncharacterized β-galactosidase of Eubacterium, Meiothermus ruber DSM1279. Temperature and pH optima of Mbgl were 65 °C and 8.0 respectively, and it retained 80 % activity even at pH 10.0. Mbgl was active as a homotetramer, recognized β-(1,4)-D-galactoside as the preferred glycosidic bond, and preferentially hydrolyzed pNPgal with K(m) 3.33 mM and k(cat) 2,000 s(-1). It displayed high transglycosylation activity with wide acceptor specificity including hexoses and pentoses leading to the formation of prebiotic galacto-oligosaccharides whereas its lactose hydrolysis potential was low.

Function and 3D structure of the N-glycans on glycoproteins

Glycosylation is one of the most common post-translational modifications in eukaryotic cells and plays important roles in many biological processes, such as the immune response and protein quality control systems. It has been notoriously difficult to study glycoproteins by X-ray crystallography since the glycan moieties usually have a heterogeneous chemical structure and conformation, and are often mobile. Nonetheless, recent technical advances in glycoprotein crystallography have accelerated the accumulation of 3D structural information. Statistical analysis of “snapshots” of glycoproteins can provide clues to understanding their structural and dynamic aspects. In this review, we provide an overview of crystallographic analyses of glycoproteins, in which electron density of the glycan moiety is clearly observed. These well-defined N-glycan structures are in most cases attributed to carbohydrate-protein and/or carbohydrate-carbohydrate interactions and may function as “molecular glue” to help stabilize inter- and intra-molecular interactions. However, the more mobile N-glycans on cell surface receptors, the electron density of which is usually missing on X-ray crystallography, seem to guide the partner ligand to its binding site and prevent irregular protein aggregation by covering oligomerization sites away from the ligand-binding site.

Fungal enzyme sets for plant polysaccharide degradation

Enzymatic degradation of plant polysaccharides has many industrial applications, such as within the paper, food, and feed industry and for sustainable production of fuels and chemicals. Cellulose, hemicelluloses, and pectins are the main components of plant cell wall polysaccharides. These polysaccharides are often tightly packed, contain many different sugar residues, and are branched with a diversity of structures. To enable efficient degradation of these polysaccharides, fungi produce an extensive set of carbohydrate-active enzymes. The variety of the enzyme set differs between fungi and often corresponds to the requirements of its habitat. Carbohydrate-active enzymes can be organized in different families based on the amino acid sequence of the structurally related catalytic modules. Fungal enzymes involved in plant polysaccharide degradation are assigned to at least 35 glycoside hydrolase families, three carbohydrate esterase families and six polysaccharide lyase families. This mini-review will discuss the enzymes needed for complete degradation of plant polysaccharides and will give an overview of the latest developments concerning fungal carbohydrate-active enzymes and their corresponding families.

Crystal structures of Trichoderma reesei β-galactosidase reveal conformational changes in the active site

We have determined the crystal structure of Trichoderma reesei (Hypocrea jecorina) β-galactosidase (Tr-β-gal) at a 1.2Å resolution and its complex structures with galactose, IPTG and PETG at 1.5, 1.75 and 1.4Å resolutions, respectively. Tr-β-gal is a potential enzyme for lactose hydrolysis in the dairy industry and belongs to family 35 of the glycoside hydrolases (GH-35). The high resolution crystal structures of this six-domain enzyme revealed interesting features about the structure of Tr-β-gal. We discovered conformational changes in the two loop regions in the active site, implicating a conformational selection-mechanism for the enzyme. In addition, the Glu200, an acid/base catalyst showed two different conformations which undoubtedly affect the pK(a) value of this residue and the catalytic mechanism. The electron density showed extensive glycosylation, suggesting a structure stabilizing role for glycans. The longest glycan showed an electron density that extends to the eighth monosaccharide unit in the extended chain. The Tr-β-gal structure also showed a well-ordered structure for a unique octaserine motif on the surface loop of the fifth domain.

Metabolic engineering strategies for the improvement of cellulase production by Hypocrea jecorina

Hypocrea jecorina (= Trichoderma reesei) is the main industrial source of cellulases and hemicellulases used to depolymerise plant biomass to simple sugars that are converted to chemical intermediates and biofuels, such as ethanol. Cellulases are formed adaptively, and several positive (XYR1, ACE2, HAP2/3/5) and negative (ACE1, CRE1) components involved in this regulation are now known. In addition, its complete genome sequence has been recently published, thus making the organism susceptible to targeted improvement by metabolic engineering. In this review, we summarise current knowledge about how cellulase biosynthesis is regulated, and outline recent approaches and suitable strategies for facilitating the targeted improvement of cellulase production by genetic engineering.

Crystallization and preliminary diffraction analysis of a beta-galactosidase from Trichoderma reesei

An extracellular beta-galactosidase from Trichoderma reesei was crystallized from sodium cacodylate buffer using polyethylene glycol (PEG) as a precipant. Crystals grown by homogenous streak-seeding belonged to space group P1, with unit-cell parameters a = 67.3, b = 69.1, c = 81.5 A, alpha = 109.1, beta = 97.3, gamma = 114.5 degrees . The crystals diffracted to 1.8 A resolution using a rotating-anode generator and to 1.2 A resolution using a synchrotron source. On the basis of the Matthews coefficient (V(M) = 3.16 A(3) Da(-1)), one molecule is estimated to be present in the asymmetric unit. The aim of the determination of the crystal structure is to increase the understanding of this industrially significant enzyme.