Expression and characterization of an endo-1,4-β-galactanase from Emericella nidulans in Pichia pastoris for enzymatic design of potentially prebiotic oligosaccharides from potato galactans

Bioinformatics-based identification of GH12 endoxyloglucanases in citrus-pathogenic Penicillium spp

Millions of tons of citrus peel waste are produced every year as a byproduct of the juice industry. Citrus peel is rich in pectin and xyloglucan, but while the pectin is extracted for use in the food industry, the xyloglucan is currently not valorized. To target hydrolytic degradation of citrus peel xyloglucan into oligosaccharides, we have used bioinformatics to identify three glycoside hydrolase 12 (GH12) endoxyloglucanases (EC 3.2.1.151) from the citrus fruit pathogens Penicillium italicum GL-Gan1 and Penicillium digitatum Pd1 and characterized them on xyloglucan obtained by alkaline extraction from citrus peel. The enzymes displayed pH-temperature optima of pH 4.6-5.3 and 35-37°C. PdGH12 from P. digitatum and PiGH12A from P. italicum share 84% sequence identity and displayed similar kinetics, although kcat was highest for PdGH12. In contrast, PiGH12B from P. italicum, which has the otherwise conserved Trp in subsite -4 replaced with a Tyr, displayed a 3 times higher KM and a 4 times lower kcat/KM than PiGH12A, but was the most thermostable enzyme of the three Penicillium-derived endoxyloglucanases. The benchmark enzyme AnGH12 from Aspergillus nidulans was more thermally stable and had a higher pH-temperature optimum than the enzymes from Penicillum spp. The difference in structure of the xyloglucan oligosaccharides extracted from citrus peel xyloglucan and tamarind xyloglucan by the new endoxyloglucanases was determined by LC-MS. The inclusion of citrus peel xyloglucan demonstrated that the endoxyloglucanases liberated fucosylated xyloglucan oligomers, implying that these enzymes have the potential to upgrade citrus peel residues to produce oligomers useful as intermediates or bioactive compounds.

Characterization of a multi-domain exo-β-1,3-galactanase from Paenibacillus xylanexedens

β-1,3-Galactanases selectively degrade β-1,3-galactan, thus it is an attractive enzyme technique to map high-galactan structure and prepare galactooligosaccharides. In this work, a gene encoding exo-β-1,3-galactanase (PxGal43) was screened form Paenibacillus xylanexedens, consisting of a GH43 domain, a CBM32 domain and α-L-arabinofuranosidase B (AbfB) domain. Using β-1,3-galactan (AG-II-P) as substrate, the recombined enzyme expressed in Escherichia coli BL21 (DE3) exhibited an optimal activity at pH 7.0 and 30 °C. The enzyme was thermostable, retaining >70 % activity after incubating at 50 °C for 2 h. In addition, it showed high tolerance to various metal ions, denaturants and detergents. Substrate specificity indicated that PxGal43 hydrolysis only β-1,3-linked galactosyl oligosaccharides and polysaccharides, releasing galactose as an exo-acting manner. The function of the CBM32 and AbfB domain was revealed by their sequential deletion and suggested that their connection to the catalytic domain was crucial for the oligomerization, catalytic activity, substrate binding and thermal stability of PxGal43. The substrate docking and site-directed mutagenesis proposed that Glu191, Gln244, Asp138 and Glu81 served as the catalytic acid, catalytic base, pKa modulator, and substrate identifier in PxGal43, respectively. These results provide a better understanding and optimization of multi-domain bacterial GH43 β-1,3-galactanase for the degradation of arabinogalactan.

Preparation of β-galacto-oligosaccharides using a novel endo-1,4-β-galactanase from Penicillium oxalicum

Endo-1,4-β-galactanase is an indispensable tool for preparing prebiotic β-galacto-oligosaccharides (β-GOS) from pectic galactan resources. In the present study, a novel endo-1,4-β-galactanase (PoβGal53) belonging to glycoside hydrolase family 53 from Penicillium oxalicum sp. 68 was cloned and expressed in Pichia pastoris GS115. Upon purification by affinity chromatography, recombinant PoβGal53 exhibited a single band on SDS-PAGE with a molecular weight of 45.0 kDa. Using potato galactan as substrate, PoβGal53 showed optimal reaction conditions of pH 4.0, 40 °C, and was thermostable, retaining >80 % activity after incubating below 45 °C for 12 h. Significantly, PoβGal53 exhibited relatively conserved substrate specificity for (1 → 4)-β-D-galactan with an activity of 6244 ± 282 U/mg. In this regard, the enzyme is in effect the most efficient endo-1,4-β-galactanase identified to date. By using PoβGal53, β-GOS monomers were prepared from potato galactan and separated using medium pressure liquid chromatography. HPAEC-PAD, MALDI-TOF-MS and ESI-MS/MS analyses demonstrated that these β-GOS species ranged from 1,4-β-D-galactobiose to 1,4-β-D-galactooctaose (DP 2-8) with high purity. This work provides not only a highly active tool for enzymatic degradation of pectic galactan, but an efficient protocol for preparing β-GOS.

Enzymatic production of a suite of human milk oligosaccharides directly in milk

Human milk oligosaccharides (HMOs) denote specific glycans in human breast milk. They function as prebiotics, immune modulating, and antimicrobial agents in the gut of breastfed infants, and certain HMOs even promote the cognitive development of the baby. HMOs are virtually absent in cow's milk and hence in infant formula, which provides a huge incentive for identifying ways in which HMOs can be produced to improve infant formulas. Here, we show that different sialylated and fucosylated HMOs can be generated in cow's milk via different simultaneous enzymatic transglycosylation reactions catalyzed by an engineered sialidase (EC 3.2.1.18, from Trypanosoma rangeli) and an 1,2-α-L-fucosidase (EC 3.2.1.63, from Tannerella forsinthia) acting on the lactose in the milk and on casein glycomacropeptide, two types of commercially available HMOs, i.e. 2'-fucosyllactose and lacto-N-neotetraose, added to the milk. We also outline the details of the individual reactions in aqueous systems, demonstrate that the enzymatic reactions can be accomplished at 5 °C, and validate the products formed by LC-MS and NMR analysis. Enzymatic production of HMOs directly in milk provides opportunities for enriching milk and infant formulas and extends the use of enzymatic transglycosylation reactions to synthesis of HMOs in milk and eventually in other beverages.

Discovery of novel secretome CAZymes from Penicillium sclerotigenum by bioinformatics and explorative proteomics analyses during sweet potato pectin digestion

Novel selective enzymatic refining of sweet potato processing residues requires judicious enzyme selection and enzyme discovery. We prepared a pectinaceous cell wall polysaccharide fraction from sweet potato using an enzymatic a treatment to preserve the natural linkages and substitutions. Polysaccharide composition and linkage analysis data confirmed the pectinaceous polysaccharide fraction to be a rhamnogalacturonan I-rich fraction with a high content of arabinogalactan Type I. We hypothesized that the post-harvest tuber pathogenic fungus Penicillium sclerotigenum would harbor novel enzymes targeting selective sweet potato pectin modification. As part of the study, we also report the first genome sequence of P. sclerotigenum. We incubated the sweet potato pectinaceous fraction with P. sclerotigenum. Using proteomics accompanied by CUPP-bioinformatics analysis, we observed induced expression of 23 pectin-associated degradative enzymes. We also identified six abundantly secreted, induced proteins that do not correspond to known CAZymes, but which we suggest as novel enzymes involved in pectin degradation. For validation, the predicted CUPP grouping of putative CAZymes and the exo-proteome data obtained for P. sclerotigenum during growth on sweet potato pectin were compared with proteomics and transcriptomics data reported previously for pectin-associated CAZymes from Aspergillus niger strain NRRL3. The data infer that P. sclerotigenum has the capacity to express several novel enzymes that may provide novel opportunities for sweet potato pectin modification and valorization of sweet potato starch processing residues. In addition, the methodological approach employed represents an integrative systematic strategy for enzyme discovery.

Cloning, Expression, and Characterization of Endo-β-1,6-galactanase PoGal30 from Penicillium oxalicum

Because β-1,6-galactans are significant components in arabinogalactans from plant cell walls, identifying selective endo-β-1,6-galactanases is crucial to degrading these polysaccharides and to analyzing and modifying their structures. Here, we cloned and expressed in E. coli a novel endo-β-1,6-galactanase in the glycosidic hydrolase family 30 (GH30) from Penicillium oxalicum. Our recombinant PoGal30 hydrolase (1464 bp gene) that contains an N-terminal His-tag for purification by nickel affinity chromatography has a specific activity of 3.8 U/mg on the substrate de-arabinosylated gum Arabic (dGA) polysaccharide. The enzyme has 487 residues with a molecular mass of 60 kDa, an isoelectric point of 6, and functional pH and temperature optima of pH 2.5 to pH 5.0 and 40 °C, respectively. While the activity of PoGal30 is activated by Mg²⁺ (5 or 50 mmol/L), it is completely inhibited by Cu²⁺ and Fe³⁺ (50 mmol/L) and partially inhibited by Hg²⁺, EDTA, and SDS (50 mmol/L). The enzyme demonstrates high specificity towards β-1,6-galactosidic linkages in dGA, but is inactive against aryl-glycosides and galactobioses with different linkages. Using PoGal30 is, therefore, an effective approach to analyzing the fine structure of polysaccharides and preparing bioactive oligosaccharides.

Interaction between four galactans with different structural characteristics and gut microbiota

Human gut microbiota played a key role in maintaining and regulating host health. Gut microbiota composition could be altered by daily diet and related nutrients. Diet polysaccharide, an important dietary nutrient, was one kind of biological macromolecules linked by the glycosidic bonds. Galactans were widely used in foods due to their gelling, thickening and stabilizing properties. Recently, effects of different galactans on gut microbiota have attracted much attention. This review described the structural characteristics of 4 kinds of galactans, including porphyran, agarose, carrageenan, and arabinogalactan, along with the effects of different galactans on gut microbiota and production of short-chain fatty acids. The ability of gut microbiota to utilize galactans with different structural characteristics and related degradation mechanism were also summarized. All these four galactans could be used by gut Bacteroides. Besides, the porphyran could be utilized by Lactobacillus and Bifidobacterium, while the arabinogalactan could be utilized by Lactobacillus, Bifidobacterium and Roseburia. Four galactans with significant difference in molecular weight/degree of polymerization, glycosidic linkage, esterification, branching and monosaccharide composition required gut microbes which could utilize them have corresponding genes encoding the corresponding enzymes for decomposition. This review could help to understand the relationship between galactans with different structural characteristics and gut microbiota, and provide information for potential use of galactans as functional foods.

Engineering the substrate binding site of the hyperthermostable archaeal endo-β-1,4-galactanase from Ignisphaera aggregans

BACKGROUND

Endo-β-1,4-galactanases are glycoside hydrolases (GH) from the GH53 family belonging to the largest clan of GHs, clan GH-A. GHs are ubiquitous and involved in a myriad of biological functions as well as being widely used industrially. Endo-β-1,4-galactanases, in particular hydrolyse galactan and arabinogalactan in pectin, a major component of the primary plant cell wall, with important functions in plant defence and application in the food and other industries. Here, we explore the family's biological diversity by characterizing the first archaeal and hyperthermophilic GH53 galactanase, and utilize it as a scaffold for engineering enzymes with different product lengths.

RESULTS

A galactanase gene was identified in the genome of the anaerobic hyperthermophilic archaeon Ignisphaera aggregans, and the isolated catalytic domain expressed and characterized (IaGal). IaGal presents the typical (βα)₈ barrel structure of clan GH-A enzymes, with catalytic carboxylates at the end of the 4th and 7th barrel strands. Its activity optimum of at least 95 °C and melting point over 100 °C indicate extreme thermostability, a very advantageous property for industrial applications. If enzyme depletion is reduced, so is the need for re-addition, and thus costs. The main stabilizing features of IaGal compared to other structurally characterized members are π-π and cation-π interactions. The length of the substrate binding site-and thus produced oligosaccharide products-is intermediate compared to previously characterized galactanases. Variants inspired by the structural diversity in the GH53 family were rationally designed to shorten or extend the substrate binding groove, in order to modulate product length. Subsite-deleted variants produced shorter products than IaGal, as do the fungal galactanases inspiring the design. IaGal variants engineered with a longer binding site produced a less expected degradation pattern, though still different from that of wild-type IaGal. All variants remained extremely stable.

CONCLUSIONS

We have characterized in detail the most thermophilic endo-β-1,4-galactanase known to date and successfully engineered it to modify the degradation profile, while maintaining much of its desirable thermostability. This is an important achievement as oligosaccharide products length is an important property for industrial and natural GHs alike.

Effect of Potato Pulp Pectic Polysaccharide on the Stability of Acidified Milk Drinks

In order to broaden the application of potato pulp pectic polysaccharide (PPP) in stabilizing acidified milk drinks (AMDs) and investigate the stabilizing effect and physical properties of AMDs prepared with PPP, a comparative study was made among PPP, commercial high methoxyl pectin (HMP) and low methoxyl pectin (LMP). The zeta potential, rheology, particle size and serum separation of AMDs were evaluated after preparing with PPP, HMP and LMP, respectively. Results indicated that PPP led to lower serum separation than LMP (14.65% for AMDs prepared with 0.5% PPP compared to 25.05% for AMDs prepared with 0.5% LMP), but still higher than HMP (9.09% for AMDs prepared with 0.5% HMP). However, narrower particle size distribution and lower viscosity of AMDs was achieved by PPP than by LMP and HMP. PPP can electrostatically adsorb on the surface of casein and its abundant neutral sugar side chains would provide steric hindrance to prevent casein flocculation in AMDs. Our results might provide some new ideas for the application of PPP in improving the stability of AMDs.

Enzyme-Catalyzed Production of Potato Galactan-Oligosaccharides and Its Optimization by Response Surface Methodology

This work shows an optimized enzymatic hydrolysis of high molecular weight potato galactan yielding pectic galactan-oligosaccharides (PGOs), where endo-β-1,4-galactanase (galactanase) from Cellvibrio japonicus and Clostridium thermocellum was used. For this, response surface methodology (RSM) by central composite design (CCD) was applied. The parameters varied were temperature (°C), pH, incubation time (min), and enzyme/substrate ratio (U/mg). The optimized conditions for the production of low degree of polymerization (DP) PGOs were obtained for each enzyme by spectrophotometric assay and confirmed by chromatography. The optimal conditions predicted for the use of C. japonicus galactanase to obtain PGOs of DP = 2 were T = 51.8 °C, pH 5, E/S = 0.508 U/mg, and t = 77.5 min. For DP = 3, they were T = 21 °C, pH 9, E/S = 0.484 U/mg, and t = 12.5 min; and for DP = 4, they were T = 21 °C, pH 5, E/S = 0.462 U/mg, and t = 12.5 min. The efficiency results were 51.3% for substrate hydrolysis. C. thermocellum galactanase had a lower yield (35.7%) and optimized conditions predicted for PGOs of DP = 2 were T = 60 °C, pH 5, E/S = 0.525 U/mg, and time = 148 min; DP = 3 were T = 59.7 °C, pH 5, E/S = 0.506 U/mg, and time = 12.5 min; and DP = 4, were T = 34.5 °C, pH 11, E/S = 0.525 U/mg, and time = 222.5 min. Fourier transformed infrared (FT-IR) and nuclear magnetic resonance (NMR) characterizations of PGOs are presented.

l-Arabinose induces d-galactose catabolism via the Leloir pathway in Aspergillus nidulans

l-Arabinose and d-galactose are the principal constituents of l-arabinogalactan, and also co-occur in other hemicelluloses and pectins. In this work we hypothesized that similar to the induction of relevant glycoside hydrolases by monomers liberated from these plant heteropolymers, their respective catabolisms in saprophytic and phytopathogenic fungi may respond to the presence of the other sugar to promote synergistic use of the complex growth substrate. We showed that these two sugars are indeed consumed simultaneously by Aspergillus nidulans, while l-arabinose is utilised faster in the presence than in the absence of d-galactose. Furthermore, the first two genes of the Leloir pathway for d-galactose catabolism - encoding d-galactose 1-epimerase and galactokinase - are induced more rapidly by l-arabinose than by d-galactose eventhough deletion mutants thereof grow as well as a wild type strain on the pentose. d-Galactose 1-epimerase is hyperinduced by l-arabinose, d-xylose and l-arabitol but not by xylitol. The results suggest that in A. nidulans, l-arabinose and d-xylose - both requiring NADPH for their catabolisation - actively promote the enzyme infrastructure necessary to convert β-d-galactopyranose via the Leloir pathway with its α-anomer specific enzymes, into β-d-glucose-6-phosphate (the starting substrate of the oxidative part of the pentose phosphate pathway) even in the absence of d-galactose.

Comparison of traditional field retting and Phlebia radiata Cel 26 retting of hemp fibres for fibre-reinforced composites

Classical field retting and controlled fungal retting of hemp using Phlebia radiata Cel 26 (a mutant with low cellulose degrading ability) were compared with pure pectinase treatment with regard to mechanical properties of the produced fibre/epoxy composites. For field retting a classification of the microbial evolution (by gene sequencing) and enzyme profiles were conducted. By phylogenetic frequency mapping, different types of fungi, many belonging to the Ascomycota phylum were found on the fibres during the first 2 weeks of field retting, and thereafter, different types of bacteria, notably Proteobacteria, also proliferated on the field retted fibres. Extracts from field retted fibres exhibited high glucanase activities, while extracts from P. radiata Cel 26 retted fibres showed high polygalacturonase and laccase activities. As a result, fungal retting gave a significantly higher glucan content in the fibres than field retting (77 vs. 67%) and caused a higher removal of pectin as indicated by lower galacturonan content of fibres (1.6%) after fibres were retted for 20 days with P. radiata Cel 26 compared to a galacturonan content of 3.6% for field retted fibres. Effective fibre stiffness increased slightly after retting with P. radiata Cel 26 from 65 to 67 GPa, while it decreased after field retting to 52 GPa. Effective fibre strength could not be determined similarly due to variations in fibre fracture strain and fibre-matrix adhesion. A maximum composite strength with 50 vol% fibres of 307 MPa was obtained using P. radiata Cel 26 compared to 248 MPa with field retting.

Emerging concepts in the nutraceutical and functional properties of pectin-A Review

Pectin is a structural heteropolysaccharide found ubiquitously in terrestrial plants. It finds diverse food applications such as that of a gelling agent, stabilizer, and fat replacer. In the pharmaceutical arena, pectin exhibits a number of functions, from decreasing blood fat to combating various types of cancers. This review shows the shift of pectin from its conventional roles to its progressive applications. Insights into the advances in the production of pectin, the role it plays as a nutraceutical, possible prebiotic potential and a delivery vehicle for probiotics, and food applications are highlighted. Bioactive and functional properties of pectin are discussed and how the structural built up defines them, is emphasized. As a biopolymer, the applications of pectin in active packaging are also mentioned.

Penicillium purpurogenum produces a highly stable endo-β-(1,4)-galactanase

The polysaccharides of galactose present in the pectin of the plant cell wall are degraded by endo-β-1,4-galactanases. The filamentous fungus Penicillium purpurogenum, which grows on a number of natural carbon sources, among them sugar beet pulp which contains pectin, has a gene (ppgal1) coding an endo-β-1,4-galactanase (PpGAL1). This enzyme was expressed heterologously in Pichia pastoris. It has a molecular mass of 38 kDa, a pH optimum of 4-4.5, and an optimal temperature of 60 °C. It is 100 % stable for up to 24 h at pH 4-4.5 and 40 °C. These stability properties, which exceed those from other endo-β-1,4-galactanases reported to date, make it particularly suitable for industrial processes requiring acidic conditions and temperatures up to 40 °C. PpGAL1 is, therefore, a potentially effective tool in the food industry and in other biotechnological applications.

Enzymatic generation of galactose-rich oligosaccharides/oligomers from potato rhamnogalacturonan I pectic polysaccharides

Potato pulp by-product rich in galactan-rich rhamnogalacturonan I (RG I) was investigated as a new source of oligosaccharides with potential prebiotic properties. The efficiency of selected monocomponent enzymes and multi-enzymatic preparations to generate oligosaccharides/oligomers from potato RG I was evaluated. These overall results of yield were dependent on the activity profile of the multi-enzymatic preparations. Highest oligo-RG I yield of 93.9% was achieved using multi-enzymatic preparation (Depol 670L) with higher hydrolytic activity toward side chains of RG I as compared to its backbone. Main oligo-RG I products were oligosaccharides with DP of 2-12 (79.8-100%), while the oligomers with DP of 13-70 comprised smaller proportion (0.0-20.2%). Galactose (58.9-91.2%, w/w) was the main monosaccharide of oligo-RG I, while arabinose represented 0.0-12.1%. An understanding of the relationship between the activity profile of multi-enzymatic preparations and the yield/DP of oligo-RG I was achieved. This is expected to provide the capability to generate galacto- and galacto(arabino) oligosaccharides and their corresponding oligomers from an abundant by-product.

Monoclonal antibody-based analysis of cell wall remodeling during xylogenesis

Xylogenesis, a process by which woody tissues are formed, entails qualitative and quantitative changes in the cell wall. However, the molecular events that underlie these changes are not completely understood. Previously, we have isolated two monoclonal antibodies, referred to as XD3 and XD27, by subtractive screening of a phage-display library of antibodies raised against a wall fraction of Zinnia elegans xylogenic culture cells. Here we report the biochemical and immunohistochemical characterization of those antibodies. The antibody XD3 recognized (1→4)-β-D-galactan in pectin fraction. During xylogenesis, the XD3 epitope was localized to the primary wall of tracheary-element precursor cells, which undergo substantial cell elongation, and was absent from mature tracheary elements. XD27 recognized an arabinogalactan protein that was bound strongly to a germin-like protein. The XD27 epitope was localized to pre-lignified secondary walls of tracheary elements. Thus these cell-wall-directed monoclonal antibodies revealed two molecular events during xylogenesis. The biological significance of these events is discussed in relation to current views of the plant cell wall.

Effect of mutations on the thermostability of Aspergillus aculeatus β-1,4-galactanase

New variants of β-1,4-galactanase from the mesophilic organism Aspergillus aculeatus were designed using the structure of β-1,4-galactanase from the thermophile organism Myceliophthora thermophila as a template. Some of the variants were generated using PROPKA 3.0, a validated pKa prediction tool, to test its usefulness as an enzyme design tool. The PROPKA designed variants were D182N and S185D/Q188T, G104D/A156R. Variants Y295F and G306A were designed by a consensus approach, as a complementary and validated design method. D58N was a stabilizing mutation predicted by both methods. The predictions were experimentally validated by measurements of the melting temperature (Tm ) by differential scanning calorimetry. We found that the Tm is elevated by 1.1 °C for G306A, slightly increased (in the range of 0.34 to 0.65 °C) for D182N, D58N, Y295F and unchanged or decreased for S185D/Q188T and G104D/A156R. The Tm changes were in the range predicted by PROPKA. Given the experimental errors, only the D58N and G306A show significant increase in thermodynamic stability. Given the practical importance of kinetic stability, the kinetics of the irreversible enzyme inactivation process were also investigated for the wild-type and three variants and found to be biphasic. The half-lives of thermal inactivation were approximately doubled in G306A, unchanged for D182N and, disappointingly, a lot lower for D58N. In conclusion, this study tests a new method for estimating Tm changes for mutants, adds to the available data on the effect of substitutions on protein thermostability and identifies an interesting thermostabilizing mutation, which may be beneficial also in other galactanases.

A novel non-blue laccase from Bacillus amyloliquefaciens: secretory expression and characterization

Laccases are copper-containing enzymes which possess a promising potential in many industrial and environmental applications. Here we describe the cloning, extracellular expression and characterization of a novel non-blue laccase from Bacillus amyloliquefaciens in Pichia pastoris. The recombinant enzyme was secreted into the culture supernatant with high activity. It lacks the absorption band at 610 nm typical for blue laccases. However, electron paramagnetic resonance (EPR) spectrum proved the existence of type 1 copper center that was not detectable in the UV-visible spectrum. Metal content analysis revealed that the enzyme contains two copper ions, one iron ion and one zinc ion per protein molecular, suggesting that it is a novel non-blue laccase. The pH and temperature optima of the recombinant laccase were 6.6 and 60°C, respectively, and it was stable at pH 9.0 for 10 days. The enzyme activity was slightly activated by NaCl with concentration up to 200 mM. The purified laccase showed high efficiency in decolorizing reactive black 5 and indigo carmine, achieving more than 93% decolorization after 1h. The extreme robustness of the recombinant B. amyloliquefaciens laccase offers several advantages over most fungal laccases in various industrial applications.

Performance of microbial phytases for gastric inositol phosphate degradation

Microbial phytases catalyze dephosphorylation of phytic acid, thereby potentially releasing chelated iron and improving human iron absorption from cereal-based diets. For this catalysis to take place in vivo, the phytase must be robust to low pH and proteolysis in the gastric ventricle. This study compares the robustness of five different microbial phytases, evaluating thermal stability, activity retention, and extent of dephosphorylation of phytic acid in a simulated low-pH/pepsin gastric environment and examines secondary protein structural changes at low pH via circular dichroism. The Peniophora lycii phytase was found to be the most thermostable, but the least robust enzyme in gastric conditions, whereas the Aspergillus niger and Escherichia coli phytases proved to be most resistant to gastric conditions. The phytase from Citrobacter braakii showed intermediate robustness. The extent of loss of secondary structure at low pH correlated positively with the extent of activity loss at low pH.

In situ prebiotics for weaning piglets: in vitro production and fermentation of potato galacto-rhamnogalacturonan

Postweaning diarrhea (PWD) in pigs is a leading cause of economic loss in pork production worldwide. The current practice of using antibiotics and zinc to treat PWD is unsustainable due to the potential of antibiotic resistance and ecological disturbance, and novel methods are required. In this study, an in vitro model was used to test the possibility of producing prebiotic fiber in situ in the gastrointestinal (GI) tract of the piglet and the prebiotic activity of the resulting fiber in the terminal ileum. Soluble fiber was successfully produced from potato pulp, an industrial waste product, with the minimal enzyme dose in a simulated upper GI tract model extracting 26.9% of the initial dry matter. The fiber was rich in galactose and galacturonic acid and was fermented at 2.5, 5, or 10 g/liter in a glucose-free medium inoculated with the gut contents of piglet terminal ileum. Fermentations of 5 g/liter inulin or 5 g/liter of a purified potato fiber were used as controls. The fibers showed high fermentability, evident by a dose-dependent drop in pH and an increase in the organic acid content, with lactate in particular being increased. Deep sequencing showed a significant increase in the numbers of Lactobacillus and Veillonella organisms and an insignificant increase in the numbers of Clostridium organisms as well as a decrease in the numbers of Streptococcus organisms. Multivariate analysis showed clustering of the treatment groups, with the group treated with purified potato fiber being clearly separated from the other groups, as the microbiota composition was 60% Lactobacillus and almost free of Clostridium. For animal studies, a dosage corresponding to the 5-g/liter treatment is suggested.

Rational design of a new Trypanosoma rangeli trans-sialidase for efficient sialylation of glycans

This paper reports rational engineering of Trypanosoma rangeli sialidase to develop an effective enzyme for a potentially important type of reactivity: production of sialylated prebiotic glycans. The Trypanosoma cruzi trans-sialidase and the homologous T. rangeli sialidase has previously been used to investigate the structural requirements for trans-sialidase activity. We observed that the T. cruzi trans-sialidase has a seven-amino-acid motif (197–203) at the border of the substrate binding cleft. The motif differs substantially in chemical properties and substitution probability from the homologous sialidase, and we hypothesised that this motif is important for trans-sialidase activity. The 197–203 motif is strongly positively charged with a marked change in hydrogen bond donor capacity as compared to the sialidase. To investigate the role of this motif, we expressed and characterised a T. rangeli sialidase mutant, Tr13. Conditions for efficient trans-sialylation were determined, and Tr13's acceptor specificity demonstrated promiscuity with respect to the acceptor molecule enabling sialylation of glycans containing terminal galactose and glucose and even monomers of glucose and fucose. Sialic acid is important in association with human milk oligosaccharides, and Tr13 was shown to sialylate a number of established and potential prebiotics. Initial evaluation of prebiotic potential using pure cultures demonstrated, albeit not selectively, growth of Bifidobacteria. Since the 197–203 motif stands out in the native trans-sialidase, is markedly different from the wild-type sialidase compared to previous mutants, and is shown here to confer efficient and broad trans-sialidase activity, we suggest that this motif can serve as a framework for future optimization of trans-sialylation towards prebiotic production.

The binding of zinc ions to Emericella nidulans endo-β-1,4-galactanase is essential for crystal formation

A novel Emericella nidulans endo-β-1,4-galactanase (EnGAL) demonstrates a strong capacity to generate high levels of very potent prebiotic oligosaccharides from potato pulp, a by-product of the agricultural potato-starch industry. EnGAL belongs to glycoside hydrolase family 53 and shows high (72.5%) sequence identity to an endo-β-1,4-galactanase from Aspergillus aculeatus. Diffraction data extending to 2.0 Å resolution were collected from a crystal of EnGAL grown from conditions containing 0.2 M zinc acetate. The crystal structure showed a high similarity between EnGAL and other endo-β-1,4-galactanases belonging to GH53. It also revealed 15 zinc ions bound to the protein, one of which is located in the active site, where it is coordinated by residues Glu136 and Glu246 which comprise the catalytic machinery. The majority of the zinc ions are located on the surface of the enzyme, in some cases with side chains from two different molecules as ligands, thus explaining why the presence of zinc ions was essential for crystallization.

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.

Innovative analytical tools to characterize prebiotic carbohydrates of functional food interest

Functional foods are one of the most interesting areas of research and innovation in the food industry. A functional food or functional ingredient is considered to be any food or food component that provides health benefits beyond basic nutrition. Recently, consumers have shown interest in natural bioactive compounds as functional ingredients in the diet owing to their various beneficial effects for health. Water-soluble fibers and nondigestible oligosaccharides and polysaccharides can be defined as functional food ingredients. Fructooligosaccharides (FOS) and inulin are resistant to direct metabolism by the host and reach the caecocolon, where they are used by selected groups of beneficial bacteria. Furthermore, they are able to improve physical and structural properties of food, such as hydration, oil-holding capacity, viscosity, texture, sensory characteristics, and shelf-life. This article reviews major innovative analytical developments to screen and identify FOS, inulins, and the most employed nonstarch carbohydrates added or naturally present in functional food formulations. High-performance anion-exchange chromatography with pulsed electrochemical detection (HPAEC-PED) is one of the most employed analytical techniques for the characterization of those molecules. Mass spectrometry is also of great help, in particularly matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS), which is able to provide extensive information regarding the molecular weight and length profiles of oligosaccharides and polysaccharides. Moreover, MALDI-TOF-MS in combination with HPAEC-PED has been shown to be of great value for the complementary information it can provide. Some other techniques, such as NMR spectroscopy, are also discussed, with relevant examples of recent applications. A number of articles have appeared in the literature in recent years regarding the analysis of inulin, FOS, and other carbohydrates of interest in the field and they are critically reviewed.

Enzymatic depolymerization of gum tragacanth: bifidogenic potential of low molecular weight oligosaccharides

Gum tragacanth derived from the plant "goat's horn" (Astragalus sp.) has a long history of use as a stabilizing, viscosity-enhancing agent in food emulsions. The gum contains pectinaceous arabinogalactans and fucose-substituted xylogalacturonans. In this work, gum tragacanth from Astragalus gossypinus was enzymatically depolymerized using Aspergillus niger pectinases (Pectinex BE Color). The enzymatically degraded products were divided into three molecular weight fractions via membrane separation: HAG1 < 2 kDa; 2 kDa < HAG2 < 10 kDa; HAG3 > 10 kDa. Compositional and linkage analyses showed that these three fractions also varied with respect to composition and structural elements: HAG1 and HAG2 were enriched in arabinose, galactose, and galacturonic acid, but low in fucose and xylose, whereas HAG3 was high in (terminal) xylose, fucose, and 1,4-bonded galacturonic acid, but low in arabinose and galactose content. The growth-stimulating potential of the three enzymatically produced gum tragacanth fractions was evaluated via growth assessment on seven different probiotic strains in single-culture fermentations on Bifidobacterium longum subsp. longum (two strains), B. longum subsp. infantis (three strains), Lactobacillus acidophilus , B. lactis, and on one pathogenic strain of Clostridium perfringens . The fractions HAG1 and HAG2 consistently promoted higher growth of the probiotic strains than HAG3, especially of the three B. longum subsp. infantis strains, and the growth promotion on HAG1 and HAG2 was better than that on galactan (control). HAG3 completely inhibited the growth of the C. perfringens strain. Tragacanth gum is thus a potential source of prebiotic carbohydrates that exert no viscosity effects and which may find use as natural functional food ingredients.

Functional characterization of the galactan utilization system of Geobacillus stearothermophilus

Type I galactan is a pectic polysaccharide composed of β-1,4 linked units of d-galactose and is part of the main plant cell wall polysaccharides, which are the most abundant sources of renewable carbon in the biosphere. The thermophilic bacterium Geobacillus stearothermophilus T-6 possesses an extensive system for the utilization of plant cell wall polysaccharides, including a 9.4-kb gene cluster, ganREFGBA, which encodes galactan-utilization elements. Based on enzyme activity assays, the ganEFGBA genes, which probably constitute an operon, are induced by short galactosaccharides but not by galactose. GanA is a glycoside hydrolase family 53 β-1,4-galactanase, active on high molecular weight galactan, producing galactotetraose as the main product. Homology modelling of the active site residues suggests that the enzyme can accommodate at least eight galactose molecules (at subsites -4 to +4) in the active site. GanB is a glycoside hydrolase family 42 β-galactosidase capable of hydrolyzing short β-1,4 galactosaccharides into galactose. Applying both GanA and GanB on galactan resulted in the full degradation of the polymer into galactose. The ganEFG genes encode an ATP-binding cassette sugar transport system whose sugar-binding lipoprotein, GanE, was shown to bind galacto-oligosaccharides. The utilization of galactan by G. stearothermophilus involves the extracellular galactanase GanA cleaving galactan into galacto-oligosaccharides that enter the cell via a specific transport system GanEFG. The galacto-oligosaccharides are further degraded by the intracellular β-galactosidase GanB into galactose, which is then metabolized into UDP-glucose via the Leloir pathway by the galKET gene products.

DATABASE

Nucleotide sequence data have been deposited in the GenBank database under the accession number JF327803.