Purification and partial characterization of two xylanases that differ in hydrolysis of soluble and insoluble xylan fractions

Characterization of Thermophilic Lignocellulolytic Microorganisms in Composting

Composting involves the selection of a microbiota capable of resisting the high temperatures generated during the process and degrading the lignocellulose. A deep understanding of the thermophilic microbial community involved in such biotransformation is valuable to improve composting efficiency and to provide thermostable biomass-degrading enzymes for biorefinery. This study investigated the lignocellulose-degrading thermophilic microbial culturome at all the stages of plant waste composting, focusing on the dynamics, enzymes, and thermotolerance of each member of such a community. The results revealed that 58% of holocellulose (cellulose plus hemicellulose) and 7% of lignin were degraded at the end of composting. The whole fungal thermophilic population exhibited lignocellulose-degrading activity, whereas roughly 8-10% of thermophilic bacteria had this trait, although exclusively for hemicellulose degradation (xylan-degrading). Because of the prevalence of both groups, their enzymatic activity, and the wide spectrum of thermotolerance, they play a key role in the breakdown of hemicellulose during the entire process, whereas the degradation of cellulose and lignin is restricted to the activity of a few thermophilic fungi that persists at the end of the process. The xylanolytic bacterial isolates (159 strains) included mostly members of Firmicutes (96%) as well as a few representatives of Actinobacteria (2%) and Proteobacteria (2%). The most prevalent species were Bacillus licheniformis and Aeribacillus pallidus. Thermophilic fungi (27 strains) comprised only four species, namely Thermomyces lanuginosus, Talaromyces thermophilus, Aspergillus fumigatus, and Gibellulopsis nigrescens, of whom A. fumigatus and T. lanuginosus dominated. Several strains of the same species evolved distinctly at the stages of composting showing phenotypes with different thermotolerance and new enzyme expression, even not previously described for the species, as a response to the changing composting environment. Strains of Bacillus thermoamylovorans, Geobacillus thermodenitrificans, T. lanuginosus, and A. fumigatus exhibiting considerable enzyme activities were selected as potential candidates for the production of thermozymes. This study lays a foundation to further investigate the mechanisms of adaptation and acquisition of new traits among thermophilic lignocellulolytic microorganisms during composting as well as their potential utility in biotechnological processing.

Role of lignin and thermophilic lignocellulolytic bacteria in the evolution of humification indices and enzymatic activities during compost production

This study aimed to evaluate the effect of lignin content and thermophilic lignocellulolytic bacteria bioaugmentation on composting process. Treatments including bioaugmentation with thermophilic lignocellulolytic bacteria isolates such as Paenibacillus validus, Paenibacillus koreensis, Bacillus nealsonii, a mixture of the three mentioned bacterial isolates and control were compared at two level of organic media (high lignin content and low lignin content) in the form of nested factorial design. Several indices such as humification and enzymatic activities were monitored to evaluate the composting rate. The results revealed that high lignin treatments displayed higher ligninase, xylanase, protease and urease enzymatic activities compared to low lignin treatments. On the other hand, low lignin treatments showed higher level of humification indices, cellulase, beta-glucosidase and alkaline phosphomonoesterase enzymatic activities in comparison with high lignin treatments. Also, all measured enzymatic activities are at their highest between the second and the tenth weeks; however, this trend decreased to reach a steady point from the 18th weeks to the 24th weeks, but for urease enzymatic activity, a totally different trend in high and low lignin treatments was observed. Moreover, the highest humification indices as well as the cellulase and β-glucosidase enzymatic activities were associated to the Bacillus nealsonii isolate and the full consortium. They also displayed the highest ligninase, xylanase, protease, and urease and phosphatase activities. The efficient isolates shortened the time required for completing the composting process for about 2 to 4 weeks compared to the control treatments. For all measured indices, the control treatment had the lowest values.

Uncovering new indicators to predict stability, maturity and biodiversity of compost on an industrial scale

Currently, the metagenomic study of the composting process has gained great importance since it has allowed the identification of the existence of microorganisms that, until now, had not been isolated during the process by traditional techniques. However, it is still complex to determine which bioindicators could reveal the degree of maturity and stability of a particular compost. Thereby, the main objective of this work was to demonstrate the possible correlation between traditional parameters of maturity and stability of compost, with other indicators of biodiversity in products highly heterogeneous from composting processes on an industrial scale. The results demonstrated the enormous influence of the raw materials in characterizing the products obtained. Even so, important relationships were established between the Chao1 and Shannon indexes, and certain parameters related to the maturity, stability and toxicity of the samples, such as nitrification index, humification rate, phenolic content, germination index or oxygen consumption.

Enzymatic profiles associated with the evolution of the lignocellulosic fraction during industrial-scale composting of anthropogenic waste: Comparative analysis

In the new European Waste Law, composting is proposed as one of the best options to properly manage organic waste of anthropogenic origin. Currently, the massive generation of this type of waste, as well as its heterogeneity, makes difficult in many cases control this process of degradation on an industrial scale. In this work, 15 facilities were selected based on 5 types of organic waste: Urban Solid Waste, Vegetable Waste, Sewage Sludges, Agrifood Waste and "Alpeorujo". The samples were collected in different thermal phases. The results revealed very different physicochemical and enzymatic profiles, as well as different degrees of humification depending on the process and the raw materials. However, parameters such as β-glucosidase, amylase, lignin/holocellulose ratio and humification rate showed similar trends in all cases. All of them could act as important indicators to evaluate the quality of a composting process, despite the heterogeneity of the starting materials.

Evolution of enzymatic activities and carbon fractions throughout composting of plant waste

Many alternatives for the proper disposal of horticultural plant wastes have been studied, and composting is one of the most attractive due to its insignificant environmental impact and low cost. The quality of compost for agronomical use is related to the degree of organic matter maturation and stabilization. Traditional parameters as well as temperature, ratio C/N, cationic exchange capacity, extractable carbon, or evolution of humificated substances have been successfully used to assess compost maturity and stability. However, microorganisms frequently isolated during composting release a wide range of hydrolytic enzymes, whose activity could apparently give interesting information on the rate of decomposition of organic matter and, therefore, on the product stability. The aim of this work was to study the evolution of some important enzymatic activities during composting of agricultural wastes and their comparison with other chemical parameters commonly employed as quality and maturity indexes, to establish a relationship between the degradation intensity of specific organic carbon fractions throughout the process. In this work, the chemical and biochemical parameters of plant wastes were studied along a composting process of 189 days to evaluate their importance as tools for compost characterization. Results showed an intense enzymatic activity during the first 2-3 weeks of composting (bio-oxidative phase), because of the availability of easily decomposable organic compounds. From a biological point of view, a less intense phase was observed between second and third month of composting (mesophilic or cooling phase). Finally, chemical humification parameters were more closely associated with the period between 119 and 189 days (maturation phase). Significant correlations between the enzymatic activities as well as between enzyme activities and other more traditional parameters were also highlighted, indicating that both kind of indexes can be a reliable tool to determine the degree of stability and maturation of horticultural plant wastes based-compost.

Evaluation of the xylan breakdown potential of eight mesophilic endoxylanases

In biomass degradation using simultaneous saccharification and fermentation (SSF), there is a need for efficient biomass degrading enzymes that can work at lower temperatures suitable for yeast fermentation. As xylan is an important lignocellulosic biomass constituent, this study aimed at investigating the possible differences in xylan breakdown potential of endoxylanases using eight different endoxylanases at conditions relevant for SSF. Both solubilising and degrading capacities of the endoxylanases were investigated using water-insoluble and water-soluble oat spelt xylan as model substrates for biomass xylan. Results showed that selecting for combinations of endoxylanases that are efficient at solubilising xylan on the one hand and degrading it to large extent on the other hand, coupled to high specific activities, seems the best option for complete xylan breakdown in lignocellulosic biomass conversion using SSF.

Microbial population dynamics and enzyme activities in composting processes with different starting materials

A biological comparison based on differences in the starting material for composting processes was made. Mesophilic aerobic microbiota, fungi, actinomycetes and hemicellulolytic microorganisms reached significantly higher levels in the MSW final product. The population of cellulolytic microorganisms did not show a clear trend, although it was more numerous in the HW piles. Counts for N(2)-fixing and ammonifying bacteria were significantly higher in the SS pile at the early stages of the process, while populations tended to become equal as time progressed. The lowest populations were detected for nitrifying bacteria, with higher but not always significant levels for the SS pile. beta-Glucosidase and phosphatase activities were higher in the SS pile at the early stages. Protease reached its maximum activity during the bio-oxidative phase and final stages in the HW and MSW piles, respectively. Dehydrogenase activity, with an occasional high level for the MSW at the beginning of the process, was almost inexistent since the end of the bio-oxidative phase. On the contrary, urease showed higher levels at the final stage of the process, with the MSW pile showing the greatest levels most of the time. According to these results, the nature of the starting material causes differences in biological parameters.

Xylanase inhibitors bind to nonstarch polysaccharides

This study is an in-depth investigation of the interaction between polysaccharides and the proteinaceous xylanase inhibitors, Triticum aestivum xylanase inhibitor (TAXI), xylanase inhibitor protein (XIP), and thaumatin-like xylanase inhibitor (TLXI). The binding affinities of all three known types of xylanase inhibitors from wheat are studied by measuring the residual xylanase inhibition activity after incubation of the inhibitors in the presence of different polysaccharides, such as beta-glucans and (arabino)xylans. The binding affinities of all three xylanase inhibitors for (arabino)xylans increased with a decreasing arabinose/xylose ratio (A/X ratio). This phenomenon was observed both with water-extractable and water-unextractable (arabino)xylans. The inhibitors also interacted with different soluble and insoluble beta-glucans. None of the inhibitors tested had the ability to hydrolyze the polysaccharides investigated. The present findings contribute to the unraveling of the function of xylanase inhibitors in nature and to the prediction of the effect of added xylanases in cereal-based biotechnological processes, such as bread making and gluten-starch separation.

TLXI, a novel type of xylanase inhibitor from wheat (Triticum aestivum) belonging to the thaumatin family

Wheat (Triticum aestivum) contains a previously unknown type of xylanase (EC 3.2.1.8) inhibitor, which is described in the present paper for the first time. Based on its >60% similarity to TLPs (thaumatin-like proteins) and the fact that it contains the Prosite PS00316 thaumatin family signature, it is referred to as TLXI (thaumatin-like xylanase inhibitor). TLXI is a basic (pI> or =9.3 in isoelectric focusing) protein with a molecular mass of approx. 18-kDa (determined by SDS/PAGE) and it occurs in wheat with varying extents of glycosylation. The TLXI gene sequence encodes a 26-amino-acid signal sequence followed by a 151-amino-acid mature protein with a calculated molecular mass of 15.6-kDa and pI of 8.38. The mature TLXI protein was expressed successfully in Pichia pastoris, resulting in a 21-kDa (determined by SDS/PAGE) recombinant protein (rTLXI). Polyclonal antibodies raised against TLXI purified from wheat react with epitopes of rTLXI as well as with those of thaumatin, demonstrating high structural similarity between these three proteins. TLXI has a unique inhibition specificity. It is a non-competitive inhibitor of a number of glycoside hydrolase family 11 xylanases, but it is inactive towards glycoside hydrolase family 10 xylanases. Progress curves show that TLXI is a slow tight-binding inhibitor, with a K(i) of approx. 60-nM. Except for zeamatin, an alpha-amylase/trypsin inhibitor from maize (Zea mays), no other enzyme inhibitor is currently known among the TLPs. TLXI thus represents a novel type of inhibitor within this group of proteins.

Biochemical properties of genetic recombinant xylanase II

The aim of this study was to overexpress the xylanase II gene of Trichoderma reesei in Escherichia coli and determine the characteristics of the recombinant enzyme. Recombinant xylanase II gene was constructed by ligating the cDNA of xylanase, obtained from reverse transcriptase-polymerase chain reaction, and fused with NusA protein of pET-431b plasmid. An Ni2+-NTA affinity column was used to further purify the recombinant xylanase II. The molecular mass of the recombinant enzyme measured by sodium dodecyl sulfate-polyacrylamide gel electrophoresis was approx 76 kDa (including 55 kDa of NusA and 21 kDa of xylanase II), and the isoelectric point and specific activity were 7.5 and 225 U/mg, respectively. The optimal reaction temperature and pH for the recombinant enzyme were 50 degrees C and 4.0, respectively. The recombinant enzyme was stable at a pH range of 5.0-10.0 and maintained 95% residual activity after incubating at 30-35 degrees C for 30 min. The kinetic parameters KM and Vmax of the recombinant xylanase II were 13.8 mg/mL and 336 micromol/(mg.min), respectively, using birchwood xylan as the substrate.

Effect of inoculation in composting processes: modifications in lignocellulosic fraction

Three microbial isolates, identified as Bacillus shackletonni, Streptomyces thermovulgaris and Ureibacillus thermosphaericus were tested as inoculants in composting processes in relation to their capacity to improve lignocellulose degradation. Different wastes from agricultural activities were used as raw material for the heaps: pepper plant waste (PPW) as the main component and olive-oil mill waste (OMW), almond shell (AS), pruning waste (PW) and rice straw (RS) as additives. Cellulose was more extensively degraded than hemicellulose and lignin, although the use of inoculants (B. shackletonni and S. thermovulgaris) improved the action of the autochthonous microbiota just in the AS heaps. A higher efficiency was observed for lignin, since lower concentrations of this polymer were detected in the inoculated heaps in relation to control heaps. U. thermosphaericus was the most efficient microorganism since inoculation with this strain decreased the final lignin content in a range between 17.23% and 24.34%. S. thermovulgaris and B. shackletonni led to a higher reduction of the lignin levels in the OMW and PW heaps (14.25% and 19.07% less lignin than control heaps) and OMW (13%), respectively. The composting process can therefore be improved by means of inoculation if the microorganisms used for this purpose are appropriate for the characteristics of the raw material.

Purification and characterization of two endo-1,4-beta-xylanases from Antarctic krill, Euphausia superba Dana

Two Euphausia superba Dana endo-1.4-beta-xylanases (A, and B), hydrolysing xylan in the same manner as the enzyme classified as EC 3.2.1.8, were isolated and purified. (2) The enzymes were distinguished by their molecular mass and charge, affinities towards the oat xylan (Km of 4.1 and 7.7 mg ml(-1), respectively), values of activation energy in oat xylan hydrolysis (35.5 and 42.5 kJ mol(-1), respectively), as well as the way in which they split the substrate. (3) In vitro they showed the same optimal temperature (37-40 degrees C), optimal pH (5.7-6.0), very low thermostability, and were stabilized and activated by Ca2+ and Mg2+ ions, as well as by some unidentified substances with molecular mass less than 17 kDa, present in crude extracts of krill.

Influence of ferulic acid on the production of feruloyl esterases by Aspergillus niger

Extracellular feruloyl esterases from the filamentous fungus Aspergillus niger are induced by growth on oat spelt xylan (OSX), which contains no detectable esterified ferulic acid. FAE-III accounted for most of the feruloyl esterase activity. Addition of free ferulic acid to OSX at the start of the culture induced FAE-III secretion a further 2.3-fold, and also induced other feruloyl esterases which could not be ascribed to FAE-III. Wheat bran-(WB)-grown cultures, containing 1% (m/v) esterlinked ferulic acid, gave almost identical FAE-III and total feruloyl esterase activities as the cultures grown on OSX plus ferulic acid. De-esterification of WB yielded less total feruloyl esterase, and 2.4-fold less FAE-III, compared to untreated WB. A slightly modified form of FAE-III was produced on de-esterified WB. These results show that production of FAE-III does not absolutely require ferulic acid. However, production is stimulated by the presence of free ferulic acid through increased expression, and is reduced by the removal of esterified ferulic acid from the growth substrate.

Xylanolytic enzymes from fungi and bacteria

The development of new analytical techniques and the commercial availability of new substrates have led to the purification and characterization of a large number of xylan-degrading enzymes. Furthermore, the introduction of recombinant DNA technology has resulted in the selection of xylanolytic enzymes that are more suitable for industrial applications. For a successful integration of xylanases in industrial processes, a detailed understanding of the mechanism of enzyme action is, however, required. This review gives an overview of various xylanolytic enzyme systems from bacteria and fungi that have been described recently in more detail.