Plant Hemicellulose Research Articles

Mechanistic insights on the conversion of sugars into 5-hydroxymethylfurfural

A sustainable bio-economy requires the efficient utilization of all of the components of biomass. After glucose, mannose and galactose are the most abundant 6-carbon sugars in plant hemicellulose; lactose is a common dairy byproduct. Here, we demonstrate the conversion of these sugars into 5-hydroxymethylfurfural (HMF), a key intermediate for renewable fuels and chemicals. Chromium salts enable the efficient conversion of mannose into HMF but are less effective for galactose, lactose, and the related hexulose, tagatose. Isotopic labeling studies and the reactivity of psicose and sorbose indicate that the key aldose-to-ketose conversion occurs via a 1,2-hydride shift, which is followed by dehydration of the furanose tautomer. These data valorize mannose, galatactose, and lactose as sources of HMF, and inform the design of new catalysts for the conversion of aldoses to ketoses.

Mode of action of glycoside hydrolase family 5 glucuronoxylan xylanohydrolase from Erwinia chrysanthemi

The mode of action of xylanase A from a phytopathogenic bacterium, Erwinia chrysanthemi, classified in glycoside hydrolase family 5, was investigated on xylooligosaccharides and polysaccharides using TLC, MALDI-TOF MS and enzyme treatment with exoglycosidases. The hydrolytic action of xylanase A was found to be absolutely dependent on the presence of 4-O-methyl-D-glucuronosyl (MeGlcA) side residues in both oligosaccharides and polysaccharides. Neutral linear beta-1,4-xylooligosaccharides and esterified aldouronic acids were resistant towards enzymatic action. Aldouronic acids of the structure MeGlcA(3)Xyl(3) (aldotetraouronic acid), MeGlcA(3)Xyl(4) (aldopentaouronic acid) and MeGlcA(3)Xyl(5) (aldohexaouronic acid) were cleaved with the enzyme to give xylose from the reducing end and products shorter by one xylopyranosyl residue: MeGlcA(2)Xyl(2), MeGlcA(2)Xyl(3) and MeGlcA(2)Xyl(4). As a rule, the enzyme attacked the second glycosidic linkage following the MeGlcA branch towards the reducing end. Depending on the distribution of MeGlcA residues on the glucuronoxylan main chain, the enzyme generated series of shorter and longer aldouronic acids of backbone polymerization degree 3-14, in which the MeGlcA is linked exclusively to the second xylopyranosyl residue from the reducing end. Upon incubation with beta-xylosidase, all acidic hydrolysis products of acidic oligosaccharides and hardwood glucuronoxylans were converted to aldotriouronic acid, MeGlcA(2)Xyl(2). In agreement with this mode of action, xylose and unsubstituted oligosaccharides were essentially absent in the hydrolysates. The E. chrysanthemi xylanase A thus appears to be an excellent biocatalyst for the production of large acidic oligosaccharides from glucuronoxylans as well as an invaluable tool for determination of the distribution of MeGlcA residues along the main chain of this major plant hemicellulose.

The Quality of Stemwood of Pinus sylvestris in an Alkalised Environment

The impact of long-term dust pollution emitted from a cement plant on soil chemistry, and the concentrations of plant nutrients, lignin, cellulose and hemicellulose in the stemwood of 80–85-year-old Pinus sylvestris was investigated at different distances from the emission source. It was found that alkaline cement dust (pH 12.3–12.6) emissions for over 40 years resulted in an alkalisation (pH 6.7–7.9) of the polluted soil compared to a pH value of 3.8 in unpolluted soil. There were also nutrient imbalances in the soil, as well as certain disturbances in mineral nutrition processes and accumulation of nutrients in the tree stems. The average concentrations of K, Ca and Mg in stems were higher and those of N and P lower than in the unpolluted area. The lignin (L) content in stemwood increased, hemicellulose (Hc) decreased, while cellulose (Ce) did not change. A variation in the partitioning of L, Ce, Hc and nutrients between different sections of stems and between trees from different sample plots was found. L, Ce and Hc were not related to the internal K, Ca and Mg concentrations. Correlations were established between L, Ce, or Hc content and C content, and between L and Hc content in stem tissues. The contents of wood components were not related to N or P in the alkalised areas, but seemed to be more associated with P than with N. Alterations in the arbitrarily chosen ratio L/(Ce + Hc) indicated changes in wood quality, and a negative correlation with N/P was found in stem tissue in the polluted area, while positive correlations with N/Mg and Ca/Mg were found in the control area.

Thermostability of endo-1,4-β-xylanase II from Trichoderma reesei studied by electrospray ionization Fourier-transform ion cyclotron resonance MS, hydrogen/deuterium-exchange reactions and dynamic light scattering

Endo-1,4-β-xylanase II (XYNII) from Trichoderma reesei is a 21kDa enzyme that catalyses the hydrolysis of xylan, the major plant hemicellulose. It has various applications in the paper, food and feed industries. Previous thermostability studies have revealed a significant decrease in enzymic activity of the protein at elevated temperatures in citrate buffer [Tenkanen, Puls and Poutanen (1992) Enzyme Microb. Technol. 14, 566–574]. Here, thermostability of XYNII was investigated using both conventional and nanoelectrospray ionization Fourier-transform ion cyclotron resonance MS and hydrogen/deuterium (H/D)-exchange reactions. In addition, dynamic light scattering (DLS) was used as a comparative method to observe possible changes in both tertiary and quaternary structures of the protein. We observed a significant irreversible conformational change and dimerization when the protein was exposed to heat. H/D exchange revealed two distinct monomeric protein populations in a narrow transition temperature region. The conformational change in both the water and buffered solutions occurred in the same temperature region where enzymic-activity loss had previously been observed. Approx. 10–30% of the protein was specifically dimerized when exposed to the heat treatment. However, adding methanol to the solution markedly lowered the transition temperature of conformational change as well as increased the dimerization up to 90%. DLS studies in water confirmed the change in conformation observed by electrospray ionization MS. We propose that the conformational change is responsible for the loss of enzymic activity at temperatures over 50°C and that the functioning of the active site in the enzyme is unfeasible in a new, more labile solution conformation.

Thermostability of endo-1,4-beta-xylanase II from Trichoderma reesei studied by electrospray ionization Fourier-transform ion cyclotron resonance MS, hydrogen/deuterium-exchange reactions and dynamic light scattering.

Endo-1,4-beta-xylanase II (XYNII) from Trichoderma reesei is a 21 kDa enzyme that catalyses the hydrolysis of xylan, the major plant hemicellulose. It has various applications in the paper, food and feed industries. Previous thermostability studies have revealed a significant decrease in enzymic activity of the protein at elevated temperatures in citrate buffer [Tenkanen, Puls and Poutanen (1992) Enzyme Microb. Technol. 14, 566-574]. Here, thermostability of XYNII was investigated using both conventional and nanoelectrospray ionization Fourier-transform ion cyclotron resonance MS and hydrogen/deuterium (H/D)-exchange reactions. In addition, dynamic light scattering (DLS) was used as a comparative method to observe possible changes in both tertiary and quaternary structures of the protein. We observed a significant irreversible conformational change and dimerization when the protein was exposed to heat. H/D exchange revealed two distinct monomeric protein populations in a narrow transition temperature region. The conformational change in both the water and buffered solutions occurred in the same temperature region where enzymic-activity loss had previously been observed. Approx. 10-30% of the protein was specifically dimerized when exposed to the heat treatment. However, adding methanol to the solution markedly lowered the transition temperature of conformational change as well as increased the dimerization up to 90%. DLS studies in water confirmed the change in conformation observed by electrospray ionization MS. We propose that the conformational change is responsible for the loss of enzymic activity at temperatures over 50 degrees C and that the functioning of the active site in the enzyme is unfeasible in a new, more labile solution conformation.

Hyperthermophilic xylanases

Xylan is a major component of plant hemicellulose and is the second most abundant polysaccharide in nature. It is a complex polymer consisting of a β -D-1,4-linked xylanopyranoside backbone, which can be acetylated or substituted with arabinosyl and glucuronic acid side groups. The complete enzymatic hydrolysis of xylan into assimilable sugars requires a number of enzymes, including endo- β -1,4-xylanase. More than 80 sequences of the genes coding for xylanases have been reported and many more xylanases from bacteria and fungi have been purified and characterized. A striking feature of many microbial xylanolytic systems is the presence of multiple family 10 and/or 11 xylanases that are produced as discrete gene products. This chapter focuses on xylanases from bacteria that grow above 80° and is essentially confined to the enzymes that have been cloned from Dictyoglomus and Thermotoga . The Thermotoga xylanase described in 1991 remains the most stable xylanase known. Only one Archeal xylanase is known, from Thermococcus zilligii isolate AN1, and so far, the gene for this enzyme has been refractory to cloning so the information is for the nonrecombinant form.

The quality of the forage eaten by Norwegian reindeer on South Georgia in summer

The chemical composition and digestibility of plants selected by Norwegian reindeer (Rangifer t. tarandus) on the sub-Antarctic island of South Georgia (SG) were investigated in the austral summer and compared with two qualities of standard grasses of Phleum pratense of European origin. Paridiochola flabellata, Poa pratense, Poa annua, Deschampsia antarctka, and Phleum alpinum collected on SG contained 14.8, 17.6, 22.8, 16.1 and 10.1% respectively of crude protein of dry matter (DM). Aceana magellanica also collected on SG contained 19.8% of crude protein and 18.8% of water-soluble carbohydrates (WSC) of DM, while the tussock grass P. flabellata, contained as much as 29-3% of WSC of DM. Total plant cell-wall contents (CWC), including cellulose, hemi-cellulose and lignin in P. flabellata, P. pratense, P. annua and P. alpinum were 53.2, 49.6, 41.7 and 40.4% of DM respectively, while A. magellanica contained only 17.5% of DM CWC. The lignin concentrations of plants analysed varied between 1.2 and 3.2% of DM. Mean in vitro dry matter digestibility (IVDMD) of selected plants ranged from 70% in P. flabellata to 83% in P. alpinum after 48 h incubation in rumen fluid from these reindeer. In contrast, the IVDMD of the poor and high quality standard grass Phleum pratense were 54% and 73% of DM, respectively. The forage eaten by reindeer on SG in summer was of high quality, with low lignin content, moderate protein concentration and high degradability in rumen fluid.

Horizontal gene transfer of glycosyl hydrolases of the rumen fungi.

By combining analyses of G + C content and patterns of codon usage and constructing phylogenetic trees, we describe the gene transfer of an endoglucanase (celA) from the rumen bacteria Fibrobacter succinogenes to the rumen fungi Orpinomyces joyonii. The strong similarity between different glycosyl hydrolases of rumen fungi and bacteria suggests that most, if not all, of the glycosyl hydrolases of rumen fungi that play an important role in the degradation of cellulose and other plant polysaccharides were acquired by horizontal gene transfer events. This acquisition allows fungi to establish a habitat within a new environmental niche: the rumen of the herbivorous mammals for which cellulose and plant hemicellulose constitute the main raw nutritive substrate.

Microbial Xylanolytic Enzyme System: Properties and Applications

The chapter introduces xylan as a major component of plant hemicellulose. It is the most abundant renewable polysaccharide in nature after cellulose. The chapter describes its structure and its interaction with plant cell walls. Due to the complex structure of xylans, several different enzymes are needed for their enzymatic degradation and modification. Properties and production of different enzymes—namely, xylanases, β-Xylosidases, α-Arabinosidases, α-Glucuronidases, and esterases are discussed in the chapter. Application of xylanases in pulp and paper making is reviewed in the chapter. The process involves some steps: prebleaching of kraft pulps, enzymatic debarking, fiber modification, production of dissolving pulp, removal of shives, and retting of flax fibers. The potential for use of xylanolytic enzymes in the food and feed industries is also high. The main aim in the application of xylanolytic enzymes is the hydrolysis of hemicellulosic substrate for production of fermentable sugars. The knowledge gathered on the hydrolysis mechanism of hemicelluloses, especially xylans, is greatly promoted the rapid application of these enzymes in new areas. Attention is focused on recent advances and on several aspects that were not covered in earlier reviews.

Stereoselective synthesis of regioisomers of aldobiouronic acid.

alpha-Glucuronidase is a very important enzyme for the complete hydrolysis of plant hemicellulose, but the substrate specificity of the enzyme has not previously been reported. In this connection, the three regioisomers of O-(alpha-D-glucopyranosyluronic acid)-D-xylose (aldobiouronic acid), 2-O-(alpha-D-glucopyranosyluronic acid)-D-xylose (13), 3-O-(alpha-D-glucopyranosyluronic acid)-D-xylose (14), and 4-O-(alpha-D-glucopyranosyluronic acid)-D-xylose (15), were stereoselectively synthesized to clarify the substrate specificity.

Growth of Rumen Bacteria on Plant Cell Wall Polysaccharides

SUMMARY: Three strains of Butyrivibrio fibrisolvens were isolated from the rumen contents of cattle feeding on red (Trifolium pratense L.) or white (T. repens L.) clover. The substrates used in these isolations were plant hemicellulose fractions other than simple insoluble xylan. The strains showed some differences in their ability to grow on various plant polysaccharides and to secrete polysaccharases specific to these polymers. The same type of rumen contents yielded, on polygalacturonic acid media, a strain of Lachnospira multiparus which grew only on pectin and secreted as sole polysaccharase a polygalacturonase. Only one of the three B. fibrisolvens strains grew vigorously on polygalacturonic acid and its polygalacturonase appeared to be different to that of the L. multiparus.

Digestibility of Forage Hemicellulose and Pectin by Rumen Bacteria in Vitro and the Effect of Lignification Thereon1,2

Summary Four maturity stages of timothy, three of alfalfa, and two of orchard grass were used as substrates to study the rate and extent of hemicellulose fermentation by rumen bacteria in vitro. In all cases, hemicellulose fermentation decreased as the plant matured. When the forage particle size was markedly reduced by ball-milling, an attempt to remove any barrier type of inhibition by lignin, the extent of hemicellulose fermentation increased. This increase became larger with advancing maturity of the forage. The rate and extent of pectin digestion were estimated with three stages of alfalfa. As with the hemicellulose, both the rate and extent of pectin fermentation decreased as the alfalfa matured and the effect of maturity ivas lessened considerably after reduction of the alfalfa particle size by ball-milling. The experimental evidence presented in this paper indicates that both hemicellulose and pectin digestion are influenced by the maturity of the forage, and suggests that this effect is the result of lignin forming a physical barrier between the plant hemicellulose or pectin and the rumen bacteria.