Molecular and biotechnological aspects of xylanases

Abstract

Hemicellulolytic microorganisms play a significant role in nature by recycling hemicellulose, one of the main components of plant polysaccharides. Xylanases (EC 3.2.1.8) catalyze the hydrolysis of xylan, the major constituent of hemicellulose. The use of these enzymes could greatly improve the overall economics of processing lignocellulosic materials for the generation of liquid fuels and chemicals. Recently cellulase-free xylanases have received great attention in the development of environmentally friendly technologies in the paper and pulp industry. In microorganisms that produce xylanases low molecular mass fragments of xylan and their positional isomers play a key role in regulating its biosynthesis. Xylanase and cellulase production appear to be regulated separately, although the pleiotropy of mutations, which causes the elimination of both genes, suggests some linkage in the synthesis of the two enzymes. Xylanases are found in a cornucopia of organisms and the genes encoding them have been cloned in homologous and heterologous hosts with the objectives of overproducing the enzyme and altering its properties to suit commercial applications. Sequence analyses of xylanases have revealed distinct catalytic and cellulose binding domains, with a separate non-catalytic domain that has been reported to confer enhanced thermostability in some xylanases. Analyses of three-dimensional structures and the properties of mutants have revealed the involvement of specific tyrosine and tryptophan residues in the substrate binding site and of glutamate and aspartate residues in the catalytic mechanism. Many lines of evidence suggest that xylanases operate via a double displacement mechanism in which the anomeric configuration is retained, although some of the enzymes catalyze single displacement reactions with inversion of configuration. Based on a dendrogram obtained from amino acid sequence similarities the evolutionary relationship between xylanases is assessed. In addition the properties of xylanases from extremophilic organisms have been evaluated in terms of biotechnological applications.

Simultaneous saccharification and fermentation of hemicellulose to butanol by a non-sporulating Clostridium species

“…The binding of insoluble materials to cells plays an important role in the efficiency of the enzymatic hydrolysis of insoluble lignocellulosic biomass. Therefore, these results suggest that strain MF28 has an essential component that anchors cells to xylan, such as a xylan-binding domain (XBD) (Kulkarni et al, 1999). To further understand the binding domains, quantitative reverse transcription PCR was conducted to measure the expression of putative binding genes in strain MF28.…”

“…5C). Naturally occurring ''xylano somes'' are self-assembled complexes present on the cell surface, comprising a multimodular non-catalytic scaffold in protein and containing at least one carbohydrate-binding domain (e.g., XBD) which targets the catalytic domains toward saccharification of complex biomass substrates (Kulkarni et al, 1999). The high expression of members of the xbd gene family in strain MF28 strongly suggests that a xylan-binding domain/module contributes to carbohydrate-cell binding.…”

Wood production, wood technology, and biotechnological impacts

“…In order to meet these demands, the genes of the enzymes in question are overexpressed in heterologous hosts like ascomycetous yeasts (Saccharomyces cerevisiae, Pichia pastoris) or filamentous ascomycetes (Aspergillus spp., Trichoderma sp.). This approach is successfully applied with hydrolytic enzymes from ascomycetous species (Kulkarni et al 1999), but was repeatedly shown to be problematic for enzymes from wood-decaying basidiomycetes due to differences in protein glycosylation (e.g. see , Larrondo et al 2003a.…”

“…Important for this is that the enzymes are as much as possible cellulase-free (Christov et al 1999). Also for applications in the textile industry, presence of cellulases is unwanted in xylanase preparations in order not to damage the celluose (Kulkarni et al 1999. Production conditions, in particular media components, can favour which types of enzymes are obtained from the fungi and help to avoid or reduce impurities with unwanted enzymes (Haltrich et al 1996).…”

Isolation of Moderately Halophilic Pseudoalteromonas Producing Extracellular Hydrolytic Enzymes from Persian Gulf

“…Extracellular hydrolytic enzymes such as amylases, proteases, lipases and DNases, have quite diverse potential usages in different areas such as food industry, feed additive, biomedical sciences and chemical industries [5][6][7][8]. Industrial processes are carried out under specific physical and chemical conditions which cannot always be adjusted to the optimal values required for the activity of the available enzymes.…”