Purification and characterization of beta-xylosidase activities from the yeast Arxula adeninivorans

β-Xylosidases: Structural Diversity, Catalytic Mechanism, and Inhibition by Monosaccharides

Xylan, a prominent component of cellulosic biomass, has a high potential for degradation into reducing sugars, and subsequent conversion into bioethanol. This process requires a range of xylanolytic enzymes. Among them, β-xylosidases are crucial, because they hydrolyze more glycosidic bonds than any of the other xylanolytic enzymes. They also enhance the efficiency of the process by degrading xylooligosaccharides, which are potent inhibitors of other hemicellulose-/xylan-converting enzymes. On the other hand, the β-xylosidase itself is also inhibited by monosaccharides that may be generated in high concentrations during the saccharification process. Structurally, β-xylosidases are diverse enzymes with different substrate specificities and enzyme mechanisms. Here, we review the structural diversity and catalytic mechanisms of β-xylosidases, and discuss their inhibition by monosaccharides.

Characterization of the AXDH gene and the encoded xylitol dehydrogenase from the dimorphic yeast Arxula adeninivorans

The xylitol dehydrogenase-encoding Arxula adeninivorans AXDH gene was isolated and characterized. The gene includes a coding sequence of 1107 bp encoding a putative 368 amino acid protein of 40.3 kDa. The identity of the gene was confirmed by a high degree of homology of the derived amino acid sequence to that of xylitol dehydrogenases from different sources. The gene activity was regulated by carbon source. In media supplemented with xylitol, D-sorbitol and D-xylose induction of the AXDH gene and intracellular accumulation of the encoded xylitol dehydrogenase was observed. This activation pattern was confirmed by analysis of AXDH promoter-GFP gene fusions. The enzyme characteristics were analysed from isolates of native strains as well as from those of recombinant strains expressing the AXDH gene under control of the strong A. adeninivorans-derived TEF1 promoter. For both proteins, a molecular mass of ca. 80 kDa was determined corresponding to a dimeric structure, an optimum pH at 7.5 and a temperature optimum at 35 degrees C. The enzyme oxidizes polyols like xylitol and D-sorbitol whereas the reduction reaction is preferred when providing D-xylulose, D-ribulose and L-sorbose as substrates. Enzyme activity exclusively depends on NAD+ or NADH as coenzymes.

Purification and biochemical properties of a thermostable xylose-tolerant β-D-xylosidase from Scytalidium thermophilum

The thermophilic fungus Scytalidium thermophilum produced large amounts of periplasmic beta- D-xylosidase activity when grown on xylan as carbon source. The presence of glucose in the fresh culture medium drastically reduced the level of beta- D-xylosidase activity, while cycloheximide prevented induction of the enzyme by xylan. The mycelial beta-xylosidase induced by xylan was purified using a procedure that included heating at 50 degrees C, ammonium sulfate fractioning (30-75%), and chromatography on Sephadex G-100 and DEAE-Sephadex A-50. The purified beta- D-xylosidase is a monomer with an estimated molecular mass of 45 kDa (SDS-PAGE) or 38 kDa (gel filtration). The enzyme is a neutral protein (pI 7.1), with a carbohydrate content of 12% and optima of temperature and pH of 60 degrees C and 5.0, respectively. beta- D-Xylosidase activity is strongly stimulated and protected against heat inactivation by calcium ions. In the absence of substrate, the enzyme is stable for 1 h at 60 degrees C and has half-lives of 11 and 30 min at 65 degrees C in the absence or presence of calcium, respectively. The purified beta- D-xylosidase hydrolyzed p-nitrophenol-beta- D-xylopyranoside and p-nitrophenol-beta- D-glucopyranoside, exhibiting apparent K(m) and V(max) values of 1.3 mM, 88 micromol min(-1) protein(-1) and 0.5 mM, 20 micromol min(-1) protein(-1), respectively. The purified enzyme hydrolyzed xylobiose, xylotriose, and xylotetraose, and is therefore a true beta- D-xylosidase. Enzyme activity was completely insensitive to xylose, which inhibits most beta-xylosidases, at concentrations up to 200 mM. Its thermal stability and high xylose tolerance qualify this enzyme for industrial applications. The high tolerance of S. thermophilum beta-xylosidase to xylose inhibition is a positive characteristic that distinguishes this enzyme from all others described in the literature.

Molecular characterisation and expression analysis of the first hemicellulase gene (bxl1) encoding beta-xylosidase from the thermophilic fungus Talaromyces emersonii

The gene coding for beta-xylosidase, bxl1, has been cloned from the thermophilic filamentous fungus, Talaromyces emersonii. This is the first report of a hemicellulase gene from this novel source. At the genomic level, bxl1 consists of an open reading frame of 2388 nucleotides with no introns that encodes a putative protein of 796 amino acids. The bxl1 translation product contains a signal peptide of 21 amino acids that yields a mature protein of 775 amino acids, with a predicted molecular mass of 86.8 kDa. The deduced amino acid sequence of bxl1 exhibits considerable homology with the primary structures of the Aspergillus niger, Aspergillus nidulans, Aspergillus oryzae, and Trichoderma reesei beta-xylosidase gene products, and with some beta-glucosidases, all of which have been classified as Family 3 glycosyl hydrolases. Northern blot analysis of the bxl1 gene indicates that it is induced by xylan and methyl-beta-D-xylopyranoside. D-Xylose induced expression of bxl1 but was shown to repress induction of the gene at high concentrations. The presence of six CreA binding sites in the upstream regulatory sequence (URS) of the bxl1 gene indicates that the observed repression by D-glucose may be mediated, at least partly, by this catabolite repressor.

Degradation of xylan to D-xylose by recombinant Saccharomyces cerevisiae coexpressing the Aspergillus niger beta-xylosidase (xlnD) and the Trichoderma reesei xylanase II (xyn2) genes

The beta-xylosidase-encoding xlnD gene of Aspergillus niger 90196 was amplified by the PCR technique from first-strand cDNA synthesized on mRNA isolated from the fungus. The nucleotide sequence of the cDNA fragment was verified to contain a 2,412-bp open reading frame that encodes a 804-amino-acid propeptide. The 778-amino-acid mature protein, with a putative molecular mass of 85.1 kDa, was fused in frame with the Saccharomyces cerevisiae mating factor alpha1 signal peptide (MFalpha1(s)) to ensure correct posttranslational processing in yeast. The fusion protein was designated Xlo2. The recombinant beta-xylosidase showed optimum activity at 60 degrees C and pH 3.2 and optimum stability at 50 degrees C. The K(i(app)) value for D-xylose and xylobiose for the recombinant beta-xylosidase was determined to be 8.33 and 6.41 mM, respectively. The XLO2 fusion gene and the XYN2 beta-xylanase gene from Trichoderma reesei, located on URA3-based multicopy shuttle vectors, were successfully expressed and coexpressed in the yeast Saccharomyces cerevisiae under the control of the alcohol dehydrogenase II gene (ADH2) promoter and terminator. These recombinant S. cerevisiae strains produced 1,577 nkat/ml of beta-xylanase activity when expressing only the beta-xylanase and 860 nkat/ml when coexpressing the beta-xylanase with the beta-xylosidase. The maximum beta-xylosidase activity was 5.3 nkat/ml when expressed on its own and 3.5 nkat/ml when coexpressed with the beta-xylanase. Coproduction of the beta-xylanase and beta-xylosidase enabled S. cerevisiae to degrade birchwood xylan to D-xylose.

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.

Comparative biochemical, genetical and immunological studies of glucoamylase producing Arxula adeninivorans yeast strains

Seven Arxula adeninivorans strains originating from The Netherlands (CSIR 1136, CSIR 1138 and CBS 8244T), South Africa (CSIR 1147, CSIR 1148 and CSIR 1149) and Siberia (Ls3) were compared concerning their secretory glucoamylase. Within the spectrum of secretory polypeptides obtained by SDS-polyacrylamide gel electrophoresis, the glucoamylase corresponding polypeptide, could be identified by means of specific antibodies directed against the glucoamylase of strain Ls3. The molecular masses of the glucoamylases vary from 84 to 95 kDa. After endoglycosidase F treatment of the secretory proteins, the N-linked carbohydrate content for each glucoamylase could be quantified at about 25%. Glucoamylase activity staining of secretory proteins separated under nondenaturing conditions revealed one glucoamylase active protein for the Dutch strains and two active forms for the strains originating from South Africa and Siberia. Highest activities of glucoamylase can be measured at the end of the exponential growth phase for each strain. The Dutch strain CSIR 1138 secretes the highest activity. Optimum values for temperature and pH were determined and compared. By means of pulsed field gel electrophoresis and Southern analysis, the glucoamylase corresponding gene could be localized on the second of the four chromosomes of each yeast strain.

Characterization of Arxula adeninivorans strains from different habitats

Some Arxula adeninivorans strains selected from wood hydrolysates in Siberia, from soil in South Africa and from maize silage and soil in The Netherlands were compared. DNA-fingerprinting, pulse field gel electrophoresis as well as analysis of secretory proteins have been chosen to describe the similarities among the strains. Combination of the three methods allowed identification of each strain. Strains from the same origin show extensive similarities. The results of the DNA-fingerprints indicate that the strain isolated in Siberia belongs to the group of strains originated from South Africa. However, it differed in the molecular weight of the third chromosome and in the pattern of secretory proteins from the South African isolates.