Improved synthesis of an aldobiouronic acid related to hardwood xylans, and preparation of a derivative thereof suitable for linking to proteins

Glucuronoyl Esterase Screening and Characterization Assays Utilizing Commercially Available Benzyl Glucuronic Acid Ester

Research on glucuronoyl esterases (GEs) has been hampered by the lack of enzyme assays based on easily obtainable substrates. While benzyl d-glucuronic acid ester (BnGlcA) is a commercially available substrate that can be used for GE assays, several considerations regarding substrate instability, limited solubility and low apparent affinities should be made. In this work we discuss the factors that are important when using BnGlcA for assaying GE activity and show how these can be applied when designing BnGlcA-based GE assays for different applications: a thin-layer chromatography assay for qualitative activity detection, a coupled-enzyme spectrophotometric assay that can be used for high-throughput screening or general activity determinations and a HPLC-based detection method allowing kinetic determinations. The three-level experimental procedure not merely facilitates routine, fast and simple biochemical characterizations but it can also give rise to the discovery of different GEs through an extensive screening of heterologous Genomic and Metagenomic expression libraries.

Glucuronoyl esterases are active on the polymeric substrate methyl esterified glucuronoxylan

Alkali extracted beechwood glucuronoxylan methyl ester prepared by esterification of 4-O-methyl-D-glucuronic acid side residues by methanol was found to serve as substrate of microbial glucuronoyl esterases from Ruminococcus flavefaciens, Schizophyllum commune and Trichoderma reesei. The enzymatic deesterification was monitored by (1)H NMR spectroscopy and evaluated on the basis of the decrease of the signal of the ester methyl group and increase of the signal of methanol. The results show for the first time the action of enzymes on polymeric substrate, which imitates more closely the natural substrate in plant cell walls than the low molecular mass artificial substrates used up to present.

Functional expression of a thermophilic glucuronyl esterase from Sporotrichum thermophile: identification of the nucleophilic serine

A glucuronyl esterase (GE) from the thermophilic fungus Sporotrichum thermophile, belonging to the carbohydrate esterase family 15 (CE-15), was functionally expressed in the methylotrophic yeast Pichia pastoris. The putative GE gene ge2 from the genomic DNA was successfully cloned in frame with the sequence for the Saccharomyces cerevisiae alpha-factor secretion signal under the transcriptional control of the alcohol oxidase (AOX1) promoter and integrated in P. pastoris X-33 to confirm that the encoded enzyme StGE2 exhibits esterase activity. The enzyme was active on substrates containing glucuronic acid methyl ester, showing optimal activity at pH 7.0 and 55 degrees C. The esterase displayed broad pH range stability between 4-10 and temperature stability up to 50 degrees C, rendering StGE2 a strong candidate for future biotechnological applications that require robust biocatalysts. ClustalW alignment of StGE2 with characterized GEs and selected homologous sequences, members of CE-15 family, revealed a novel consensus sequence G-C-S-R-X-G that features the characteristic serine residue involved in the generally conserved catalytic mechanism of the esterase family. The putative serine has been mutated, and the corresponding enzyme has been expressed in P. pastoris to prove that the candidate nucleophilic residue is responsible for catalyzing the enzymatic reaction.

Two glucuronoyl esterases of Phanerochaete chrysosporium

The white-rot fungus Phanerochaete chrysosporium produces glucuronoyl esterase, a recently discovered carbohydrate esterase, during growth on sugar beet pulp. Two putative genes encoding this enzyme, ge1 and ge2, were isolated and cloned. Heterologous expression in Aspergillus vadensis, Pycnoporus cinnabarinus and Schizophyllum commune resulted in extracellular glucuronoyl esterase activity, demonstrating that these genes encode this enzymatic function. The amino acid sequence of GE1 was used to identify homologous genes in the genomes of twenty-four fungi. Approximately half of the genomes, both from ascomycetes and basidiomycetes, contained putative orthologues, but their presence could not be assigned to any of fungal class or subclass. Comparison of the amino acid sequences of identified and putative glucuronoyl esterases to other types of carbohydrate esterases (CE) confirmed that they form a separate family of CEs. These enzymes are interesting candidates for biotechnological applications such as the separation of lignin and hemicellulose.

Identification of genes encoding microbial glucuronoyl esterases

One type of covalent linkages connecting lignin and hemicellulose in plant cell walls is the ester linkage between 4-O-methyl-D-glucuronic acid of glucuronoxylan and lignin alcohols. An enzyme that could hydrolyze such linkages, named glucuronoyl esterase, occurs in the cellulolytic system of the wood-rotting fungus Schizophyllum commune. Here we report partial amino acid sequences of the enzyme and the results of subsequent search for homologous genes in sequenced genomes. The homologous genes of unknown functions were found in genomes of several filamentous fungi and one bacterium. The gene corresponding to the cip2 gene of Hypocrea jecorina (Trichoderma reesei), known to be up-regulated under conditions of induction of cellulolytic and hemicellulolytic enzymes, was over-expressed in H. jecorina. The product of the cip2 gene was purified to homogeneity and shown to exhibit glucuronoyl esterase activity.

Synthetic esters recognized by glucuronoyl esterase from Schizophyllum commune

Glucuronoyl esterase is a novel carbohydrate esterase recently discovered in the cellulolytic system of the wood-rotting fungus Schizophyllum commune on the basis of its ability to hydrolyze methyl ester of 4-O-methyl-D-glucuronic acid. This substrate was not fully corresponding to the anticipated function of the enzyme to hydrolyze esters between xylan-bound 4-O-methyl-D-glucuronic acid and lignin alcohols occurring in plant cell walls. In this work we showed that the enzyme was capable of hydrolyzing two synthetic compounds that mimic the ester linkages described in lignin-carbohydrate complexes, esters of 4-O-methyl-D-glucuronic and D-glucuronic acid with 3-(4-methoxyphenyl)propyl alcohol. A comparison of kinetics of hydrolysis of methyl and 3-(4-methoxyphenyl)propyl esters indicated that the glucuronoyl esterase recognizes the uronic acid part of the substrates better than the alcohol type. The catalytic efficiency of the enzyme was much higher with the ester of 4-O-methyl-D-glucuronic acid than with that of D-glucuronic acid. Examination of the action of glucuronoyl esterase on a series of methyl esters of 4-O-methyl-D-glucopyranuronosyl residues alpha-1,2-linked to xylose and several xylooligosaccharides suggested that the rate of deesterification is independent of the character of the carbohydrate part glycosylated by the 4-O-methyl-D-glucuronic acid.

Glucuronoyl esterase--novel carbohydrate esterase produced by Schizophyllum commune

The cellulolytic system of the wood-rotting fungus Schizophyllum commune contains an esterase that hydrolyzes methyl ester of 4-O-methyl-d-glucuronic acid. The enzyme, called glucuronoyl esterase, was purified to electrophoretic homogeneity from a cellulose-spent culture fluid. Its substrate specificity was examined on a number of substrates of other carbohydrate esterases such as acetylxylan esterase, feruloyl esterase and pectin methylesterase. The glucuronoyl esterase attacks exclusively the esters of MeGlcA. The methyl ester of free or glycosidically linked MeGlcA was not hydrolysed by other carbohydrate esterases. The results suggest that we have discovered a new type of carbohydrate esterase that might be involved in disruption of ester linkages connecting hemicellulose and lignin in plant cell walls.

Synthesis and molecular structure of methyl 4-O-methyl-alpha-D-glucopyranuronate

A method for the preparation of methyl 4-O-methyl-α-D-glucopyranuronate and its single crystal X-ray structure determination are reported. The molecule adopts an almost ideal ⁴C₁ (^OC₃) conformation.

A chromogenic substrate for a beta-xylosidase-coupled assay of alpha-glucuronidase

4-Nitrophenyl 2-(4-O-methyl-alpha-d-glucopyranuronosyl)-beta-d-xylopyranoside obtained on deesterification of 4-nitrophenyl 2-O-(methyl 4-O-methyl-alpha-d-glucopyranosyluronate)-beta-d-xylopyranoside (Hirsch et al., Carbohydr. Res. 310, 145-149, 1998) was found to be an excellent substrate for the measurement of hemicellulolytic alpha-glucuronidase activity. A new precise alpha-glucuronidase assay was developed by coupling the alpha-glucuronidase-catalyzed formation of 4-nitrophenyl beta-d-xylopyranoside with its efficient hydrolysis by beta-xylosidase. A recombinant strain of Saccharomyces cerevisiae, harboring and expressing the beta-xylosidase gene xlnD of Aspergillus niger under control of the alcohol dehydrogenase II promoter on a multicopy plasmid, was used as a source of beta-xylosidase. The activity values of beta-xylosidase in the assay required to achieve a steady-state rate of 4-nitrophenol formation shortly after starting the alpha-glucuronidase reaction were obtained both experimentally and by calculation using the kinetics of coupled enzyme reactions.