Enzymatic synthesis of alkyl ?-D-xylosides by transxylosylation and reverse hydrolysis

Chemistry of xylopyranosides

Xylose is one of the few monosaccharidic building blocks that are used by mammalian cells. In comparison with other monosaccharides, xylose is rather unusual and, so far, only found in two different mammalian structures, i.e. in the Notch receptor and as the linker between protein and glycosaminoglycan (GAG) chains in proteoglycans. Interestingly, simple soluble xylopyranosides can not only initiate the biosynthesis of soluble GAG chains but also function as inhibitors of important enzymes in the biosynthesis of proteoglycans. Furthermore, xylose is a major constituent of hemicellulosic xylans and thus one of the most abundant carbohydrates on Earth. Altogether, this has spurred a strong interest in xylose chemistry. The scope of this review is to describe synthesis of xylopyranosyl donors, as well as protective group chemistry, modifications, and conformational analysis of xylose.

β-Xylopyranosides: synthesis and applications

In recent years, β-xylopyranosides have attracted interest due to the development of biomass-derived molecules. This review focuses on general routes for the preparation of β-xylopyranosides by chemical and enzymatic pathways and their main uses.

A typical endo-xylanase from Streptomyces rameus L2001 and its unique characteristics in xylooligosaccharide production

The activity of the extracellular xylanase produced by Streptomyces rameus L2001 against different xylans and xylooligosaccharides (XOS) was investigated. The main products of hydrolysis of birchwood xylan and oat-spelt xylan by the S. rameus L2001 xylanase were xylobiose (X2) and xylotriose (X3), suggesting that this is an endo-acting xylanase. This was confirmed by analysis of XOS degradation products. The enzyme hardly hydrolyzed X2 and X3, but hydrolyzed xylotetraose (X4) and xylopentaose (X5) producing mainly X2 and X3 through transglycosylation. Depending on the substrate, different quantities of reducing sugars were produced by the xylanase: 150 mg/g from corncob, 105 mg/g from bean culms, and 133 mg/g from bagasse. With the bagasse substrate, the xylanase yielded 2.36, 2.76, 2.03, and 2.17 mg/mL of X2, X3, X4, and X5, respectively. The structure of xylobiose and xylotriose from the hydrolysis of corncob xylan was identified by MS and NMR. The production of XOS from various agricultural wastes has potential industrial applications. This is the first report of XOS production by S. rameus L2001.

Synthesis of sugar fatty acid esters using a β-xylosidase from Aspergillus awamori for transxylosylation

Aspergillus awamori K4 β-xylosidase has broad acceptor specificity. It has been used to synthesize a sugar fatty acid ester via its transxylosylation activity. One xylosyl residue was initially transferred to hexamethylene glycol as a linker with a yield of 0.36 g/g xylobiose. Linoleic acid was subsequently linked to one terminal hydroxyl side of the transfer product hydroxyhexyl xyloside through an esterification reaction catalyzed by a lipase. The synthesis of hexyl linoleoyl xyloside was confirmed by TOF-MS analysis. The binding with a linker improved the esterification reaction because of the hydrophobic hexamethylene chain and also prevented steric hindrance by the xylosyl residue. This sugar fatty acid ester synthesis method using transglycosylation should facilitate the production of emulsifiers or surfactants with various functions.

Characteristics of transxylosylation by β-xylosidase from Aspergillus awamori K4

The Aspergillus awamori K4 beta-xylosidase gene (Xaw1) sequence was deduced by sequencing RT-PCR and PCR products. The ORF was 2,412 bp and the predicted peptide was 804 amino acids long, corresponding to a molecular weight of 87,156 Da. The mature protein was 778 amino acids long with a molecular weight of 84,632 Da. A homology search of the amino acid sequence revealed that it was very similar to the Aspergillus niger beta-xylosidase gene with only five amino acid differences. K4 beta-xylosidase had the same catalytic mechanism as family 3 beta-glucosidases, involving Asp in region A. At an early stage in the reaction with xylobiose and xylotriose, the hydrolysis rate was much lower than the transxylosylation rate, decreasing gradually as the substrate concentration increased, whereas the transxylosylation rate increased greatly. Aspergillus awamori K4 beta-xylosidase had broad acceptor specificity toward alcohols, hydroxybenzenealcohols, sugar alcohols and disaccharides. A consensus portion involving the hydroxymethyl group of the acceptor was confirmed in the major transfer products 1(4)-O-beta-D-xylosyl erythritol, (2-hydroxyl)-phenyl-methyl-beta-D-xylopyranoside, 6S-O-beta-D-xylosyl maltitol (S: sorbitol residue) and 6G-O-beta-D-xylosyl palatinose (G: glucosyl residue). This might suggest that the methylene in the hydroxymethyl group facilitates base-catalyzed hydroxyl group attack of the anomeric center of the xylosyl-enzyme intermediate.

Transglycosylation reaction of xylanase B from the hyperthermophilic Thermotoga maritima with the ability of synthesis of tertiary alkyl beta-D-xylobiosides and xylosides

The recombinant xylanase B (XynB) of Thermotoga maritima MSB8 was characterized and was found to cleave p-nitrophenyl beta-D-xyloside via the transglycosylation reaction in the previous study. XynB was activated in the presence of alcohols, and XynB activity was increased by iso-propanol (2M) to 2.1-fold. This type of activation was investigated and was shown to be due to the transglycosylation activity with p-nitrophenyl beta-D-xylobioside being converted to alkyl beta-D-xylobiosides in the presence of XynB and alcohols. Through the transglycosylation reaction, alkyl beta-xylosides and xylobiosides were simultaneously produced in the presence of xylan and alcohols. Primary alcohols were found to be the best acceptors. The highest yields of alkyl beta-xylosides and xylobiosides were 33% and 50% of the total sugar, respectively. XynB showed a great ability to transfer xylose and xylobiose to secondary alcohol acceptors, and was unique for being able to synthesize the tertiary alkyl beta-xylosides and xylobiosides with high yields of 18.2% and 11.6% of the total sugar, respectively. This is the first report of a xylanase with the ability to synthesize tertiary alkyl beta-xylosides and xylobiosides. The specificity of the beta-linkage was confirmed by the proton nuclear magnetic resonance ((1)H NMR). Thus, XynB of T. maritima appears to be an ideal enzyme for the synthesis of useful alkyl beta-xylosides and xylobiosides.

Synthesis of alkyl-alpha-L-rhamnosides by water soluble alcohols enzymatic glycosylation

The synthesis of alkyl-alpha-rhamnosides by alpha-rhamnosidase was studied using rhamnose and rhamnosides, particularly the flavonoid naringin, as glycosylation agents, and water soluble alcohols as acceptors. The reaction products were analyzed by HPLC chromatography and identified by 13C y 1H NMR. The glycosylation of alcohols by reverse hydrolysis was maximum for 40% methanol, 30% ethanol, 10% propanol and 20% isopropanol. Under optimum conditions the yield of rhamnose to alkyl-alpha-rhamnoside transformation decreased from 68% for methyl-alpha-rhamnoside to 10% for isopropyl-alpha-rhamnoside. The time course of rhamnosylations produced using naringin as the donor was comparable with that of the reverse hydrolysis obtained at the same molar concentration of the donor. The flavonoids and their derivatives remaining in the solution after the glycosylation were removed by ion exchange QEAE chromatography at pH 10. These results indicate that both, reverse hydrolysis and glycosylation by naringin are acceptable procedures for the enzymatic synthesis of short chain length alkyl-alpha-L-rhamnosides.

Enzymatic synthesis of aroma compound xylosides using transfer reaction by Trichoderma longibrachiatum xylanase

Enzymatic synthesis of aroma compound xylosides was performed by Trichoderma longibrachiatum xylanase. Information concerning the nature of xylosides present in the reaction medium was obtained by GC-EI-MS, by GC-NCI-MS of TFA derivatives, and by positive FAB-MS of the reaction mixtures. Moreover, the structures of isolated benzyl beta-D-xylopyranoside and 4-O-beta-xylopyranosyl-beta-D-xylopyranoside were established by (1)H and (13)C NMR and heteronuclear two-dimensional ((1)H-(13)C) chemical shift correlation. The results obtained for hexyl and benzyl alcohol xylosides indicated that a reaction implying a transfer of one to two or three xylose units from xylan was involved. The enzyme was able to recognize xylobiose, xylotriose, and xylan as xylose donors. Benzyl xyloside, produced independently of xylobioside and xylotrioside, was found as the major kinetic product of the reaction. Benzyl xyloside was produced in higher quantities and at a higher rate than that obtained for the di- and trixyloside derivatives. The maximum production for benzyl xyloside, 1.29 g/L, was obtained in the presence of hexane (50%) used as cosolvent. Xylosides and xylobiosides of several aroma compounds, (Z)-hex-3-en-1-ol, heptan-2-ol, geraniol, nerol, and citronellol, were synthesized in different amounts, from 850 mg/L for (Z)-hex-3-en-1-yl xylosides to 1.5 mg/L for citronellyl xylosides. No synthesis occurred when menthol, linalool, and eugenol were used as acceptors.

Analysis of glycosidase-catalyzed transglycosylation reaction using probabilistic model

General mechanism of transglycosylation reaction by glycosidases contains branched paths to form and destroy the glycosylated intermediate. The probabilistic model was applied for the simulation and analysis of the transglycosylation mechanism. The model is composed of a single enzyme molecule and finite amounts of substrates and water molecules mimicking the possible smallest enzyme-catalyzed reaction system in a microcompartment. Using random numbers and probabilities, progress of distribution of reactants and products can be simulated and predicted with minimum adjustable parameters. Experimental data of beta-xylosidase and beta-glucosidase reactions were quantitatively analyzed with the simple scheme. Since the algorithm and simulation procedures are simple, the model is applicable to related complicated enzyme mechanisms containing many branched reaction paths.

Effective extraction and purification of beta-xylosidase from Trichoderma koningii fermentation culture by aqueous two-phase partitioning

Effective extraction of protein from bulk medium is an important technique in bioresearch. In the present study, we describe an extracellular beta-xylosidase from the fermentation supernatant of Trichoderma koningii G-39 that was successfully extracted and purified simultaneously in a single step by using an aqueous two-phase partitioning method. This two-phase system was prepared by dissolving suitable amount of poly(ethylene glycol) (PEG) and sodium dihydrogenphosphate (NaH(2)PO(4)) in aqueous solution. beta-Xylosidase was recovered with high yield and high concentration in the bottom salt-rich phase when 25% (w/v) PEG 1500 and 20-25% (w/v) NaH(2)PO(4) were applied. Based on a 1-liter scale extraction, the purity of the enzyme was enhanced at least 33-fold. The total activity increased 422% in comparison with that in the untreated filtrate. The effectiveness and simplicity may make this technique potentially useful in various applications. The transxylosylation activity of the enzyme purified by this technique was also investigated.

Effect of beta-galactosidase hydration on alcoholysis reaction in organic one-phase liquid systems

Alcoholysis reactions were performed in organic one-phase liquid systems with E. coli beta-galactosidase to produce heptyl-beta-galactoside from lactose and 1-heptanol. The reaction rate was highly dependent on the amount of water solubilized in the alcohol. A larger amount of water led to a system of two liquid phases in which the alcoholysis rate was 73% faster than in the one-phase system. No hydrolysis reaction of either lactose or product was observed in one-phase liquid systems up to 20 h, independent of the water content. Solubility of lactose in the organic phase increased with the water content in the system and the reaction followed the Michaelis-Menten model. Water activity was calculated for heptanol containing different amounts of water and the obtained values were used to estimate the hydration of beta-galactosidase from known models. Enzyme activity correlated with sorbed water, similar to the behavior reported for lysozyme in low water environments. It is concluded that water contribution to enzyme hydration dominates the rate of reaction compared to its effect on lactose solubilization.

Anomer-selective glucosylation of l-menthol by yeast alpha-glucosidase

l-Menthol was glucosylated by the alpha-glucosidase (EC 3.2.1.20) of Saccharomyces cerevisiae using maltose as the glucosyl donor. When 50 mg of l-menthol and 1.6 M maltose in 10 mM citrate-phosphate buffer (pH 5.5) were incubated at 45 degrees C, l-menthyl alpha-D-glucopyranoside (alpha-MenG) was alpha-anomer-selectively formed as a product. The specificity of the alpha-linkage was confirmed by 13C-NMR analysis. In the reaction mixture after 2 h, alpha-MenG was mainly accumulated in a crystalline form and the concentration of dissolved alpha-MenG was constant at 1.4 mM. The molar conversion yield of alpha-MenG produced based on the supplied l-menthol was maximally 30.7% at 48 h of reaction.

Factors affecting the specificity of beta-glucosidase from Fusarium oxysporum in enzymatic synthesis of alkyl-beta-D-glucosides

Fusarium oxysporum beta-glucosidase has been used to catalyze the production of alkyl-beta-D-glucosides from various disaccharides, based on the transglucosylation reaction, in the presence of primary, secondary and tertiary alcohols as glucosyl acceptors. Primary alcohols were found to be the best acceptors. The influence of the glucosyl donor concentration, as well as the enzyme specificity towards the cleaved glucosidic bond and the aglucone part of the donor, have also been investigated. The enzyme does not exhibit regiospecificity and seems to be unspecific towards the aglucone part. The specificity of the beta linkage has been confirmed by proton nuclear magnetic resonance (1H NMR) analysis.

Enzymatic synthesis of trisaccharides and alkyl beta-D-glucosides by the transglycosylation reaction of beta-glucosidase from Fusarium oxysporum

Purified beta-glucosidase from Fusarium oxysporum catalyses hydrolysis and transglycosylation reactions. By utilizing the transglycosylation reaction, trisaccharides and alkyl beta-D-glucosides were synthesized under optimal conditions in the presence of various disaccharides and alcohols. The yields of trisaccharides and alkyl beta-D-glucosides were 22-37% and 10-33% of the total sugar, respectively. The enzyme retained 70-80% of its original activity in the presence of 25% (w/v) methanol, ethanol and propanol. Thus, beta-glucosidase from F. oxysporum appears to be an ideal enzyme for the synthesis of useful trisaccharides and alkyl beta-D-glucosides.