Xylo-oligosaccharides

Another reason is that xylo-oligosaccharides of defined structure are very important substrates that serve as model compounds for the optimization of hydrolytic processes and in enzymic assays. The enormous development... 

This article describes the further purification of this endo-xylanase and the measurement of its isoelectric point, partial amino acid sequence, activity and stability at various pHs and tempCTatures, and characteristic products when it hydrolyzes linear xylo-oligosaccharides. 

Xylo-oligosaccharides. Samples of X4 through X7 of 98% purity or better and of X3 and X9 of 90% or better were prepared by acidic hydrolysis of larchwood xylan and preparative chromatography, as previously described (i). 

Essentially pure xylo-oligosaccharides in 0.5% concentration were incubated with Xylanase II at 50°C in 0.05M sodium citrate buffer, pH 5.0 for X5 through X7 and pH 6.05 for Xg and X9, and samples after varying periods were submitted to HPLC. With X5, X2 and X3 were the major products, with smaller amounts of X4 and D-xylose produced. Incubation of gave X3 as the major product, followed by X4 and X2. With prolonged incubation the X4 was consumed to form X2 and smaller amounts of D-xylose and X3. Hydrolysis of X7 led to the formation of X4 and X3, with smaller amounts of X5, and X2, and a very small amount of Xg. Tlie course... 

Preliminary kinetic studies indicate that X4 hydrolysis was extremely slow, while larger straight-chain substrates were cleaved at rates that increased with increasing chain length. It is possible that branched-chain xylo-oligosaccharides were formed during incubation with Xylanase II their rates of formation and hydrolysis are as yet unknown. 

The xylanolytic enzyme system of Trichoderma reesei, a well-known producer of cellulolytic enzymes, is versatile and well suited for the total hydrolysis of different xylans. It consists of two major, specific and several non-specific xylanases, at least one / -xylosidase, a-arabinosidase and a-glucuronidase and at least two acetyl esterases. The hydrolysis of polymeric xylans starts by the action of endoxylanases. The side-groupcleaving enzymes have their highest activities towards soluble, short xylo-oligosaccharides, and make the substituted oligosaccharides again accessible for xylanases and / -xylosidase. 

The / -xylosidase of T. reesei was a rather large, probably dimeric enzyme and, like most other / -xylosidases, had an acidic pi-value (Table I). A / -xylosidase purified earlier from T. viride with a molecular weight of 101 kDa also had an isoelectric point pi 4.45 (5). In addition to p-nitrophenyl-/ -xylopyranoside and xylo-oligosaccharides, the / -xylosidase of T. reesei also showed activity with p-nitrophenyl-a-arabinofuranoside as substrate, but did not hydrolyze arabinan or release arabinose from arabinoxylan. The purified a-arabinosidase, however, was capable of effecting both the latter hydrolyses (26, Table II). The isoelectric point of the a-arabinosidase of T. reesei was pi 7.5 and its molecular weight 53 kDa (Table I). 

Figure 2. Hydrolysis products of beechwood O-acetylglucuronoxylan (Fig. 2a), beechwood glucuronoxylan (Fig. 2b), and wheat straw arabinoxylan (Fig. 2c), as analyzed by gel permeation chromatography. The hydrolysis was carried out at pH 5 at 45°C for 24 h using 10.000 nkat of the 20 kDa xylanase of T. reesei. X = xylose X2 = xylobiose Xmga — 4-O-methyl-glucuronosyl substituted xylo-oligosaccharides X = xylo-oligosaccharides DP > 20.

EC 3.1.1.72 acetylxylan esterase Deacetylation of xylans and xylo-oligosaccharides... 

Bray, M. R. and Clarke, A. J., Action pattern of xylo-oligosaccharide hydrolysis by Schizophyllum commune xylanase A. Eur J Biochem 1992,204 (1), 191 6. 

The xylanase produced by Agaricus bisporus158 degraded rice-straw hemicellulose, initially to L-arabinose and the xylo-oligosaccharides of d.p. 3= 2. D-Xylose was liberated upon prolonged hydrolysis. 

Aspergillus niger van Tieghem247,248 produced three D-xylanases, namely, D-xylanase I, II, and III, two of which (II and III) degraded rice-straw arabinoxylan to D-xylose, L-arabinose, and a mixture of arabinoxylo- and xylo-oligosaccharides as follows. 

Degradation products arising from the hydrolysis of arabinoxylo-and xylo-oligosaccharides of d.p. 3-6 and 2-5, respectively, by the action of a D-xylanase (HC I) from Ceratocystis paradoxa are shown in Table XXIV. Xylotetraose (X4) was the smallest xylo-oligosaccharide attacked, and both it and X5 yielded X2 and X3. The mechanism proposed228 for the hydrolysis of X4 by HC I is as follows. 

Degradation Products Arising from the Hydrolysis of Arabinoxylo- and Xylo-oligosaccharides by a D-Xylanase (HCI) from C. paradoxa... 

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