Xylobiose hydrolysis

When xylobiose was tested as a substrate, deviation from Michaelis-Menten kinetics consistent with transglycosylation activity was observed at higher substrate concentrations (up to 16 mM xylobiose was tested). Capillary electrophoresis analysis of the reaction products from xylobiose hydrolysis showed production of significant xylose and xylotriose, corroborating the transglycosylation activity inferred by the kinetic data. Thus, under the... 

Fig. 2 pH dqiendence of steady-state kinetic parameters for 1,4-3-D-xylobiose hydrolysis catalyzed by SXA at 25°C. Initial rates were determined from reactions in buffers of constant ionic strength (7==0.3 M) and kinetic parameters were determined by fitting initial-rate data to Eq. 1 SEs ( ) are indicated, a kcat vs pH. The curve was generated by fitting values vs pH to Eq. 5 pXa=7.57 0.03, pH-independent ka = 2 2. b kcJKm vs pH. The curve was generated by fitting k lK values vs pH to Eq. 6 pXai=4.86 0.06,... 

This enzyme [EC 3.2.1.37], also referred to as xylan 1,4-)3-xylosidase and exo-l,4-/3-xylosidase, catalyzes the hydrolysis of l,4-)3-D-xylans resulting in the removal of successive o-xylose residues from the nonreducing termini. Xylobiose can also be utilized as a substrate. The sheep liver enzyme has been reported to exhibit other exoglycosidase activities. 

The hydrolysis of acetylated xylo-oligomers from the steaming extract of birchwood using an enzyme mixture containing only xylanase and / -xylosidase was very limited (Table III). Chemical deacetylation showed that the substrate specificities of xylanase and / -xylosidase with respect to substrate DP overlapped, and that /J-xylosidase also hydrolyzed longer oligosaccharides than xylobiose to xylose. The addition of acetyl esterase to the hydrolysis of acetylated xylo-oligomers enhanced xylose production (Table III), but deacetylation was still incomplete (the acetyl content of... 

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.

The complete degradation of hemicellulose becomes more complex than that of cellulase, since substituent-hydrolyzing activities are also necessary. With heteroxylans, apart from endo-l,4- 3-xylanase, which catalyzes the hydrolysis of internal 3-l,4-xylan links and P-xylosidase, which catalyzes the hydrolysis of xylooligossacharides, mainly xylobiose into xylose, other enzymes must act to accomplish complete hydrolysis, such as acetyl xylan esterase, a-glucuronidase, and a-L-arabinofuranosidase (1). 

Very recently, in the first thorough study, the hydrolysis of o-nitrophenyl xylobioside by ten mutants of the GHIO xylanase/glycanase Cex from Cell-ulomonas DocumentIfimi was correlated with for the binding of five xylobiose... 

Table 1 Michaelis-Menten kinetic parameters for hydrolysis of p-NP-AiaF, p-NP-AraP, p- and o-NP-XylP, and xylobiose.

Fig. 7 Progress curves of l,4- 3-D-xylobiose (X2) hydrolysis catalyzed by SXA. Reactions contained 100 mM succinate-NaOH, pH 5.3 at 25 °C. Curves were generated from the KINSIM calculations, assuming rapid equilibrium binding, with the indicated [enzyme] and [X2] and the following as inputs + X2 <=> X2...

A measure of the numerical sequences of residues in a wheat arabinoxylan that accommodate enzymolysis by a Streptomyces xylanase, is given by the pattern of periodate oxidation of the hydrolysis fragments. For the observed release of xylobiose, for example, the sequence required is at least four consecutive D-xylopyranose residues having no L-arabinofuranosyl branches. Polysaccharides from soil contain considerable proportions of (1 4)-linked glucose and xylose residues that resist periodate oxidation, for both chemical and physical reasons that remain unsolved. ... 

The kinetics of the hydrolysis of D-xylo-oligosaccharides, ranging from di- to penta-saccharide, in dilute sulphuric acid have been studied.The rate of hydrolysis of the glycosidic bond in 4-0-j3-D-xylopyranosyl-D-xylose was similar to that of the same bond at the reducing end of the other oligosaccharides and was 1.8 times the rate of each internal bond. The yield of D-xylose from each oligosaccharide could be calculated from this ratio and from the empirical equation which describes the rate constant for the above xylobiose. 

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