Hemicellulase Hemicellulose

The combined action of xylanase and mannanase on sprucewood holocellulose increased the hydrolysis of hemicelluloses without any detectable attack of cellulose. At the end of the experiments—i.e., after 48 hr of xylanase incubation followed by 32 hr of combined xylanase-mannanase incubation—about half the hemicelluloses present in the starting material were selectively converted into low-molecular-weight sugars. The amount of mannan removed was two times higher than after 80 hr of incubation with mannanase only. Unexpectedly, the xylan dissolution was scarcely increased by the combined action of the two hemicellulases. 

About a quarter of the mannan in the sprucewood holocellulose seems to be accessible to the mannanase. More mannan can be hydrolyzed only when the second hemicellulose becomes at least partly dissolved. The remaining mannan—less than half the initial amount— appears to be removable only together with cellulose degradation. In beechwood holocellulose, most of the main hemicellulose—i.e., the xylan —can be hydrolyzed rather selectively by the sole action of xylanase. This phenomenon could be due to the different molecular size of the two hemicellulases, mannanase about 24 A, xylanase about 18 A (JO), resulting in better diffusing conditions for the xylanase. However, the porosity of the... 

In contrast to cellulose, which is crystalline, strong, and resistant to hydrolysis, hemicellulose has a random, amorphous structure with little strength. It is easily hydrolyzed by dilute acid or base, but nature provides an arsenal of hemicellulase enzymes for its hydrolysis. Hemicellulases are commercially important because they open the structure of wood for easier bleaching and thus support the introduction of ECF or TCF methods. Many different pentoses are usually present in hemicellulose. Xylose, however, is always the predominating sugar. The pentoses are also present in rings (not shown) that can be five- or six-membered. 

The food, feed, and paper industries use hemicellulase to convert hemicellulose to useful products (Wong and Saddler, 1993). At the concentrations found in vegetable waste matter, hemicellulose has been viewed intermit-tendy as a potential enzyme substrate and as feedstock for commercial ethanol and furfural production. 

In contrast to cellulases, the hemicellulases encompass a much broader suite of activities. In addition to analogue versions of endo-, exo-, and glycosidase cellulase activities, multiple debranching activities are needed to handle the high complexity of the heterogeneous hemicelluloses (Table 33.3). The varied backbone composition of hemicelluloses also adds complexity. Xylans, xyloglucans, man-nans, and numerous other minor polysaccharide chains form the backbone for different hemicelluloses. 

J. Puls, J. Schuseil in Hemicelluloses and Hemicellulases M. P. Coughlan, G. P. Hazle-wood (eds), Portland Press, London, England, 1993, pp. 1-97. 

This chapter focuses on production of sugars from cellulosic or fibrous biomass feedstocks using conversion routes based on enzymatic hydrolysis. Section 4.2 describes the nature of cellulase and hemicellulase enzyme systems capable of depolymerizing cellulose and hemicellulose to soluble sugars, and reviews the prominent concepts of enzyme synergy and biomass recalcitrance. Performance of enzymatic hydrolysis under anticipated real-world processing conditions involving... 

The synergy between cellulases, hemicellulases, and accessory enzymes has been explored in some depth but is still not fully understood. Studies utilizing purified cellulases and hemicellulases have shown that when some of the hemicellulose component in lignocellulose remains intact following pretreatment, the enzymatic hydrolysis of the remaining xylan and substituted xylan moieties improves cellulose hydrolysis by cellulases (Selig et al., 2008, 2009). This improvement in cellulose... 

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