دانلود رایگان مقاله به سمت تفکیک آنزیمی ساختار گیاه پیچیده زیلان

Abstract

Significant progress over the past few years has been achieved in the enzymology of microbial degradation and saccharification of plant xylan, after cellulose being the most abundant natural renewable polysaccharide. Several new types of xylan depolymerizing and debranching enzymes have been described in microorganisms. Despite the increasing variety of known glycoside hydrolases and carbohydrate esterases, some xylan structures still appear quite recalcitrant. This review focuses on the mode of action of different types of depolymerizing endoxylanases and their cooperation with β-xylosidase and accessory enzymes in breakdown of complex highly branched xylan structures. Emphasis is placed on the enzymatic hydrolysis of alkali-extracted deesterified polysaccharide as well as acetylated xylan isolated from plant cell walls under non-alkaline conditions. It is also shown how the combination of selected endoxylanases and debranching enzymes can determine the nature of prebiotic xylooligosaccharides or lead to complete hydrolysis of the polysaccharide. The article also highlights the possibility for discovery of novel xylanolytic enzymes, construction of multifunctional chimeric enzymes and xylanosomes in parallel with increasing knowledge on the fine structure of the polysaccharide.

3. Conclusions and future prospects

Despite obvious progress in understanding the structure of plant xylans and microbial enzymes of their degradation, we can recognize several aspects in both lines of research that require further clarification and deeper understanding. Minimal attention has been paid to learning more about the nature of the residues of the polysaccharide which are resistant to further enzyme digestion. The recent work suggesting two different xylan conformations in one xylan molecule in Arabidopsis as a consequence of density of substituents (Busse-Wicher et al., 2014), leads to an idea that the more resistant portion of the polysaccharide could correspond to more densely substituted parts of the xylan main chain. Structural studies of these recalcitrant domains could certainly stimulate the search for novel enzymes. In this connection we should mention certain confusion in the literature concerning the presence of D-galactose vs L-galactose in highly branched xylans. Clarification of this point could define the requirement of α-L-galactosidases. Recent discovery of unique xylanases catalyzing efficient hydrolysis of superbranched corn fiber xylan (Rogowski et al., 2015) could open a new chapter in the field of enzymatic breakdown of corn fiber hemicellulose. We are anxious to see the impact of this new development.