Views: 3 Author: Site Editor Publish Time: 2023-04-28 Origin: Site
Xylan is a hemicellulose, a polysaccharide mainly composed of xylose residues.It is present in secondary cell walls of plants, dicots and all cell walls of grasses.Xylan is the third most abundant biopolymer on Earth, after cellulose and chitin.
Xylans are polysaccharides composed of β-1,4-linked residues of xylose (a pentose sugar) and side chains of α-arabinofuranose and/or α-glucuronic acid.Based on substituent groups, xylans can be divided into three classes: i) glucuronoxylan (GX) ii) neutral arabinoxylan (AX) and iii) glucuronoxylan (GAX ).Cross-linking of cellulose microfibrils and lignin facilitated by ferulic acid residues in some cases.
Plant cell structure
Xylans play an important role in the integrity of plant cell walls and in increasing the resistance of cell walls to enzymatic digestion; thus, they help plants defend against herbivores and pathogens (biotic stress).Xylan also plays an important role in plant growth and development.Typically, hardwoods contain 10-35% xylan, while softwoods contain 10-15%.The main xylan constituent in hardwoods is O-acetyl-4-O-methylglucuronoxylans,while arabino-4-O-methylglucuronoxylans is the main constituent in softwoods.In general, softwood xylans differ from hardwood xylans by the absence of acetyl groups and the presence of arabinose units attached to the xylan backbone via α-(1,3)-glycosidic bonds.
Some macrophyte green algae contain xylans (particularly homoxylans especially in the genera Codium and Bryopsis which replace cellulose in the cell wall matrix. Also,it replaces the inner fibrous cell wall layer of cellulose in some red algae.
The quality of cereal flour and the firmness of dough is influenced by its xylan content and thus plays an important role in the bakery industry.The main component of xylan can be converted into xylitol (a xylose derivative), which is used as a natural food sweetener, helps reduce tooth decay, and can be used as a sugar substitute for diabetic patients.High proportion of xylan in poultry feed.Xylan is one of the most important antinutritional factors in commonly used feed ingredients.Xylo-oligosaccharides produced from xylans are considered "functional foods" or dietary fibers because of their potential prebiotic properties.
Several glycosyltransferases are involved in xylan biosynthesis.In eukaryotes, GTs account for approximately 1% to 2% of gene products.GTs are assembled into complexes present in the Golgi apparatus.However, the xylan synthase complex has not been isolated from Arabidopsis tissue (dicots).The first gene involved in xylan biosynthesis was identified in the xylem mutant (irx) of Arabidopsis thaliana due to mutations in some genes affecting xylan biosynthesis.As a result, the plant grows abnormally due to thinning and weakening of secondary xylem cell walls.