Xylans

Xylans are a family of hemicelluloses that occur widely in plant cell walls. They are polysaccharides built on a backbone of β-1,4-linked d-xylopyranose residues and are typically decorated with various side chains and substituents. The most common backbones are glucuronoxylan, arabinoxylan, and glucuronoarabinoxylan; xylans may also be acetylated. Substitutions include arabinose and glucuronic acid, often in the form of methyl esters in some plant species, and acetyl groups that can affect solubility and enzyme accessibility. In grasses, ferulic or diferulic acid residues can link xylan chains to lignin, contributing to lignin cross-links.

Occurrence and function: Xylans are major hemicelluloses in plant cell walls, alongside cellulose and lignin. They provide structural support and influence cell wall porosity by interacting with cellulose microfibrils and lignin. The exact composition and substitutions vary by plant type: hardwoods tend to have glucuronoxylans with glucuronic acid substitutions; cereals and grasses often have arabinoxylans with arabinose side chains and ferulic cross-links.

Industrial and nutritional relevance: Xylans are targets for enzymatic degradation by xylanases and accessory enzymes, a step in pulp bleaching and in the conversion of plant biomass to biofuels and value-added products. In human and animal nutrition, xylans contribute dietary fiber; humans lack the enzymes to digest them, but gut microbiota can ferment them into short-chain fatty acids.