Xylanases targeting unusual glycosidic bonds: Unlocking prebiotic xylooligosaccharides from macroalgae

The growing interest in functional foods and prebiotics has encouraged the search for novel bioactive ingredients. Macroalgae (seaweeds) represent an underexplored source of unique xylans with β-1,3 and β-1,3/1,4 glycosidic linkages, that can be enzymatically hydrolysed into novel xylooligosaccharides (XOS). These oligosaccharides, characterized by linkage patterns different from those in terrestrial biomass, may exert distinct biological effects with potential biotechnological applications in food, nutraceuticals, and health. However, current knowledge regarding these xylans and their hydrolytic enzymes remains limited. This review therefore provides a comprehensive and up-to-date overview of macroalgal species containing β-1,3-xylan and β-1,3/1,4-mixed-linkage xylan (MLX), together with the extraction methodologies currently employed to obtain these xylans as substrates for enzymatic studies. It also highlights the specific enzymes involved in their hydrolysis -endo-1,3-β-xylanases and β-1,3/1,4-mixed-linkage xylanases (MLXases)- and discuss their biochemical, kinetic and structural features as well as the current insights into substrate specificity and catalytic mechanisms. Finally, the review examines current advances in the enzymatic production of XOS from macroalgal xylans and summarizes the bioactive properties of β-1,3-XOS and β-1,3/1,4-mixed-linkage XOS (MLXOS) reported to date.