Xyloglucan is a

hemicellulose A hemicellulose (also known as polyose) is one of a number of heteropolymer, heteropolymers (matrix polysaccharides), such as arabinoxylans, present along with cellulose in almost all embryophyte, terrestrial plant cell walls.Scheller HV, Ulvskov H ...

that occurs in the primary cell wall of all

vascular plant Vascular plants (), also called tracheophytes () or collectively Tracheophyta (), form a large group of land plants ( accepted known species) that have lignified tissues (the xylem) for conducting water and minerals throughout the plant. They al ...

s; however, all enzymes responsible for xyloglucan metabolism are found in

Charophyceae Charophyceae is a class of charophyte green algae. AlgaeBase places it in division Charophyta. Extant (living) species are placed in a single order Charales, commonly known as "stoneworts" and "brittleworts". Fossil members of the class may be ...

algae.LEV Del Bem and M Vincentz (2010) Evolution of xyloglucan-related genes. ''BMC Evolutionary Biology'', 10:340, 1-17 In many dicotyledonous plants, it is the most abundant hemicellulose in the primary cell wall. Xyloglucan binds to the surface of

cellulose Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wall ...

microfibril A microfibril is a very fine fibril, or fiber-like strand, consisting of glycoproteins and cellulose. It is usually, but not always, used as a general term in describing the structure of protein fiber, e.g. hair and sperm tail. Its most frequently ...

s and may link them together. It is the substrate of xyloglucan endotransglycosylase, which cuts and ligates xyloglucans, as a means of integrating new xyloglucans into the cell wall. It is also thought to be the substrate of alpha-

expansin Expansins are a family of closely related nonenzymatic proteins found in the plant cell wall, with important roles in plant cell growth, fruit softening, abscission, emergence of root hairs, pollen tube invasion of the stigma and style, meristem fun ...

, which promotes cell wall enlargement.

Chemistry

Xyloglucan has a backbone of β1→4-linked

glucose Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, using ...

residues, most of which are substituted with 1-6 linked

xylose Xylose ( grc, ξύλον, , "wood") is a sugar first isolated from wood, and named for it. Xylose is classified as a monosaccharide of the aldopentose type, which means that it contains five carbon atoms and includes an aldehyde functional gro ...

sidechains. The xylose residues are often capped with a

galactose Galactose (, '' galacto-'' + '' -ose'', "milk sugar"), sometimes abbreviated Gal, is a monosaccharide sugar that is about as sweet as glucose, and about 65% as sweet as sucrose. It is an aldohexose and a C-4 epimer of glucose. A galactose molec ...

residue sometimes followed by a

fucose Fucose is a hexose deoxy sugar with the chemical formula C6H12O5. It is found on ''N''-linked glycans on the mammalian, insect and plant cell surface. Fucose is the fundamental sub-unit of the seaweed polysaccharide fucoidan. The α(1→3) li ...

residue. The specific structure of xyloglucan differs between plant families.

Biosynthesis

Xyloglucan is synthesized in Golgi trans cisternae and in the trans Golgi network (TGN) and is transported to the

cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment ( ...

vesicles Vesicle may refer to: ; In cellular biology or chemistry * Vesicle (biology and chemistry), a supramolecular assembly of lipid molecules, like a cell membrane * Synaptic vesicle ; In human embryology * Vesicle (embryology), bulge-like features o ...

, where it is expelled and adsorbs on nascent cellulosic microfibrils.

Metabolism in the human gut

The human genome doesn’t contain the genes coding for xyloglucan degradation even though xyloglucans are an important component of most human diets. Recent studies have shown that a discrete genetic locus confers xyloglucan metabolism in selected human gut

Bacteroidota The phylum Bacteroidota (synonym Bacteroidetes) is composed of three large classes of Gram-negative, nonsporeforming, anaerobic or aerobic, and rod-shaped bacteria that are widely distributed in the environment, including in soil, sediments, and ...

. This findings reveals that the metabolism of even highly abundant components of

dietary fiber Dietary fiber (in British English fibre) or roughage is the portion of plant-derived food that cannot be completely broken down by human digestive enzymes. Dietary fibers are diverse in chemical composition, and can be grouped generally by the ...

maybe mediated by niche species. The metabolism of xyloglucans is the result of the concerted action of several enzymes and membrane transporters. However, given the high diversity of composition of xyloglucans from different plant sources, there is a keystone enzyme, an endo-xyloglucanase called BoGH5A, that has the ability to cleave a range of xyloglucans to generate short xyloglucans ready for uptake. A detailed analysis of the structure and function of the enzyme has revealed the presence of a domain called the BACON domain whose primary function in BoGH5A may be to distance the catalytic module from the cell surface and confer additional mobility to the catalytic domain to attack the polysaccharide. A broad active-site cleft engendering binding plasticity is the key feature allowing BoGH5A which allows it to accommodate a wide range of natural XyGs. The prevalence of XyGs in the human diet suggests that the mechanism by which bacteria degrade these complex polysaccharides is highly important to human energy acquisition. Moreover, the rarity of XyG metabolism highlights the significance of ''Bacteroides ovatus'' and other proficient XyG-degrading Bacteroidota as key members of the human gut microbial consortium.

References

{{Reflist Polysaccharides