Xyloglucan is a

hemicellulose A hemicellulose (also known as polyose) is one of a number of heteropolymers (matrix polysaccharides), such as arabinoxylans, present along with cellulose in almost all embryophyte, terrestrial plant cell walls. Cellulose is crystalline, strong, an ...

that occurs in the

primary cell wall A cell wall is a structural layer that surrounds some cell types, found immediately outside the cell membrane. It can be tough, flexible, and sometimes rigid. Primarily, it provides the cell with structural support, shape, protection, and functi ...

of all

vascular plant Vascular plants (), also called tracheophytes (, ) or collectively tracheophyta (; ), are plants that have lignin, lignified tissues (the xylem) for conducting water and minerals throughout the plant. They also have a specialized non-lignified Ti ...

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 p ...

algae.LEV Del Bem and M Vincentz (2010) Evolution of xyloglucan-related genes. ''BMC Evolutionary Biology'', 10:340, 1-17 In many

dicotyledon The dicotyledons, also known as dicots (or, more rarely, dicotyls), are one of the two groups into which all the flowering plants (angiosperms) were formerly divided. The name refers to one of the typical characteristics of the group: namely, ...

ous 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 chemical formula, formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of glycosidic bond, β(1→4) linked glucose, D-glucose units. Cellulose is an important s ...

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 o ...

s and may link them together. It is the

substrate Substrate may refer to: Physical layers *Substrate (biology), the natural environment in which an organism lives, or the surface or medium on which an organism grows or is attached ** Substrate (aquatic environment), the earthy material that exi ...

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 sugar with the Chemical formula#Molecular formula, molecular formula , which is often abbreviated as Glc. It is overall the most abundant monosaccharide, a subcategory of carbohydrates. It is mainly made by plants and most algae d ...

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

xylose Xylose ( , , "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 group. It is deriv ...

sidechains. The xylose residues are often capped with a

galactose Galactose (, ''wikt:galacto-, galacto-'' + ''wikt:-ose#Suffix 2, -ose'', ), sometimes abbreviated Gal, is a monosaccharide sugar that is about as sweetness, sweet as glucose, and about 65% as sweet as sucrose. It is an aldohexose and a C-4 epime ...

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) l ...

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 or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extr ...

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 a neuron, synaptic vesicles (or neurotransmitter vesicles) s ...

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

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

dietary fiber Dietary fiber (fibre in English in the Commonwealth of Nations, Commonwealth English) 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 co ...

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