Hemicellulose is a term used to designate a very diverse group of polysaccharides present in the cell walls of many plants and that represent more than a third of the biomass of said structures.
The concept was proposed by Johann Heinrich Schulze to designate polysaccharides other than starch and in association with cellulose that were extractable from the cell walls of higher plants by using alkaline solutions.
These polysaccharides are composed of glucan skeletons linked by β-1,4 bonds that have different glycosylated substituents and that are capable of interacting with each other and with cellulose fibers through hydrogen bonds (non-covalent interactions).
Unlike cellulose, which forms tightly packed microfibers, hemicelluloses have rather amorphous structures, which are soluble in aqueous solutions.
Since more than a third of the dry weight of plant cells corresponds to hemicelluloses, much interest currently exists in the production of biofuels and other chemical compounds by processing these polysaccharides.
Classification and structure
Hemicelluloses are currently divided into four structurally different classes of molecules: xylanes, D-manno glycans, β-glucans, and xyloglycans. These three types of hemicelluloses have different distribution and localization patterns, as well as other important differences.
They are the main hemicellulocytic components present in the secondary cell walls of dicotyledonous plants. They represent more than 25% of the biomass of woody and herbaceous plants and about 50% in some species of monocotyledons.
Xylanes are heteropolymers composed of D-xylopyranose linked by β-1,4 bonds and which can have short branches. This group is subdivided into homoxylanes and heteroxylanes, among which are glucuronoxylans and other complex polysaccharides.
These molecules can be isolated from different plant sources: from flaxseed fiber, from beet pulp, from sugarcane bagasse, from wheat bran and others.
Its molecular weight can vary considerably, depending on the type of xylan and the plant species. The range found in nature usually ranges from 5,000 g / mol to more than 350,000 g / mol, but it depends a lot on the degree of hydration and other factors.
This type of polysaccharide is found in higher plants in the form of galactomannans and glucomannan, which are composed of linear chains of D-mannopyranose linked by β-1,4 bonds and by residues of D-mannopyranose and D-glucopyranose linked by β bonds. -1.4, respectively.
Both types of hand glycans can have D-galactopyranose residues attached to the backbone of the molecule at different positions.
Galactomannans are found in the endosperm of some nuts and dates, they are insoluble in water and of similar conformation to that of cellulose. Glucomannan, on the other hand, are the main hemicellulocytic components of the cell walls of softwoods.
Glucans are the hemicellulocytic components of cereal grains and are found predominantly in grasses and poaceae in general. In these plants, β-glucans are the main molecules associated with cellulose microfibers during cell growth.
Its structure is linear and consists of glucopyranose residues linked through mixed β-1,4 (70%) and β-1,3 (30%) bonds. The molecular weights reported for cereals vary between 0.065 to 3 x 10e6 g / mol, but there are differences relative to the species where they are studied.
This hemicellulocytic polysaccharide is found in higher plants and is one of the most abundant structural materials of cell walls. In dicotyledonous angiosperms it represents more than 20% of wall polysaccharides, while in grasses and other monocots it represents up to 5%.
Xyloglycans are composed of a cellulose-like backbone, composed of glucopyranose units linked by β-1,4 bonds, which is linked to α-D-xylopyranose residues through its carbon at position 6.
These polysaccharides are tightly bound to the cellulose microfibers of the cell wall through hydrogen bonds, contributing to the stabilization of the cellulocyte network.
Most membrane polysaccharides are synthesized from very specific activated nucleotide sugars.
These sugars are used by glycosyltransferase enzymes in the Golgi complex, responsible for the formation of glycosidic bonds between the monomers and the synthesis of the polymer in question.
The cellulocyte skeleton of xyloglycans is synthesized by members of the family of proteins responsible for cellulose synthesis, encoded by the CSLC genetic family.
Just as its composition varies depending on the plant species studied, the functions of hemicelluloses also. The main ones are:
In the formation of the cell wall of plants and other organisms with cells similar to plant cells, the different classes of hemicelluloses fulfill essential functions in structural matters thanks to their ability to associate non-covalently with cellulose.
Xylan, one of the types of hemicelluloses, are especially important in the hardening of the secondary cell walls developed by some plant species.
In some plant species such as tamarind, the seeds, instead of starch, store xyloglucans that are mobilized thanks to the action of the enzymes present in the cell wall and this occurs during the germination processes, where energy is supplied to the embryo contained in the seed.
Functions and commercial importance
The hemicelluloses stored in the seeds such as tamarind are exploited commercially for the production of additives that are used in the food industry.
Examples of these additives are tamarind gum »and« guar gum »or« guaran »(extracted from a kind of legume).
In the bakery industry, the presence of arabinoxylans can affect the quality of the products obtained, in the same way that, due to their characteristic viscosity, they also affect beer production.
The presence of certain types of celluloses in some plant tissues can greatly affect the use of these tissues for the production of biofuels.
Usually, the addition of hemicellulosic enzymes is a common practice to overcome these drawbacks. But with the advent of molecular biology and other highly useful techniques, some researchers are working on the design of transgenic plants that produce specific types of hemicelluloses.
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