Views: 41 Author: Site Editor Publish Time: 2023-02-27 Origin: Site
Beta-glucans are complex polysaccharides that are found in fungi and plants, such as Agaricus blazei, shiitake mushroom, Ganoderma lucidum, and oat. Beta-glucans display an array of potentially therapeutic properties.
In microbial sources, they are a structural component and in grain sources, they are found in the endospermic and aleuronic walls [1,2,3]. Today, beta-glucans are widely marketed as biologically active compounds that have the potential to improve health [4]. Of therapeutic importance, beta-glucans have potentially important metabolic and gastro-intestinal effects, modulating the gut microbiome, altering lipid and glucose metabolism, reducing cholesterol, leading to their investigation as potential therapies for metabolic syndrome, obesity and diet regulation, gastrointestinal conditions such as irritable bowel, and to reduce cardiovascular and diabetes risk. beta-glucans also appear to have immune-modulating effects, leading to their investigation as adjuvant agents for solid cancers and haematological malignancies, for immune-mediated conditions, such as allergic rhinitis and respiratory tract infections, and to enhance wound healing. Two glucan isolates were licensed as drugs in Japan as an immune-adjuvant therapy for cancer in 1980 [5,6].
As a result, glucan has gradually attracted more and more attention and has been widely used in food, medical treatment, skin care, feed and other industries.
Structurally, beta-glucans are comprised of glucose units linked together by several different types of beta-glycosidic linkages.beta-glucans are composed of β-D-glucose monomer units, which are held together by glycosidic linkages at differing positions (1,3), (1,4) or (1,6). This structure can be either branched or unbranched [7].
The beta-glucans source will determine if the molecule has branched structures and to what extent. The fine structure of beta-glucans can vary in meaningful ways that modify its effects and mechanisms of action. A variance will occur between glycosidic linkages, molecular weight, branching, degree of polymerization, and solubility beta-glucans from different sources will have different effects or functions [8].
A distinguishing characteristic of all beta-glucans that is necessary for biological activity is its 1,3 backbone . The degree and the specific profile of biological activity appear to be related to specific beta-glucans structural characteristics. First, side-chain frequency and length are important, with a higher degree of branching associated with greater biological activity.
Beta-glucans are classified by their source, into cereal and non-cereal beta-glucans. Cereal beta-glucans, which are 1,3 and 1,4 linked, mainly display metabolic activities, such as the ability to lower cholesterol and blood glucose and have been explored in clinical studies to target metabolic conditions. These 1,3 and 1,4 linked glucans appear to be recognized as dietary fibres after ingestion and elicit their metabolic effects via this mechanism.
Non-cereal (predominantly fungal and yeast) beta-glucans have more pronounced immunomodulatory functions and are the focus of immunomodulation and anti-cancer studies. Fungal and yeast beta-glucans have a 1,3 and 1,6 linkage structure and are recognized by some receptors including dectin 1, complement receptor 3 (CR3) and toll-like receptors (TLRs) [9,10,11,12].
Yeast beta glucan is a polysaccharide that exists in yeast cell wall and is mostly derived from Saccharomyces cerevisiae or baker's yeast.The cell wall of yeast is divided into three layers from outside to the inside, namely mannan, protein and glucan; Beta-d-glucan is the main component of dextran [13,14]. Yeast beta glucan accounts for 30 60% of yeast cell wall dry weight. About 60% of yeast beta glucan is a long chain linked by 1500 glucose residues via β-(1-3)-glycoside bonds and its molecular weight is about 240 kDa [15,16,17]. A large number of studies have shown that yeast beta glucan can activate macrophages to initiate anti-tumor [18], anti-bacterial [19], wound healing [20], antioxidant [21,22], and lipid-lowering [23] effects. In addition, yeast beta glucan is widely used in the food industry as a thickener, emulsifying stabilizer and fat substitute, because of its good water-holding, heat preservation, film-forming, and non-irritating properties [24,25,26,27].
Yeast beta glucan belongs to the class of beta glucan, and its structure includes two distinct macromolecular components comprised of consecutively (1→3)-linked β-d-glucopyranosyl residues, with small numbers of (1→6)-linked branches and a minor component with consecutive (1→6)-linkages and (1→3)-branches.
With respect to bioactive function, special attention is paid to the water-insoluble native form of yeast beta-glucans[28]. The insolubility in water, alcohols or organic solvents of yeast beta-glucans is mainly due to the intermolecular complex between chitin and the polymer chains located in the β-1,3 position that coexist in a percentage of 1% of the cell wall mass [29], but also due to the strong hydrogen bonds formed between the hydroxyl groups of glucose in the polymer chain. Another major constituent that confers rigidity are mannoproteins found on the outside of the cell wall, which are bound to the β-1,3/β-1,6 chains of β-glucan through covalent bonds [30], and a degree of polymerization greater than 100 of β-1,3-glucan makes it completely insoluble in water [31].
During extraction, some of the insoluble glucans pass into soluble form due to various methods applied in cell wall fractionation (extraction with NaOH/concentrated acids or by using DMSO/H3PO4) [32]. Their solubility increases as the degree of branching decreases [33]. A degree of polymerization less than 20 of the β-1,3/β-1,6 complex leads to a weakening in the molecular interactions and forms water-soluble compounds [34].
Molecular modification can change the polysaccharide dimensional structure, molecular weight and the substituent group types, numbers, and positions, with a profound impact on bioactivity[35].
The physical method the operation is simple, no chemical reagent is used and the environment is not polluted.However, the yield of soluble yeast glucan is low if prepared by physical methods alone.
Chemical modification is the most widely used method as it can significantly increase the water solubility and bioactivity of polysaccharides by ‘grafting on’ other moieties. Although the introduction of the new group imparts some new activity to the dextran, its original structure has been changed, so its application is limited to some extent. Moreover, the treatment method can easily form residues, and some operations are also faced with a series of shortcomings such as a complicated modification process, high modification costs and the need for special modification equipment.
The biological method is mild but environmentally friendly, it is a promising polysaccharide modification technique. Compared with chemical modification, enzymatic modification has the advantages of high specificity, high efficiency and few side effects. By controlling the enzymatic reaction conditions, polysaccharides with different structures can be produced and their physical and chemical properties could also change. It features high security and controllability.
Saccharomyces are the most important genus for the food industry including species such as Saccharomyces cerevisiae with different strains used in bakery, wine or beer making [36].Saccharomyces cerevisiae is protected by a thick and mechanically resistant cell wall mostly made from polysaccharides and glycoproteins [37,38] of which 50–55% is represented by β-1,3-glucans, and up to 15% is composed of β-1.6-glucans [39]. The thick wall delimits the plasma membrane inwards through the cytoplasmic matrix. Intracellular content includes, amongst others, a nucleus, storage organelles, the Golgi apparatus, endoplasmic reticulum, vacuoles, enzymes, lysosomes.
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