This articleis missing information about biosynthesis. Please expand the article to include this information. Further details may exist on thetalk page.(August 2019)
Cellulose is an example of a (1→4)-β-D-glucan composed ofglucose units
Beta-glucans,β-glucans comprise a group of β-D-glucosepolysaccharides (glucans) naturally occurring in the cell walls ofcereals,bacteria, andfungi, with significantly differingphysicochemical properties dependent on source. Typically, β-glucans form a linear backbone with 1–3 β-glycosidic bonds but vary with respect to molecular mass, solubility, viscosity, branching structure, and gelation properties, causing diverse physiological effects in animals.
Cereal and fungal products have been used for centuries for medicinal and cosmetic purposes; however, the specific role of β-glucan was not explored until the 20th century. β-glucans were first discovered in lichens, and shortly thereafter in barley. A particular interest in oat β-glucan arose after a cholesterol lowering effect from oat bran reported in 1981.[2]
In 1997, theFDA approved of a claim that intake of at least 3.0 g of β-glucan from oats per day decreased absorption of dietary cholesterol and reduced the risk ofcoronary heart disease. The approved health claim was later amended to include these sources of β-glucan: rolled oats (oatmeal), oat bran, whole oat flour, oatrim (the soluble fraction of alpha-amylase hydrolyzed oat bran or whole oat flour), whole grain barley and barley beta-fiber. An example of an allowed label claim: "Soluble fiber from foods such as oatmeal, as part of a diet low in saturated fat and cholesterol, may reduce the risk of heart disease. A serving of oatmeal supplies 0.75 grams of the 3.0 g of β-glucan soluble fiber necessary per day to have this effect." The claim language is in the Federal Register 21 CFR 101.81 Health Claims: "Soluble fiber from certain foods and risk of coronary heart disease (CHD)".[3]
Glucans are arranged in six-sidedD-glucose rings connected linearly at varying carbon positions depending on the source, although most commonly β-glucans include a 1-3 glycosidic link in their backbone. Although technically β-glucans are chains ofD-glucosepolysaccharides linked by β-typeglycosidic bonds, by convention not all β-D-glucose polysaccharides are categorized as β-glucans.[4]Cellulose is not conventionally considered a β-glucan, as it is insoluble and does not exhibit the same physicochemical properties as other cereal or yeast β-glucans.[5]
Glucose molecule, showing carbon numbering notation and β orientation.
Some β-glucan molecules have branching glucose side-chains attached to other positions on the mainD-glucose chain, which branch off the β-glucan backbone. In addition, these side-chains can be attached to other types of molecules, like proteins, as inpolysaccharide-K.
The most common forms of β-glucans are those comprisingD-glucose units with β-1,3 links. Yeast and fungal β-glucans contain 1-6 side branches, while cereal β-glucans contain both β-1,3 and β-1,4 backbone bonds, but no β-1,3 branching.[6] Seaweeds consist of a backbone that is primarily β-1,3-glucan, but with some β-1,6-glucan in the backbone as well as in side chains.[6]
The frequency, location, and length of the side-chains may play a role in immunomodulation. Differences in molecular weight, shape, and structure of β-glucans dictate the differences in biological activity.[7][8]
In general, β-1,3 linkages are created by1,3-beta-glucan synthase, and β-1,4 linkages are created bycellulose synthase. The process leading to β-1,6 linkages is poorly understood: although genes important in the process have been identified, not much is known about what each of them do.[9]
β-glucans form a natural component of the cell walls of bacteria, fungi, yeast, and cereals such as oat and barley. Each type of beta-glucan comprises a different molecular backbone, level of branching, and molecular weight which affects its solubility and physiological impact. One of the most common sources of β(1,3)D-glucan for supplement use is derived from the cell wall of baker's yeast (Saccharomyces cerevisiae). β-glucans found in the cell walls ofyeast contain a 1,3 glucose backbone with elongated 1,6 glucose branches.[12] Other sources includeseaweed,[13] and various mushrooms, such aslingzhi,shiitake,chaga, andmaitake, which are under preliminary research for their potentialimmune effects.[14]
In the diet, β-glucans are a source of soluble,fermentablefiber – also calledprebiotic fiber – which provides a substrate formicrobiota within thelarge intestine, increasingfecal bulk and producingshort-chain fatty acids as byproducts with wide-ranging physiological activities.[15] This fermentation impacts the expression of manygenes within the large intestine,[16] which further affectsdigestive function and cholesterol and glucose metabolism, as well as theimmune system and other systemic functions.[15][17]
Cereal β-glucans from oat, barley, wheat, and rye have been studied for their effects oncholesterol levels in people with normal cholesterol levels and in those withhypercholesterolemia.[1] Intake of oat β-glucan at daily amounts of at least 3 grams lowers total andlow-density lipoprotein cholesterol levels by 5 to 10% in people with normal or elevated blood cholesterol levels.[18]
Oats and barley differ in the ratio of trimer and tetramer 1-4 linkages. Barley has more 1-4 linkages with a degree of polymerization higher than 4. However, the majority of barley blocks remain trimers and tetramers. In oats, β-glucan is found mainly in the endosperm of the oat kernel, especially in the outer layers of that endosperm.[7]
Enterocytes facilitate the transportation of β(1,3)-glucans and similar compounds across the intestinal cell wall into the lymph, where they begin to interact with macrophages to activate immune function.[19] Radiolabeled studies have verified that both small and large fragments of β-glucans are found in the serum, which indicates that they are absorbed from the intestinal tract.[20]M cells within thePeyer's patches physically transport the insoluble whole glucan particles into thegut-associated lymphoid tissue.[21]
An assay to detect the presence of (1,3)-β-D-glucan in blood is marketed as a means of identifying invasive or disseminated fungal infections.[22][23][24] This test should be interpreted within the broader clinical context, however, as a positive test does not render a diagnosis, and a negative test does not rule out infection. False positives may occur because of fungal contaminants in the antibioticsamoxicillin-clavulanate,[25] andpiperacillin/tazobactam. False positives can also occur with contamination of clinical specimens with the bacteriaStreptococcus pneumoniae,Pseudomonas aeruginosa, andAlcaligenes faecalis, which also produce (1→3)β-D-glucan.[26] This test can aid in the detection ofAspergillus,Candida, andPneumocystis jirovecii.[27][28][29] This test cannot be used to detectMucor orRhizopus, the fungi responsible formucormycosis, as they do not produce (1,3)-beta-D-glucan.[30]
^Zeković, Djordje B. (10 October 2008). "Natural and Modified (1→3)-β-D-Glucans in Health Promotion and Disease Alleviation".Critical Reviews in Biotechnology.25 (4):205–230.doi:10.1080/07388550500376166.PMID16419618.S2CID86109922.
^Sikora, Per (14 June 2012). "Identification of high b-glucan oat lines and localization and chemical characterization of their seed kernel b-glucans".Food Chemistry.137 (1–4):83–91.doi:10.1016/j.foodchem.2012.10.007.PMID23199994.
^Obayashi T, Yoshida M, Mori T, et al. (1995). "Plasma (13)-beta-D-glucan measurement in diagnosis of invasive deep mycosis and fungal febrile episodes".Lancet.345 (8941):17–20.doi:10.1016/S0140-6736(95)91152-9.PMID7799700.S2CID27299444.
^Ostrosky-Zeichner L, Alexander BD, Kett DH, et al. (2005). "Multicenter clinical evaluation of the (1→3)β-D-glucan assay as an aid to diagnosis of fungal infections in humans".Clin Infect Dis.41 (5):654–659.doi:10.1086/432470.PMID16080087.
^Lahmer, Tobias; da Costa, Clarissa Prazeres; Held, Jürgen; Rasch, Sebastian; Ehmer, Ursula; Schmid, Roland M.; Huber, Wolfgang (4 April 2017). "Usefulness of 1,3 Beta-D-Glucan Detection in non-HIV Immunocompromised Mechanical Ventilated Critically Ill Patients with ARDS and Suspected Pneumocystis jirovecii Pneumonia".Mycopathologia.182 (7–8):701–708.doi:10.1007/s11046-017-0132-x.ISSN1573-0832.PMID28378239.S2CID3870306.
^Ostrosky-Zeichner, Luis; Alexander, Barbara D.; Kett, Daniel H.; Vazquez, Jose; Pappas, Peter G.; Saeki, Fumihiro; Ketchum, Paul A.; Wingard, John; Schiff, Robert (1 September 2005). "Multicenter clinical evaluation of the (1→3) beta-D-glucan assay as an aid to diagnosis of fungal infections in humans".Clinical Infectious Diseases.41 (5):654–659.doi:10.1086/432470.ISSN1537-6591.PMID16080087.
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