Disaccharide
A disaccharide (also called a double sugar or biose[1]) is the sugar formed when two monosaccharides (simple sugars) are joined. Like monosaccharides, disaccharides are soluble in water. Three common examples are sucrose, lactose,[2] and maltose.
Disaccharides are one of the four chemical groupings of carbohydrates (monosaccharides, disaccharides, oligosaccharides, and polysaccharides). The most common types of disaccharides—sucrose, lactose, and maltose—have twelve carbon atoms, with the general formula C12H22O11. The differences in the disaccharides are due to atomic arrangements within the molecule.[3]
The joining of simple sugars into a double sugar happens by a condensation reaction, which involves the elimination of a water molecule from the functional groups only. Breaking apart a double sugar into its two simple sugars is accomplished by hydrolysis with the help of a type of enzyme called a disaccharidase. As building the larger sugar ejects a water molecule, breaking it down consumes a water molecule. These reactions are vital in metabolism. Each disaccharide is broken down with the help of a corresponding disaccharidase (sucrase, lactase, and maltase).
Classification
There are two different types of disaccharides:
- Reducing disaccharides, in which one monosaccharide, the reducing sugar, still has a free hemiacetal unit; and non-reducing disaccharides, in which the components bond through an acetal linkage between their anomeric centers and neither monosaccharide has a free hemiacetal unit. Cellobiose and maltose are examples of reducing disaccharides.
- Sucrose and trehalose are examples of non-reducing disaccharides.
Formation
Disaccharides are formed when two monosaccharides are joined together and a molecule of water is removed, a process known as dehydration reaction. For example; milk sugar (lactose) is made from glucose and galactose whereas the sugar from sugar cane and sugar beets (sucrose) is made from glucose and fructose. Maltose, another notable disaccharide, is made up of two glucose molecules.[6] The two monosaccharides are bonded via a dehydration reaction (also called a condensation reaction or dehydration synthesis) that leads to the loss of a molecule of water and formation of a glycosidic bond.[7]
Properties
The glycosidic bond can be formed between any hydroxyl group on the component monosaccharide. So, even if both component sugars are the same (e.g., glucose), different bond combinations (regiochemistry) and stereochemistry (alpha- or beta-) result in disaccharides that are diastereoisomers with different chemical and physical properties.
Depending on the monosaccharide constituents, disaccharides are sometimes crystalline, sometimes water-soluble, and sometimes sweet-tasting and sticky-feeling.
Assimilation
Digestion involves breakdown to the monosaccharides. carbohydrate digestion
Common disaccharides
| Disaccharide | Unit 1 | Unit 2 | Bond |
|---|---|---|---|
| Sucrose (table sugar, cane sugar, beet sugar, or saccharose) | Glucose | Fructose | α(1→2)β |
| Lactulose | Galactose | Fructose | β(1→4) |
| Lactose (milk sugar) | Galactose | Glucose | β(1→4) |
| Maltose (malt sugar) | Glucose | Glucose | α(1→4) |
| Trehalose | Glucose | Glucose | α(1→1)α |
| Cellobiose | Glucose | Glucose | β(1→4) |
| Chitobiose | Glucosamine | Glucosamine | β(1→4) |
Maltose, cellobiose, and chitobiose are hydrolysis products of the polysaccharides starch, cellulose, and chitin, respectively.
Less common disaccharides include:[8]
| Disaccharide | Units | Bond |
|---|---|---|
| Kojibiose | two glucose monomers | α(1→2) [9] |
| Nigerose | two glucose monomers | α(1→3) |
| Isomaltose | two glucose monomers | α(1→6) |
| β,β-Trehalose | two glucose monomers | β(1→1)β |
| α,β-Trehalose | two glucose monomers | α(1→1)β[10] |
| Sophorose | two glucose monomers | β(1→2) |
| Laminaribiose | two glucose monomers | β(1→3) |
| Gentiobiose | two glucose monomers | β(1→6) |
| Turanose | a glucose monomer and a fructose monomer | α(1→3) |
| Maltulose | a glucose monomer and a fructose monomer | α(1→4) |
| Palatinose | a glucose monomer and a fructose monomer | α(1→6) |
| Gentiobiulose | a glucose monomer and a fructose monomer | β(1→6) |
| Mannobiose | two mannose monomers | either α(1→2), α(1→3), α(1→4), or α(1→6) |
| Melibiose | a galactose monomer and a glucose monomer | α(1→6) |
| Melibiulose | a galactose monomer and a fructose monomer | α(1→6) |
| Rutinose | a rhamnose monomer and a glucose monomer | α(1→6) |
| Rutinulose | a rhamnose monomer and a fructose monomer | β(1→6) |
| Xylobiose | two xylopyranose monomers | β(1→4) |
References
- ↑ Biose on www.merriam-webster.org
- ↑ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "disaccharides".
- ↑ Biology- A course for O Level. p. 59. ISBN 9810190964.
- ↑ "Nomenclature of Carbohydrates (Recommendations 1996)2-Carb-36 Disaccharides.".
- ↑ "Disaccharides and Oligiosaccharides". Retrieved 2008-01-29.
- ↑ Whitney, Ellie; Sharon Rady Rolfes (2011). Peggy Williams, ed. Understanding Nutrition (Twelfth ed.). California: Wadsworth, Cengage Learning. p. 100. ISBN 0-538-73465-5.
- ↑ "Glycosidic Link". OChemPal. Utah Valley University. Retrieved 11 December 2013.
- ↑ F.W.Parrish; W.B.Hahn,G.R.Mandels (July 1968). "Crypticity of Myrothecium verrucaria Spores to Maltose and Induction of Transport by Maltulose, a Common Maltose Contaminant" (PDF). J. Bacteriol. (American Society for Microbiology) 96 (1): 227–233. PMC 252277. PMID 5690932. Retrieved 2008-11-21.
- ↑ Matsuda, K.; Abe, Y; Fujioka, K (November 1957). "Kojibiose (2-O-alpha-D-Glucopyranosyl-D-Glucose): Isolation and Structure: Chemical Synthesis". Nature 180 (4593): 985–6. doi:10.1038/180985a0. PMID 13483573.
- ↑ T. Taga; Y. Miwa; Z. Min (1997). "α,β-Trehalose Monohydrate". Acta Crystallogr. C 53 (2): 234–236. doi:10.1107/S0108270196012693.
External links
- Disaccharides at the US National Library of Medicine Medical Subject Headings (MeSH)
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