Disaccharide

Sucrose, a disaccharide formed from condensation of a molecule of glucose and a molecule of fructose

Adisaccharide(also called adouble sugarorbiose)is thesugarformed when twomonosaccharidesare joined byglycosidic linkage.Like monosaccharides, disaccharides are simple sugarssolublein water. Three common examples aresucrose,lactose,andmaltose.

Disaccharides are one of the four chemical groupings ofcarbohydrates(monosaccharides, disaccharides,oligosaccharides,andpolysaccharides). The most common types of disaccharides—sucrose, lactose, and maltose—have 12carbonatoms, with the general formula C₁₂H₂₂O₁₁.The differences in these disaccharides are due toatomic arrangements within the molecule.

The joining of monosaccharides into a double sugar happens by acondensation reaction,which involves the elimination of a water molecule from thefunctional groupsonly. Breaking apart a double sugar into its two monosaccharides is accomplished byhydrolysiswith the help of a type ofenzymecalled adisaccharidase.As building the larger sugar ejects a water molecule, breaking it down consumes a water molecule. These reactions are vital inmetabolism.Each disaccharide is broken down with the help of a corresponding disaccharidase (sucrase,lactase,andmaltase).

Classification

There are two functionally different classes of disaccharides:

• Reducing disaccharides,in which one monosaccharide, thereducing sugarof the pair, still has a freehemiacetalunit that can perform as a reducingaldehydegroup;lactose,maltoseandcellobioseare examples of reducing disaccharides, each with one hemiacetal unit, the other occupied by theglycosidic bond,which prevents it from acting as areducing agent.They can easily be detected by the Woehlk test or Fearon's test onmethylamine.
• Non-reducing disaccharides,in which the component monosaccharides bond through anacetallinkage between theiranomericcenters. This results in neither monosaccharide being left with a hemiacetal unit that is free to act as a reducing agent.Sucroseandtrehaloseare examples of non-reducing disaccharides because their glycosidic bond is between their respective hemiacetal carbon atoms. The reduced chemical reactivity of the non-reducing sugars, in comparison to reducing sugars, may be an advantage where stability in storage is important.

Formation

The formation of a disaccharide molecule from twomonosaccharidemolecules proceeds by displacing ahydroxy groupfrom one molecule and ahydrogen nucleus(aproton) from the other, so that the new vacant bonds on the monosaccharides join the twomonomerstogether. Because of the removal of the water molecule from the product, the term of convenience for such a process is "dehydration reaction"(also"condensation reaction"or"dehydration synthesis"). For example, milk sugar (lactose) is a disaccharide made by condensation of one molecule of each of the monosaccharidesglucoseandgalactose,whereas the disaccharide sucrose in sugar cane and sugar beet, is a condensation product of glucose andfructose.Maltose,another common disaccharide, is condensed from two glucose molecules.

The dehydration reaction that bonds monosaccharides into disaccharides (and also bonds monosaccharides into more complexpolysaccharides) forms what are called glycosidic bonds.

Properties

The glycosidic bond can be formed between any hydroxy group on the component monosaccharide. So, even if both component sugars are the same (e.g., glucose), different bond combinations (regiochemistry) andstereochemistry(alpha-orbeta-) result in disaccharides that arediastereoisomerswith different chemical and physical properties. Depending on the monosaccharide constituents, disaccharides are sometimes crystalline, sometimes water-soluble, and sometimes sweet-tasting and sticky-feeling. Disaccharides can serve asfunctional groupsby forming glycosidic bonds with other organic compounds, formingglycosides.

Assimilation

See also:Carbohydrate digestion

Digestion of disaccharides involves breakdown into monosaccharides.

Common disaccharides

Disaccharide	Unit 1	Unit 2	Bond
Sucrose(table sugar, cane sugar, beet sugar, or saccharose)	Glucose	Fructose	α(1→2)β
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 arehydrolysisproducts of the polysaccharidesstarch,cellulose,andchitin,respectively.

Less common disaccharides include:

Disaccharide	Units	Bond
Kojibiose	Twoglucoses	α(1→2)
Nigerose	Two glucoses	α(1→3)
Isomaltose	Two glucoses	α(1→6)
β,β-Trehalose	Two glucoses	β(1→1)β
α,β-Trehalose	Two glucoses	α(1→1)β
Sophorose	Two glucoses	β(1→2)
Laminaribiose	Two glucoses	β(1→3)
Gentiobiose	Two glucoses	β(1→6)
Trehalulose	One glucose and onefructose	α(1→1)
Turanose	One glucose and one fructose	α(1→3)
Maltulose	One glucose and one fructose	α(1→4)
Leucrose	One glucose and one fructose	α(1→5)
Isomaltulose	One glucose and one fructose	α(1→6)
Gentiobiulose	One glucose and one fructose	β(1→6)
Mannobiose	Twomannoses	Either α(1→2), α(1→3), α(1→4), or α(1→6)
Melibiose	Onegalactoseand one glucose	α(1→6)
Allolactose	One galactose and one glucose	β(1→6)
Melibiulose	One galactose and one fructose	α(1→6)
Lactulose	One galactose and one fructose	β(1→4)
Rutinose	Onerhamnoseand one glucose	α(1→6)
Rutinulose	One rhamnose and one fructose	β(1→6)
Xylobiose	Two xylopyranoses	β(1→4)

External links

• Disaccharidesat the U.S. National Library of MedicineMedical Subject Headings(MeSH)