Related to disaccharides are othercarbohydrates: monosaccharides, their precursors, and the largeroligosaccharides andpolysaccharides). CThe joining of monosaccharides into a double sugar happens by acondensation reaction, shown here in the case of two hexoses:
2 C6H12O6 → C12H22O11 + H2O
Breaking apart a double sugar into its two monosaccharides is accomplished byhydrolysis with the help of a type ofenzyme called 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).
There are two functionally different classes of disaccharides:
Reducing disaccharides, in which one monosaccharide, thereducing sugar of the pair, still has a freehemiacetal unit that can perform as a reducingaldehyde group;lactose,maltose andcellobiose are 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.[4]
Non-reducing disaccharides, in which the component monosaccharides bond through anacetal linkage between theiranomeric centers. This results in neither monosaccharide being left with a hemiacetal unit that is free to act as a reducing agent.Sucrose andtrehalose are 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.[5][6]
The formation of a disaccharide molecule from twomonosaccharide molecules proceeds by displacing ahydroxy group from one molecule and ahydrogen nucleus (aproton) from the other, so that the new vacant bonds on the monosaccharides join the twomonomers together. 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 monosaccharidesglucose andgalactose, 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.[7]
The dehydration reaction that bonds monosaccharides into disaccharides (and also bonds monosaccharides into more complexpolysaccharides) forms what are called glycosidic bonds.[8]
