Maltose: Difference between revisions

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[[Image:Amylase reaction.png|thumb|right|260px|Amylase reaction consisting of hydrolyzing amylose, producing maltose]]

'''Maltose''' ({{IPAc-en|ˈ|m|ɔː|l|t|oʊ|s}} or {{IPAc-en|ˈ|m|ɔː|l|t|oʊ|z}}), also known as '''maltobiose''' or '''malt sugar''', is a [[disaccharide]] formed from two units of [[glucose]] joined with an α(1→4) [[glycosidic bond|bond]]. In the [[isomer]] [[isomaltose]], the two glucose molecules are joined with an α(1→6) bond. Maltose is the two-unit member of the [[amylose]] [[homologous series]], the key structural motif of [[starch]]. When [[beta-amylase]] breaks down starch, it removes two glucose units at a time, producing maltose. An example of this reaction is found in [[germination|germinating]] seeds, which is why it was named after [[malt]]. Unlike [[sucrose]], it is a [[reducing sugar]].

== History ==

Maltose was discovered by [[Augustin-Pierre Dubrunfaut]], although this discovery was not widely accepted until it was confirmed in 1872 by Irish chemist and brewer [[Cornelius O'Sullivan]]. Its name comes from [[malt]], combined with the suffix '[[-ose]]' which is used in names of sugars.

== Structure and nomenclature ==

Carbohydrates are generally divided into [[monosaccharide]]s, [[oligosaccharide]]s, and [[polysaccharide]]s depending on the number of sugar subunits. Maltose, with two sugar units, is a disaccharide, which falls under oligosaccharides. Glucose is a [[hexose]]: a monosaccharide containing six carbon atoms. The two glucose units are in the [[pyranose]] form and are joined by an [[glycosidic bond|O-glycosidic bond]], with the first carbon (C₁) of the first [[glucose]] linked to the fourth carbon (C₄) of the second [[glucose]], indicated as (1→4). The link is characterized as α because the glycosidic bond to the anomeric carbon (C₁) is in the opposite plane from the {{chem|C|H|2|O|H}} [[substituent]] in the same ring (C₆ of the first glucose). If the glycosidic bond to the anomeric carbon (C₁) were in the same plane as the {{chem|C|H|2|O|H}} substituent, it would be classified as a '''β(1→4)''' bond, and the resulting molecule would be [[cellobiose]]. The anomeric carbon (C₁) of the second glucose molecule, which is not involved in a glycosidic bond, could be either an α- or β-anomer depending on the bond direction of the attached hydroxyl group relative to the {{chem|C|H|2|O|H}} substituent of the same ring, resulting in either α-maltose or β-maltose.

An [[isomer]] of maltose is [[isomaltose]]. This is similar to maltose but instead of a bond in the α(1→4) position, it is in the α(1→6) position, the same bond that is found at the branch points of [[glycogen]] and [[amylopectin]].

== Properties ==

Like glucose, maltose is a [[reducing sugar]], because the ring of one of the two glucose units can open to present a free [[aldehyde]] group; the other one cannot because of the nature of the glycosidic bond. Maltose can be broken down to glucose by the [[maltase]] enzyme, which catalyses the hydrolysis of the glycosidic bond.

Maltose in aqueous solution exhibits [[mutarotation]], because the α and β isomers that are formed by the different conformations of the anomeric carbon have different [[specific rotation]]s, and in aqueous solutions, these two forms are in equilibrium. Maltose can easily be detected by the Woehlk test or Fearon's test on methylamine.

It has a sweet taste, but is only about 30–60% as sweet as sugar, depending on the concentration.

== Sources and absorption ==

[[Image:Maltose syrup.jpg|thumb|right|Maltose syrup]]

Maltose is a [[malt]] component, a substance obtained when the grain is softened in water and germinates. It is also present in highly variable quantities in partially hydrolyzed starch products like [[maltodextrin]], [[corn syrup]] and acid-thinned starch.

Outside of plants, maltose is also (likely) found in [[sugarbag]].

In humans, maltose is broken down by various maltase enzymes, providing two glucose molecules that can be [[Glucose metabolism|further processed]]: either broken down to provide energy, or stored as glycogen. The lack of the [[sucrase-isomaltase]] enzyme in humans causes [[sucrose intolerance]], but complete maltose intolerance is extremely rare because there are four different maltase enzymes.

==External links==

*{{Commons category-inline}}

*[https://web.archive.org/web/20051124215824/http://www.elmhurst.edu/~chm/vchembook/546maltose.html Maltose], Elmhurst College Virtual Chembook.

{{Carbohydrates}}

{{Sugar}}

{{二次利用|date=17 June 2023}}

[[Category:Disaccharides]]