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Maltose

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Definition

noun

plural: maltoses

mal·tose, [ˈmɔːltəʊz]




A reducing disaccharide formed when two glucose monomers join together via α(1→4) glycosidic bond; the structural unit of glycogen and starch


Details

Discovery of maltose

Augustin-Pierre Dubrunfaut [1797 –1881], a French chemist, discovered maltose. He was also credited for being the first one to discover fructose. However, his discovery of maltose was not widely accepted until the chemist Cornelius O'Sullivan [1841 – 1907] confirmed it in 1872.[1] The name "maltose" comes from the word malt (i.e. germinated grain, for use in brewing, distilling, etc.) and the suffix -ose that indicates it is a sugar.


Overview

Maltose is one of the most common disaccharide carbohydrates; other examples are sucrose and lactose. Carbohydrates are a major class of biomolecules that can be classified based on the saccharide constituents. A disaccharide is a carbohydrate made up of two monosaccharides that are linked together by a glycosidic bond (glycosidic linkage).


Properties of maltose

Maltose is a white crystalline solid. Its molar mass is 342.30 g·mol−1. Its melting point is (i.e. 102 °C). It is soluble in water. Similar to sucrose and lactose, maltose has a general formula of C12H22O11. Maltose, though, is a disaccharide made up of two glucose units. The glucose components are linked together by α-1→4 glycosidic bond, which means the covalent bond forms between the α-anomeric form of Carbon-1 (C-1) on one glucose and the hydroxyl oxygen atom on C-4 on the other glucose. When the glycosidic bond is a β-(1→4), the resulting compound is cellobiose. Isomaltose is another isomer of maltose. Both of them are made up of two glucose units joined by a glycosidic bond. Nevertheless, isomaltose differs from maltose based on the glycosidic bond: α-1→4 occurs in a maltose whereas α -1→6, in isomaltose.


Maltose vs. Lactose vs. Sucrose

Maltose (malt sugar), lactose (milk sugar), and sucrose (common table sugar) are the three common dietary disaccharides. As already specified earlier, the three disaccharides have the same chemical formula: C12H22O11. All three have a glucose constituent. In maltose, two glucose units make up the compound. In lactose and sucrose though, there is only one glucose unit that combines with another monosaccharide – a galactose and a fructose, respectively. In maltose, α-(1,4) glycosidic bond joins the two sugars, i.e. between Carbon-1 and Carbon-4. In lactose, β-(1,4) glycosidic bond occurs between Carbon-1 of galactose and Carbon-4 of glucose. In sucrose, the bond forms between Carbon-1 of glucose and Carbon-2 of fructose.


Maltose and lactose are reducing sugars; sucrose is a non-reducing sugar. Maltose and lactose are reducing sugar because one of the monosaccharide constituents could present a free aldehyde group. As for sucrose, the glycosidic bond forms between the reducing ends of the two monosaccharide constituents. Thus, sucrose could not join any further with other saccharide units.


Dietary maltose does not usually occur in food, but it can be formed during the digestion of starch. Conversely, lactose usually comes from milk and dairy products whereas sucrose, usually from food sweetened by sugar extracted from sugar cane and sugar beet. The digestion of these sugars is aided by specific digestive enzymes, particularly maltase, lactase, and sucrase. In humans, these enzymes are located on the outer surface of the epithelial cells that line the small intestine. Maltase helps digest maltose, lactase (β-galactosidase in bacteria) on lactose, and sucrase on sucrose. These enzymes cleave the bond between the two monosaccharide components. Maltose is sweeter than lactose. However, of the three, sucrose is the sweetest.


Common biological reactions involving maltose

Biosynthesis

The biosynthesis of maltose involves two glucose units joined via α-1→4 glycosidic linkage. The joining of these two monosaccharides results in the release of water.


Dehydration synthesis

The further joining of several maltose compounds results in the formation of more complex carbohydrates, such as starch in plants and glycogen in animals. The process is called dehydration synthesis whereby the formation of glycosidic bonds is concomitant with the release of water.


Saccharification, digestion, and uptake

The process whereby complex carbohydrates are broken down into simpler forms is saccharification. It is the opposite of dehydration synthesis. In dehydration synthesis, the condensation reaction causes the glycosidic bond to form between the joining sugars and then water is released in the process. In saccharification, hydrolysis uses water molecule and causes the glycosidic bond to break, thereby releasing the sugar constituents.


Maltose does not often occur in food but they are obtained from the partially-hydrolyzed starch (e.g. maltodextrin and corn syrup). The digestion of starch may also provide maltose. In humans, amylase is an enzyme in the saliva and the pancreatic juice that digests starch into simpler carbohydrates, such as maltose. However, maltose, in humans, is not readily absorbed by the small intestine. It has to be further broken down into its saccharide constituents before it can be taken up by the enterocytes, into the bloodstream, and finally, to the cells of other tissues, such as liver, kidney, muscles, brain, adipose, etc. Maltose is digested and broken down into its monosaccharide units through hydrolysis with the help of the enzyme, maltase. The bond that joins the two glucose units is broken, converting maltose to two glucose units. The free glucose molecules can now be absorbed by the enterocytes (intestinal cells), released into the bloodstream, and then taken up by other cells.


Metabolic disorders

Maltose intolerance is one of the metabolic disorders associated with maltose. During digestion, the enzyme maltase is released from the gut lining to catalyze the breakdown of maltose into glucose constituents. Low maltase enzyme activity results in the undigested maltose. When the body fails to digest maltose, it draws water from the body into the intestine. This leads to diarrhea. In the colon, the gut flora metabolizes the undigested maltose. This, in turn, causes bloating and pain. Maltose intolerance is extremely rare in humans. It is typically associated with the lack of sucrase-isomaltase enzymes.


Biological importance/functions

Dietary disaccharides are consumed and digested so as to obtain simple sugars that are readily absorbed and metabolized. Maltose is one of the main sources of glucose. Glucose is a crucial nutrient since it is used chiefly in energy metabolism. Glucose is the most common form of monosaccharide that the cell uses to synthesize ATP via substrate-level phosphorylation (glycolysis) and/or oxidative phosphorylation (involving redox reactions and chemiosmosis).


Maltose forms starch. Starch and maltose are structurally similar in a sense that they are made up of glucose units. However, starch is a polymer of glucose whereas maltose is a disaccharide of glucose. Nevertheless, maltose usually comes from the digestion (or hydrolysis) of starch. In particular, two glucose units (i.e. maltose) from starch are cleaved through the catalytic activity of beta-amylase. This is what occurs, for instance, in germinating seeds.


Maltose is commercially used as a sweetener, a nutrient in infant feeding, and in bacteriological culture media. It is also used in pastries. It makes bread dough to rise when carbon dioxide is produced and released during the conversion of starch into maltose by reacting the starch with enzymes. As a sweetener, it has less sweetness than other typical sugars. However, maltose consumption is not advisable to diabetics because of its high glycemic index.



Supplementary

Etymology

  • malt + -ose (a suffix used in chemical naming of sugars)

IUPAC

  • (3R,4R,5S,6R)-6-(hydroxymethyl)-5-{[(2R,3R,4S,5S,6R)-3,4,5-trihydroxy-6-(hydroxymethyl)oxan-2-yl]oxy}oxane-2,3,4-triol

Chemical formula

  • C12H22O11

Synonym(s)

  • malt sugar
  • maltobiose

Derived terms


Further reading

Compare

See also

Mention(s)



Reference

  1. Fruton, Joseph S (1999). Proteins, Enzymes, Genes: The Interplay of Chemistry and Biology. Chelsea, Michigan: Yale University Press. p. 144. ISBN 0300153597.



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