Maltose: structure, metabolism, and biological and technological roles

Maltose, also known as malt sugar or α-D-glucopyranosyl-(1→4)-D-glucopyranose, is a disaccharide composed of two D-glucose molecules in their pyranose form. The two monosaccharides are linked by an α-(1→4) glycosidic bond, a covalent bond between C1 of one glucose unit, its hemiacetal anomeric carbon, and the oxygen atom of the hydroxyl group at C4 of the second glucose unit. The bond forms with retention of configuration at C1, remaining in the α-configuration. Because one glucose residue retains a hemiacetal carbon, maltose is a reducing sugar.[1][2]

Maltose is produced in the proximal section of the small intestine, the duodenum, as well as in germinating seeds, through the action of amylases on starch.[3]

Industrially, it is obtained through acidic or enzymatic hydrolysis of starches of various origins, often using fungal or bacterial amylases, such as those from Aspergillus oryzae or Bacillus subtilis.[4][5]

Maltose does not naturally occur in foods; it is found only in certain processed foods to which it is added during manufacturing.[6] For example, it is present in many bakery and pastry products, where it functions as a sweetener, stabilizer, and preservative.[7] For this reason, it is included among authorized food additives.[8]

In the duodenum, the α-(1→4) glycosidic bond is hydrolyzed by brush-border membrane hydrolases of the enterocytes. The released glucose is then absorbed and enters the bloodstream.[9]

Contents

Chemical properties

As with the disaccharides lactose, sucrose, and trehalose, its molecular formula is C12H22O11 and its molecular weight is 342.30 g/mol.[10]

It is highly soluble in water and has a sweet taste, although it is only about 33% as sweet as sucrose.[11]

Like monosaccharides, and like lactose among disaccharides, it is a reducing sugar, since the α-(1→4) glycosidic bond does not involve the hemiacetal (anomeric) carbon of one of the two glucose residues. This carbon is therefore free to revert to the carbonyl form; in solution, the ring can open to expose a free aldehyde group. However, it should be noted that the open-chain aldehyde form is present only in very small amounts.[2]

The anomeric carbon not involved in the glycosidic bond undergoes mutarotation; that is, both the α and β configurations are possible, although the β configuration predominates.

Structural formulas of maltose in Haworth projection, showing the α and β anomers and their interconversion via the open-chain form with exposed aldehyde group
Anomers of Maltose

Anomerism is a form of optical isomerism characteristic of carbohydrates. Two stereoisomers of a cyclic monosaccharide are defined as anomers if they differ only in the configuration at the hemiacetal or hemiketal carbon, known as the anomeric carbon or anomeric center.[1]

Intramolecular cyclization of monosaccharides converts the carbonyl carbon into an asymmetric (chiral) center. In solution, and in equilibrium with the open-chain form, two anomers can form:

  • α-isomer, if, during cyclization, the oxygen of the hydroxyl group attacks the carbon atom of the carbonyl group from top side of the sp2 plane;
  • β-isomer, if the oxygen of the hydroxyl group attacks the carbon atom of the carbonyl group from bottom side of the sp2 plane.[12]

Role

During seed germination, endospermic starch is hydrolyzed by amylases to maltose and glucose, which, along with other hydrolysis products, support the growth of germinating seedlings.[13]

The production of alcoholic beverages obtained by cereal fermentation, as well as the manufacture of foods with a high maltose content, such as glucose and maltose syrups, or bread-making, also relies on amylase activity on starch to release the disaccharide.[3][14]

In pastry making, maltose is used both as a sweetener and as a stabilizer for icings, without increasing sweetness to the same extent as sucrose. Furthermore, because it can inhibit starch retrogradation it can extend shelf life.[7] For these reasons, maltose can be classified as a preservative, stabilizer, or sweetener for food use. It is also present in many carbohydrate preparations for infant feeding.[8]

Outside the food industry, it is used, for example, as a stabilizer for immunoglobulins.[10][15]

Maltose digestion

In humans, carbohydrate digestion begins in the oral cavity with salivary alpha-amylase and continues in the duodenum through the action of hydrolases such as pancreatic α-amylase and the brush-border enzymes of enterocytes. The combined activity of these enzymes enables the hydrolysis of disaccharides, oligosaccharides, and polysaccharides into their constituent monosaccharides: fructose, glucose, and galactose. The absorption of monosaccharides occurs in the small intestine and is mediated by specific protein transporters located in the plasma membrane of enterocytes.[9]

The action of salivary and pancreatic α-amylase on the two polysaccharides that compose starch, amylose and amylopectin, produces maltose, maltotriose, an oligosaccharide composed of three glucose units linked by α-(1→4) glycosidic bonds, and, from amylopectin, α-limit dextrins, which are glucose polymers containing at least one α-(1→6) glycosidic bond.[16][17]

Although maltose, maltotriose, and α-limit dextrins can also arise from glycogen breakdown, this source plays a negligible role because, after the animal’s death, glycogen undergoes rapid degradation, mainly to glucose and lactic acid.[18]

The α-(1→4) glycosidic bond of maltose is hydrolyzed in a reaction catalyzed by two enzymes: sucrase-isomaltase (EC 3.2.1.48 and 3.2.1.10) and maltase-glucoamylase, or MAG (EC 3.2.1.20 and 3.2.1.3).[2]

Sucrase-isomaltase

Sucrase-isomaltase is a bifunctional enzyme with two active sites.[19][20] One active site, sucrase, is an α-glucosidase that hydrolyzes the glycosidic bonds of maltose, sucrose, and short α-(1→4) linked glucose oligomers containing up to six glucose units.[21] Sucrase accounts for 60–80% of small-intestine maltase activity.[22] The other active site, isomaltase, is an α-(1→6) glycosidase that hydrolyzes the α-(1→6) glycosidic bonds in α-limit dextrins.[23]

Maltase-glucoamylase

Like sucrase-isomaltase, maltase-glucoamylase is an enzyme with two active sites. One active site has high specificity for maltose, whereas the other has broad substrate specificity and acts on glucose oligomers. Both active sites catalyze the release of glucose units.[24][25][26]

Notably, α-amylase, sucrase-isomaltase, and maltase-glucoamylase act synergistically to completely digest dietary starches into glucose.[9][17]

Sucrase-isomaltase and MAG deficiency

Deficiency of a single brush-border glycosidase in enterocytes is generally due to a genetic defect, whereas the loss of all glycosidases is often the result of an intestinal infection. After the infection resolves, these enzymes gradually recover.[27]

Both sucrase and isomaltase activities may be impaired in congenital or primary sucrase-isomaltase deficiency, with accumulation, in the former case, also of undigested maltose.[28] A similar situation can occur in congenital maltase-glucoamylase deficiency, a rare condition with only a few cases reported in the literature.[29]

In both conditions, as well as during intestinal infections, undigested carbohydrates remain in the intestinal lumen, where they can be partially fermented by the gut microbiota, a component of the human microbiota. This fermentation leads to excessive production of gases such as hydrogen, carbon dioxide, and methane, and short-chain fatty acids, primarily acetic, propionic, and butyric acid.[30][31] The presence of undigested carbohydrates and their fermentation products, many of which are osmotically active solutes, causes an increase in intraluminal osmotic pressure, an influx of water into the lumen, and, consequently, diarrhea.[32]

Treatment consists of reducing or avoiding dietary maltose.[33]

References

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Biochemistry and Metabolism