The Dehydration Synthesis Reaction
When three glucose molecules link together, they undergo a chemical reaction known as dehydration synthesis, or a condensation reaction. This process is responsible for creating larger, more complex carbohydrate molecules from simpler monosaccharides. The name "dehydration" aptly describes the reaction, as a molecule of water ($H_2O$) is removed for each new bond formed between the glucose units.
For three glucose monomers to join, two separate dehydration reactions must occur. One water molecule is removed as the first two glucose units are bonded, and a second water molecule is removed as the third unit is attached. This results in a final chemical formula of $C{18}H{32}O_{16}$, which is derived from the starting three glucose molecules ($3 imes C6H{12}O6 = C{18}H{36}O{18}$) minus the two molecules of water ($2 imes H_2O = H_4O_2$).
The Glycosidic Bond
The specific covalent bonds that link the monosaccharides are called glycosidic bonds. The type and location of this bond are crucial in determining the final product and its properties. For maltotriose, the most common trisaccharide formed from glucose, the linkage is an $\alpha$-1,4 glycosidic bond. This means that the bond connects the first carbon atom of one glucose molecule to the fourth carbon atom of the next. Different glycosidic bonds produce different trisaccharides. For example, nigerotriose, another trisaccharide made of three glucose units, has an $\alpha$-1,3 glycosidic bond.
Maltotriose: A Key Trisaccharide
As the primary product of the enzymatic digestion of starch in humans, maltotriose is the most recognized answer to what three glucoses make. It is an oligosaccharide consisting of three glucose units linked by $\alpha$-1,4 glycosidic bonds. Maltotriose is also the shortest chain oligosaccharide that can be classified as a maltodextrin.
Biological Significance
In the human digestive system, the enzyme $\alpha$-amylase breaks down starch and glycogen into smaller carbohydrates, including maltotriose. The maltotriose can then be further hydrolyzed into individual glucose molecules by enzymes like maltase and glucoamylase, allowing for absorption and use as energy.
Maltotriose also plays a role in some metabolic disorders. In cases of glycogen storage disease type II, a deficiency in the enzyme responsible for glycogen breakdown can lead to increased levels of maltotriose.
Industrial Applications
Beyond its role in digestion, maltotriose has industrial applications, particularly in the brewing industry. Brewer's yeast can ferment maltotriose, contributing to the ethanol content of beer. However, some yeast strains are inefficient at fermenting maltotriose, which can result in residual sweetness in the final product. Additionally, modified maltooligosaccharides have been investigated for use in the pharmaceutical industry due to their varied properties.
How Maltotriose Compares to Other Carbohydrates
Understanding maltotriose is easier when compared to simpler carbohydrates made from glucose. Here is a comparison of maltotriose with glucose and maltose.
| Feature | Glucose | Maltose | Maltotriose |
|---|---|---|---|
| Classification | Monosaccharide | Disaccharide | Trisaccharide |
| Number of Glucose Units | 1 | 2 | 3 |
| Molecular Formula | $C6H{12}O_6$ | $C{12}H{22}O_{11}$ | $C{18}H{32}O_{16}$ |
| Synthesis Reaction | Basic monomer | Dehydration synthesis (1 bond) | Dehydration synthesis (2 bonds) |
| Digestion | Absorbed directly | Hydrolyzed by maltase | Hydrolyzed by maltase/glucoamylase |
| Source | Produced via photosynthesis | Starch digestion | Starch digestion |
| Use | Direct cellular energy | Absorbed as glucose | Absorbed as glucose |
Other Possible Trisaccharides
While maltotriose is the most common answer to what three glucoses make in a biological context, it is not the only possibility. The different ways the glucose units can be linked create other isomers, such as nigerotriose (α-1,3 linkages). Another example is cellotriose, though this is part of the larger cellulose polymer, which involves $\beta$-1,4 linkages. The specific enzyme or chemical process dictates the type of linkage and, therefore, the final trisaccharide produced.
Conclusion
In summary, when three glucose units combine, they form a trisaccharide through a process of dehydration synthesis, which releases two molecules of water. The most common product of this reaction in biological systems is maltotriose, which consists of three glucose units linked by $\alpha$-1,4 glycosidic bonds. This molecule is a key intermediate in the digestion of starch, serving as a vital energy source for the body. Understanding the formation and properties of maltotriose and other glucose-based carbohydrates is fundamental to the study of biochemistry and human metabolism.
For more information on the structure of carbohydrates, see this comprehensive guide from Khan Academy.
