5.6. Disaccharides
Saccharide dimers (disaccharides) are very common in nature. Most disaccharide compounds were identified and characterized before the IUPAC system was developed. As a result, the use of trivial names for disaccharides remains commonplace. Disaccharides may also be described using formalized IUPAC naming. These names contain all information necessary to describe the structure. They are typically far longer than the trivial name but conversely do not require (as much) memorization to be understood.
5.6.1. Nomenclature of Disaccharides
Functionally, the naming system for disaccharides simply combines the naming of each monosaccharide with the name of the glycosidic bond. Disaccharide names are typically quite long but are generally straightforward to generate or interpret.
All disaccharide names follow the same pattern. First the saccharide with the acetal of the glycosidic bond is described, then the glycosidic bond itself is described, and finally the other saccharide is described. Because the α/β stereochemistry of the acetal-containing saccharide is described in its name this portion is omitted from the glycosidic bond name. The naming scheme follows the pattern: {[α/β]-[D/L]-[trivial name -se][furan/pyran]osyl}-([carbon number of the acetal],[carbon number of the alcohol])-{[α/β]-[D/L]-[trivial name -se][furan/pyran]ose} (Figure 5.27). Consulting a chart with named monosaccharides (e.g. Figures 5.6 and 5.7) makes the process simpler but more in-depth analysis is often required.
Figure 5.27 – Examples of Naming Disaccharides.
As before, generally the naming of disaccharides requires significant practice and a combination of different approaches for determining the various parts of the name.
5.6.2. Trivial Names of Common Disaccharides
A brief set of examples of common disaccharides serves to show the variability in structure and highlight biological differences.
Sucrose is a very common disaccharide in nature (Figure 5.28). It is found in moderate to high quantities in several plants; sucrose is “table sugar”, the white sugar widely used in most households. The disaccharide is a combination of two common monosaccharides with an uncommon type of glycosidic bond: α-D-glucopyranosyl-(1,2)-β-D-fructofuranoside. The glycosidic bond is between the anomeric carbons of each saccharide.
Figure 5.28 – Structure of the Disaccharide Sucrose.
Because the functional group of the second saccharide is now an acetal instead of a hemiacetal the suffix changes from -ose to –oside. The second saccharide can also be viewed as having a glycosidic bond.
Lactose is a common disaccharide in nature (Figure 5.29). It is found in moderate to high quantities in most mammalian species’ milk. The disaccharide is comprised of a mixture of two anomers: β-D-galactopyranosyl-(1,4)-α-D-glucopyranose and β-D-galactopyranosyl-(1,4)-β-D-glucopyranose. The two isomers interconvert and exist in equilibrium.
Figure 5.29 – Structure of the Disaccharide Lactose.
Because acetals are stable under normal physiological conditions an enzyme (lactase) is needed to break the glycosidic bond during digestion. Mammalian infants naturally produce the enzyme and are able to digest the disaccharide as part of their milk-based diet. Certain populations of humans do, or do not, naturally produce this enzyme into adulthood. Lack of the enzyme results in lactose being undigested by the human. If undigested it passes into the gut directly. While passing through the gut bacteria with the enzyme are able to digest it, multiply, and result in the symptoms of lactose-intolerance.
Maltose is a common disaccharide in nature (Figure 5.30). It is a product from the digestion of starch, a set of polysaccharides commonly found in plants (see Sections 5.7.2-5.7.4). It is a dimer of glucose. The disaccharide is comprised of a mixture of two anomers: α-D-glucopyranosyl-(1,4)-α-D-glucopyranose and α-D-glucopyranosyl-(1,4)-β-D-glucopyranose. The two isomers interconvert and exist in equilibrium.
Figure 5.30 – Structure of the Disaccharide Maltose.
Because acetals are stable under normal physiological conditions an enzyme (maltase) is needed to break the glycosidic bond during digestion. This enzyme selectively breaks α-1,4 glycosidic bonds between two glucopyranoses. Humans naturally produce this enzyme and are able to digest maltose.
Cellobiose is a common disaccharide in nature (Figure 5.31). It is a product from the digestion of cellulose, a polysaccharide commonly found in plants (see Section 5.7.1). It is a dimer of glucose. The disaccharide is comprised of a mixture of two anomers: β-D-glucopyranosyl-(1,4)-α-D-glucopyranose and β-D-glucopyranosyl-(1,4)-β-D-glucopyranose. The two isomers interconvert and exist in equilibrium.
Figure 5.31 – Structure of the Disaccharide Cellobiose.
Because acetals are stable under normal physiological conditions an enzyme (beta-glucosidase) is needed to break the glycosidic bond during digestion. This enzyme selectively breaks β-1,4 glycosidic bonds between two glucopyranoses. Humans do not naturally produce this enzyme and are not able to digest cellobiose.
The sharp contrasts between cellobiose and lactose (see Section 5.6.2.2) or maltose (see Section 5.6.2.3), each of which differ from cellobiose by a single stereocentre, highlights the importance of stereochemistry and three-dimensional shape in biological systems.
