Question

Suppose that a polymer of glucose with alternating \(\alpha(1 \rightarrow 4)\) and \(\beta(1 \rightarrow 4)\) glycosidic linkages has just been discovered. Draw a Haworth projection for a repeating tetramer (two repeating dimers) of such a polysaccharide. Would you expect this polymer to have primarily a structural role or an energy-storage role in organisms? What sort of organisms, if any, could use this polysaccharide as a food source?

Short answer

The polymer likely plays a structural role, and certain microbes in the guts of herbivores could use it as a food source.

Step-by-step solution

Understand the Glycosidic Linkages

Glycosidic bonds can either be \(\alpha(1\rightarrow 4)\) or \(\beta(1\rightarrow 4)\). The alpha bond occurs when the substituent on the anomeric carbon is trans to the CH2OH group, and the beta bond is when the substituent is cis to the CH2OH group.

Draw the Glucose Monomers

Draw the two types of glucose monomers: one with an \(\alpha(1\rightarrow 4)\) linkage and one with a \(\beta(1\rightarrow 4)\) linkage. In cyclic form, this means alternating glucose units where the hydroxyl group on the anomeric carbon will be in alternating axial and equatorial positions.

Create the Repeating Dimer

Combine the two glucose monomers into a dimer, making sure to alternate between \(\alpha(1\rightarrow 4)\) and \(\beta(1\rightarrow 4)\) glycosidic bonds.

Replicate the Dimer

Draw the tetramer by replicating the dimer unit, ensuring you maintain the alternating glycosidic linkages.

Analyze the Structure's Role

This polysaccharide has both \(\alpha\) and \(\beta\) linkages, which means it likely forms a rigid structure. Therefore, it is more likely to have a structural role rather than an energy storage role, since energy-storage polysaccharides typically have only \(\alpha\) linkages.

Determine Organisms Using the Polysaccharide

Considering it has alternating \(\alpha\) and \(\beta\) linkages, organisms specializing in breaking down such compounds, such as microbes in the guts of herbivores, could use this polysaccharide as a food source.

Key concepts

Polymer of Glucose

A polymer of glucose is a large molecule consisting of many glucose units linked together. In this problem, the glucose units are connected by alternating glycosidic linkages: \(\alpha(1 \rightarrow 4)\) and \(\beta(1 \rightarrow 4)\). These types of bonds refer to how the glucose molecules are joined together. Glucose polymers can vary greatly, affecting their properties and functions, which we'll delve into in this article.

Haworth Projection

The Haworth projection provides a way to represent the three-dimensional structure of cyclic sugars on a two-dimensional plane. For glucose, the Haworth projection depicts the ring structure of the molecule, showing the orientation of the hydroxyl groups. When illustrating a repeating tetramer with alternating \(\alpha(1 \rightarrow 4)\) and \(\beta(1 \rightarrow 4)\) linkages, one must alternate the positions of these hydroxyl groups. In an alpha linkage, the substituent is trans to the \(CH_2OH\) group, whereas in a beta linkage, it is cis. This alternation provides the molecule with significant rigidity.

Structural vs. Energy-Storage Role

Polysaccharides can serve various functions depending on their linkages. Typically, polysaccharides with \(\alpha\) linkages, such as starch and glycogen, are used for energy storage because their structure allows them to be easily broken down. In contrast, those with \(\beta\) linkages, like cellulose, provide structural support as they form strong, rigid fibers. The alternating \(\alpha(1 \rightarrow 4)\) and \(\beta(1 \rightarrow 4)\) linkages in our polymer suggest that it adopts a rigid structure. Consequently, it is more likely to serve a structural role in organisms rather than merely storing energy.

Organisms and Polysaccharides

Different organisms can utilize various polysaccharides based on their digestive enzymes. For glucose polymers with alternating \(\alpha(1 \rightarrow 4)\) and \(\beta(1 \rightarrow 4)\) linkages, it requires specialized enzymes capable of breaking both types of bonds. Typically, microbes found in the guts of herbivores can break down complex polysaccharides like cellulose and the newly discovered glucose polymer. These microbes produce enzymes that can cleave both \(\alpha\) and \(\beta\) glycosidic bonds. Therefore, herbivores and certain microbes can likely utilize such a polymer as a nutrient source.