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 energystorage role in organisms? What sort of organisms, if any, could use this polysaccharide as a food source?

Short answer

Draw a tetramer with alternating \(\alpha(1 \rightarrow 4)\) and \(\beta(1 \rightarrow 4)\) linkages. The polymer likely has a structural role, usable by herbivores and some microorganisms.

Step-by-step solution

Understand the Structure

A tetramer in this context is made up of four glucose monomers. Since the polymer has alternating \(\alpha(1 \rightarrow 4)\) and \(\beta(1 \rightarrow 4)\) glycosidic linkages, visualize each unit accordingly.

Draw the Haworth Projection

To draw the Haworth projection, start by sketching two glucose monomers linked by an \(\alpha(1 \rightarrow 4)\) bond. Then, between the next two glucose units, draw a \(\beta(1 \rightarrow 4)\) linkage. Repeat this pattern to show four monomers forming a tetramer.

Analyze the Role of the Polymer

Alternating \(\alpha\) and \(\beta\) linkages suggest the polymer is less likely to coil or form dense structures, making it more suitable for structural roles rather than energy storage.

Identify Potential Consumers

Consider that many organisms do not have enzymes to digest \(\beta(1 \rightarrow 4)\) linkages, as found in cellulose. Herbivores and some microorganisms with specialized enzymes might use this polysaccharide, but not most other animals.

Key concepts

Glucose Polymer

A glucose polymer is a compound made up of repeating glucose units connected by glycosidic linkages. In our specific exercise, the discovered polymer consists of alternating \(\alpha(1 \rightarrow 4)\) and \(\beta(1 \rightarrow 4)\) glycosidic bonds. This means the glucose molecules attach through two different types of bonds, creating a unique structure. Such polymers can differ in their roles based on the kind and sequence of their linkages. For instance, amylose is a simple glucose polymer with \(\alpha(1 \rightarrow 4)\) bonds, making it prevalent for energy storage in plants.
Alternating linkages imply that the polymer does not continuously coil or compact the same way simpler structures do, potentially influencing its function in living organisms.

Haworth Projection

Haworth projections are a way to represent the cyclic structure of monosaccharides, like glucose, in a two-dimensional plane. These projections help visualize the configuration of each carbon atom in the cyclic form. To draw the repeating tetramer of our glucose polymer:
1. Sketch a glucose ring with the OH group on the anomeric carbon in the trans position for an \(\alpha(1 \rightarrow 4)\) linkage.
2. Connect it to the next glucose unit using an \(\alpha(1 \rightarrow 4)\) bond.
3. For the second glucose, position the OH group in the cis configuration for a \(\beta(1 \rightarrow 4)\) linkage.
4. Continue this alternating pattern to complete the tetramer. By doing this, you can easily visualize the configuration and arrangement of glucose units in the polymer.

Structural vs. Energy Storage Roles

Polysaccharides can serve either structural or energy storage roles based on their glycosidic linkages. Structural polysaccharides, like cellulose, involve \(\beta(1 \rightarrow 4)\) linkages, forming rigid, fibrous structures that plants use for cell walls. On the other hand, energy storage polysaccharides, such as starch and glycogen, primarily involve \(\alpha(1 \rightarrow 4)\) linkages, which create branched structures that can be broken down more easily for energy.
Given that the new polymer alternates between \(\alpha\) and \(\beta\) linkages, it won't form compact, dense structures. This suggests the polymer is more likely to serve structural purposes, providing stability or support rather than storing energy.

Organisms Digestion

The ability to digest polysaccharides depends significantly on the presence of specific enzymes. Most animals, including humans, have enzymes like amylase that break down \(\alpha(1 \rightarrow 4)\) linkages found in starch, but not the \(\beta(1 \rightarrow 4)\) linkages present in cellulose. Consequently, most animals can’t digest cellulose.
However, some herbivores and microorganisms possess specialized enzymes, such as cellulase, enabling them to break down cellulose and similar structures. For the discovered polymer with alternating linkages, organisms with enzymes capable of degrading both \(\alpha\) and \(\beta\) bonds would be necessary. Such enzymes are rare, meaning only specific organisms like certain herbivores or specialized bacteria and fungi might use this polysaccharide as a food source.