Biochemistry : Glycosidic Linkages

Study concepts, example questions & explanations for Biochemistry

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Example Questions

Example Question #1 : Glycosidic Linkages

Glucose polysaccharides are linked together at branch points in glycogen by what type of bond?

Possible Answers:

Peptide linkages

Alpha-1,4 linkages

Beta-1,6 linkages

Alpha-1,6 linkages

Beta-1,4 linkages

Correct answer:

Alpha-1,6 linkages

Explanation:

In glycogen, glucose molecules are attached one after the other by alpha-1,4 linkages. However, in order to make glycogen more compact for storage, branch points are created to created links between many shorter glucose polysaccharides. These branch points connect glucose molecules by alpha-1,6 linkages.

Example Question #2 : Glycosidic Linkages

Why does glycogen have more branches than starch?

Possible Answers:

There are less alpha-1,4 linkages

There are less alpha-1,6 linkages

None of these answers; glycogen is not more branched than starch

There are more alpha-1,4 linkages

There are more alpha-1,6 linkages

Correct answer:

There are more alpha-1,6 linkages

Explanation:

Lots of alpha-1,4 linkages allow for longer chain lengths in carbohydrates like starch and glycogen. However, it is the amount of alpha-1,6 linkages that determine the number of branches - since glycogen has many more alpha-1,6 linkages than starch does, it has more branches. This allows for easy breakdown of glycogen into glucose in the liver should there not be enough glucose in the body to supply the body's demand for energy production. Recall that glycogen phosphorylase can only break terminal alpha-1,4 glycosidic bonds; hence, with more branches there are more terminal glucose molecules that are substrates for this catabolic enzyme. 

Example Question #3 : Glycosidic Linkages

During times of glucose deprivation in the human body, the liver is able to supply glucose to the bloodstream by breaking down a large, branched polysaccharide that it holds in reserve until it is needed. Which of the following lists the correct type of glycosidic bonds found in this polysaccharide.

Possible Answers:

  for branch points and  for straight chain

 for branch points and  for straight chain

 for branch points and  for straight chain

 for branch points and  for straight chain

 for branch points and  for straight chain

Correct answer:

 for branch points and  for straight chain

Explanation:

For this question, we're told some background information about the liver's role in providing glucose homeostasis. We're told that when blood glucose levels are lowered, the liver is able to help restore glucose levels by keeping a large polysaccharide of glucose in store. In times of need, the liver can break this compound down to provide glucose to the bloodstream.

Even though the question doesn't explicitly tell us what the polysaccharide is, we should be able to infer that the compound in question is glycogen. Therefore, to answer the question, we need to know which kind of glycosidic bonds are found in glycogen.

First, let's recall that an individual glucose molecule is composed of six carbon atoms. In its ring form, glucose can exist as one of two epimers, depending on how its ring closes when transitioning from its straight chain form to its closed ring form. The anomeric carbon of the glucose molecule can be arranged in one of two ways when its ring closes. The anomeric carbon is the one that goes from being achiral to chiral as the ring closes. In the alpha configuration, the hydroxyl group attached to the anomeric carbon faces down, while in the beta configuration it faces up.

In addition to existing as either an alpha or a beta epimer, glucose also participates in glycosidic linkages using its first, fourth, and sixth carbon atom.

In glycogen, each individual glucose molecule is in the alpha configuration. Thus, we can rule out both answer choices that include beta. Moreover, the fourth carbon atom of each glucose molecule is attached to the first carbon atom (the anomeric carbon) in the next glucose molecule in the straight chain. To make branch points at various points along the straight chain, some of the glucose molecules have their sixth carbon atom attached to the anomeric carbon of other glucose molecules.

In conclusion,  glycosidic bonds are responsible for branch points and  glycosidic bonds are responsible for the straight chain.

Example Question #4 : Glycosidic Linkages

Glycogen is not a single chain of glucose units, but many chains branching off of one another. Why is the branching of glycogen important?

Possible Answers:

All of these are reasons why glycogen branching is important

Branching increases the rate of glycogen degradation

Branching increases the rate of glycogen synthesis

Branching makes glycogen more compact

Branching increases glycogen solubility

Correct answer:

All of these are reasons why glycogen branching is important

Explanation:

Because glycogen is so heavily branched, it is able to pack more glucose units together in a small space, thus it is more compact and has a greater solubility. Moreover, the branching allows for glycogen enzymes to act more efficiently on the chains of glucose, and so both degradation and synthesis have increased rates.

Example Question #5 : Glycosidic Linkages

You discover that your patient is lactose intolerant, having a mutation that does not allow them to produce an enzyme that cleaves the disaccharide lactose. If they had the lactase enzyme, which glycosidic bond would it cleave?

Possible Answers:

Galactose-beta-1,4-glucose 

Glucose-alpha-1,4-glucose

Galactose-alpha-1,4-glucose

Glucose-alpha 1,2-fructose

Glucose-beta 1,4-glucose 

Correct answer:

Galactose-beta-1,4-glucose 

Explanation:

lactose is made up of galactose and glucose and is bound via a beta 1,4 glycosidic bond. 

the enzyme lactase cleaves this bond to break down the sugar lactose. Maltose is glucose- alpha 1,4- glucose, and sucrose is glucose- alpha, 1,2- fructose. 

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