Glycogenolysis

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Biochemistry › Glycogenolysis

Questions 1 - 10
1

Which one of the following statements is correct?

Insulin causes a liver cell to convert its glycogen phosphorylase a to glycogen phosphorylase b.

Glucagon stimulates conversion of muscle glycogen phosphorylase b to muscle glycogen phosphorylase a.

Glucose stabilizes the R-state of liver glycogen phosphorylase a.

5’ AMP binds to muscle glycogen phosphorylase b and inhibits it by an allosteric mechanism.

Explanation

Insulin is released in response to high blood glucose. It causes a signaling cascade that, in addition to other things, stops glycogenolysis. This is done by converting glycogen phosphorylase from it's active "a" form to its inactive "b" configuration. The "R" state is the active state, so the presence of glucose would not trigger the breakdown of glycogen. 5' AMP would not inhibit an inactive form of an enzyme. High AMP would mean a demand for ATP, so it would convert the enzyme to its "a" form.

2

Which one of the following statements is correct?

Insulin causes a liver cell to convert its glycogen phosphorylase a to glycogen phosphorylase b.

Glucagon stimulates conversion of muscle glycogen phosphorylase b to muscle glycogen phosphorylase a.

Glucose stabilizes the R-state of liver glycogen phosphorylase a.

5’ AMP binds to muscle glycogen phosphorylase b and inhibits it by an allosteric mechanism.

Explanation

Insulin is released in response to high blood glucose. It causes a signaling cascade that, in addition to other things, stops glycogenolysis. This is done by converting glycogen phosphorylase from it's active "a" form to its inactive "b" configuration. The "R" state is the active state, so the presence of glucose would not trigger the breakdown of glycogen. 5' AMP would not inhibit an inactive form of an enzyme. High AMP would mean a demand for ATP, so it would convert the enzyme to its "a" form.

3

Which one of the following statements is incorrect?

Breakdown of glycogen in muscle produces mostly glucose, which is released into the blood.

Glycogen provides a way to store energy in tissues that consume large amounts of energy when an organism is active.

Glycogen provides a reservoir of glucose molecules that can be used to replenish the blood with glucose when food is not available.

Both the synthesis and the breakdown of glycogen are regulated.

Explanation

Glycogen is mostly stored in the liver and skeletal muscle. When it is broken down in the liver, the last enzyme, a phosphatase, removes the last phosphate group to release plain glucose into the bloodstream. In the muscle, there is no need to release the glucose, so glycogen is only broken down as far as glucose-6-phosphate. Skeletal muscle cells lack the last phosphatase required to remove the phosphate from carbon 6. This isn't an obstacle, however, because the glucose-6-phosphate is already on to the second stage of glycolysis.

4

Which one of the following statements is incorrect?

Breakdown of glycogen in muscle produces mostly glucose, which is released into the blood.

Glycogen provides a way to store energy in tissues that consume large amounts of energy when an organism is active.

Glycogen provides a reservoir of glucose molecules that can be used to replenish the blood with glucose when food is not available.

Both the synthesis and the breakdown of glycogen are regulated.

Explanation

Glycogen is mostly stored in the liver and skeletal muscle. When it is broken down in the liver, the last enzyme, a phosphatase, removes the last phosphate group to release plain glucose into the bloodstream. In the muscle, there is no need to release the glucose, so glycogen is only broken down as far as glucose-6-phosphate. Skeletal muscle cells lack the last phosphatase required to remove the phosphate from carbon 6. This isn't an obstacle, however, because the glucose-6-phosphate is already on to the second stage of glycolysis.

5

Which one of the following can store the largest total amount of Glycogen in the human body?

Skeletal muscle

Fat

Brain

Liver

Explanation

Glycogen is the storage form of glucose, and is more readily accessible than starches or fats. It is used for short-term supply of glucose and in starvation conditions is used up in a matter of hours. It is mainly stored in the liver and skeletal muscle. Glycogenolysis in the liver results in glucose release into the bloodstream, whereas in the muscle the glucose is immediately used up. The highest demand for the glucose is in the muscle, and that is where most of it is stored.

6

Which one of the following can store the largest total amount of Glycogen in the human body?

Skeletal muscle

Fat

Brain

Liver

Explanation

Glycogen is the storage form of glucose, and is more readily accessible than starches or fats. It is used for short-term supply of glucose and in starvation conditions is used up in a matter of hours. It is mainly stored in the liver and skeletal muscle. Glycogenolysis in the liver results in glucose release into the bloodstream, whereas in the muscle the glucose is immediately used up. The highest demand for the glucose is in the muscle, and that is where most of it is stored.

7

Which of the following is an example of a catabolic reaction?

Glycogenolysis

Protein synthesis

Gluconeogenesis

DNA polymerization

Explanation

A catabolic reaction is defined as a reaction used to break down a large molecule into smaller subunits. Of the following options, glycogenolysis is the only option where a larger molecule (glycogen) is broken down into smaller subunits (individual glucose molecules).

8

Which of the following is an example of a catabolic reaction?

Glycogenolysis

Protein synthesis

Gluconeogenesis

DNA polymerization

Explanation

A catabolic reaction is defined as a reaction used to break down a large molecule into smaller subunits. Of the following options, glycogenolysis is the only option where a larger molecule (glycogen) is broken down into smaller subunits (individual glucose molecules).

9

What are some characteristics of glycogen phosphorylase?

I. It is the rate-limiting enzyme of glycogenolysis

II. It breaks alpha 1,4 glycosidic bonds

III. It is activated by epinephrine

IV. It breaks alpha 1,6 glycosidic bonds

I, II, and III

I and IV

II and III

II, III, and IV

I and II

Explanation

Glycogen phosphorylase, the rate-limiting enzyme of glycogenolysis does not breaks alpha 1,6 glycosidic bonds. It releases glucose from glycogen by hydrolyzing alpha 1,4 glycosidic bonds until it reaches a branch point in the glycogen molecule. At this time, another enzyme, a debranching alpha 1,6 glycosidase hydrolyzes the alpha 1,6 glycosidic bonds. Glycogen phosphorylase is under regulation by many hormones, including insulin and glucagon, as well as epinephrine.

10

What are some characteristics of glycogen phosphorylase?

I. It is the rate-limiting enzyme of glycogenolysis

II. It breaks alpha 1,4 glycosidic bonds

III. It is activated by epinephrine

IV. It breaks alpha 1,6 glycosidic bonds

I, II, and III

I and IV

II and III

II, III, and IV

I and II

Explanation

Glycogen phosphorylase, the rate-limiting enzyme of glycogenolysis does not breaks alpha 1,6 glycosidic bonds. It releases glucose from glycogen by hydrolyzing alpha 1,4 glycosidic bonds until it reaches a branch point in the glycogen molecule. At this time, another enzyme, a debranching alpha 1,6 glycosidase hydrolyzes the alpha 1,6 glycosidic bonds. Glycogen phosphorylase is under regulation by many hormones, including insulin and glucagon, as well as epinephrine.

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