Biochemistry : Glycogenolysis Enzymes

Study concepts, example questions & explanations for Biochemistry

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Example Questions

Example Question #1 : Alternative Pathways

Why is glycogen phosphorylase alone not sufficient in in degrading glycogen?

Possible Answers:

Glycogen phosphorylase can only cleave one glycosidic bond at which point another glycogen phosphorylase enzyme must come cleave the next one

Glycogen phosphorylase can only cleave roughly ten bonds before it runs out of energy

None of these

Glycogen phosphorylase can not cleave the alpha-1,6-glycosidic bonds at glycogen branch points

Glycogen phosphorylase can only cleave alpha-1,6-glycosidic bonds, and so when it reaches a branch point it stops

Correct answer:

Glycogen phosphorylase can not cleave the alpha-1,6-glycosidic bonds at glycogen branch points

Explanation:

When glycogen phosphorylase reaches a branching point in glycogen, the bonds switch from being alpha-1,4-glycosidic bonds to alpha-1,6-glycosidic bonds.  It is unable to cleave these bonds, and so other enzymes (a transferase and a glucosidase) must come into play.

Example Question #1 : Alternative Pathways

Which enzymes are required for glycogen breakdown?

Possible Answers:

glycogen phosphorylase, glycogen debranching enzyme, phosphoglutomutase

glycogen synthase, glycogen branching enzyme, UDP-glucose pyrophosphorylase

hexokinase, glycogen synthase, phosphoglucose isomerase

glycogen phosphorylase, glycogen branching enzyme, phosphoglutomutase

glycogen synthase, glycogen debranching enzyme, UDP-glucose pyrophosphorylase

Correct answer:

glycogen phosphorylase, glycogen debranching enzyme, phosphoglutomutase

Explanation:

Glycogen is first debranched and broken down from its non-reducing end by glycogen phosphorylase to give the product G1P, which is then converted into G6P by phosphoglutomutase. Glycogen synthase, glycogen branching enzyme, and UDP-glucose pyrophosphorylase are required for glycogen synthesis.

Example Question #2 : Alternative Pathways

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

Possible Answers:

I and IV

II, III, and IV

I and II

I, II, and III

II and III

Correct answer:

I, II, and III

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.

Example Question #3 : Alternative Pathways

Which of the following enzymes is not required to breakdown glycogen into glucose-6-phosphate molecules for further metabolism?

Possible Answers:

Phosphoglucomutase

Glucosyltransferase

All of these enzymes are necessary in the breakdown of glycogen into glucose-6-phosphate molecules.

Alpha-1,6-glucosidase

Glycogen phosphorylase

Correct answer:

All of these enzymes are necessary in the breakdown of glycogen into glucose-6-phosphate molecules.

Explanation:

In order to break down glycogen into individual glucose-6-phosphate units, all of the above enzymes are required. Each plays a specific role in one of the following activities: degradation of glycogen initially, remodeling of the glycogen so that it can be acted upon by the enzymes, and conversion of glucose-1-phosphate to glucose 6-phosphate.

Example Question #4 : Alternative Pathways

Which one of the following statements is incorrect?

Possible Answers:

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

Both the synthesis and the breakdown of glycogen are regulated.

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

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

Correct answer:

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

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.

Example Question #5 : Alternative Pathways

Which one of the following statements is correct?

Possible Answers:

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

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.

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

Correct answer:

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

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.

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