Biochemistry : Alternative Pathways

Study concepts, example questions & explanations for Biochemistry

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Example Questions

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Example Question #1 : Glycogenolysis Enzymes

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

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

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

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 : Glycogenolysis Enzymes

Which enzymes are required for glycogen breakdown?

Possible Answers:

glycogen phosphorylase, glycogen branching enzyme, phosphoglutomutase

glycogen synthase, glycogen branching enzyme, UDP-glucose pyrophosphorylase

hexokinase, glycogen synthase, phosphoglucose isomerase

glycogen phosphorylase, glycogen debranching 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 #3 : 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:

II and III

I and II

I and IV

II, III, and IV

I, 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 #1 : 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

Glycogen phosphorylase

Alpha-1,6-glucosidase

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

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 #1 : Alternative Pathways

Which one of the following statements is incorrect?

Possible Answers:

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

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

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.

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 #1 : Glycogenolysis Enzymes

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.

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.

Glucose stabilizes the R-state of liver 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.

Example Question #1 : Alternative Pathways

Which of the following compounds is regenerated in the citric acid cycle?

Possible Answers:

Pyruvate

Oxaloacetate

Succinate

Citrate

Correct answer:

Oxaloacetate

Explanation:

Oxaloacetate is the four-carbon molecule that is regenerated by the enzyme malate dehydrogenase in order to continue the cycle to form citrate with acetyl-CoA in the first step of the Krebs cycle. The other answer choices are intermediates of the citric acid cycle, but only oxaloacetate is regenerated, making it a true cycle.

Example Question #1 : Glycogenolysis Regulation

Phosphorylation of glycogen phosphorylase has what effect on the enzyme?

Possible Answers:

Phosphorylation has no effect

Inactivation, conversion from glycogen phosphorylase B to glycogen phosphorylase A

Phosphorylation only confers partial activation

Activation, conversion from glycogen phosphorylase A to glycogen phosphorylase B

Activation, conversion from glycogen phosphorylase B to glycogen phosphorylase A

Correct answer:

Activation, conversion from glycogen phosphorylase B to glycogen phosphorylase A

Explanation:

Phosphorylation of glycogen phosphorylase activates it, converting it from its inactive B-form to its active A-form. 

Example Question #1 : Glycogenolysis Regulation

The process of glycogenolysis is an example of __________.

Possible Answers:

Phosphorylation

Phosphorolysis

Hydrolysis

Thiolysis

Lysis

Correct answer:

Phosphorolysis

Explanation:

Phosphorolysis is the name given to the addition of phosphate across a bond. Remember that in glycogenolysis, glycogen phosphorylase adds a phosphate across the a-1,4-glycosidic bonds between the glucose units of glycogen. The result is that glucose leaves as glucose-1-phosphate. If hydrolysis were performed instead of phosphorolysis, free glucose would be severed from glycogen and would be able to leave the cell.

Example Question #1 : Glycogenolysis

Which of the following statements about glycogen phosphorylase (GP) is incorrect?

Possible Answers:

The dephosphorylated form of glycogen phosphorylase is less active.

This enzyme is active during times of starvation.

Glucose-6-phosphate is a negative regulator of glycogen phosphorylase.

AMP is an inhibitor of glycogen phosphorylase.

Glucose is a competitive inhibitor of glycogen phosphorylase.

Correct answer:

AMP is an inhibitor of glycogen phosphorylase.

Explanation:

AMP is an activator of GP, whereas ATP is an inhibitor of GP. GP cleaves the alpha 1-4 glycosidic bond between a terminal glucose molecule and the rest of the glycogen straight chain, yielding glucose-1-phosphate during glycogenolysis.

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