Biochemistry : Citric Acid Cycle Enzymes

Study concepts, example questions & explanations for Biochemistry

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Example Questions

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Example Question #194 : Catabolic Pathways And Metabolism

Which enzyme catalyzes the conversion of citrate to isocitrate?

Possible Answers:

Phosphate

Aconitase

Aldolase

Citrate synthase

Citrate isomerase

Correct answer:

Aconitase

Explanation:

Aconitase is the enzyme that catalyzes the conversion of citrate to isocitrate. This essential enzyme is vital in energy production, as it acts like an iron regulatory protein. The conversion of citrate to isocitrate is important since it is needed to react with isocitrate dehydrogenase.

Example Question #195 : Catabolic Pathways And Metabolism

What is the name of the enzyme that incorporate Acetyl-CoA into the citric acid cycle?

Possible Answers:

Citrate synthase

Thiolase

Pyruvate carboxylase

Pyruvate dehydrogenase

Acetyl-CoA carboxylase

Correct answer:

Citrate synthase

Explanation:

Citrate synthase is the first enzyme of the citric acid cycle. Its role is to condense acetyl-CoA onto oxaloacetate in order to generate citrate.

Acetyl-CoA carboxylase is an enzyme that attaches a carboxyl group to acetyl-CoA in order to generate malonyl-CoA, which plays a role in fatty acid synthesis by contributing two carbons at a time to the growing hydrocarbon chain. Moreover, malonyl-CoA also serves a regulatory role in the breakdown and synthesis of fatty acids. Since it is a major precursor to the synthesis of fatty acids, high levels of it inhibit the breakdown of fatty acids by preventing fatty acids from entering the mitochondria, where they are broken down via beta-oxidation. Thus, malonyl-CoA allows fatty acids to be synethesized without simultaneously being degraded.

Thiolase is an enzyme that condenses two molecules of acetyl-CoA into acetoacetyl-CoA. This molecule is an important intermediate in two important pathways. One is the production of ketone bodies, while the other is the mevalonate pathway, which is an important series of reactions that synthesizes many compounds, such as cholesterol.

Pyruvate dehydrogenase is an enzyme complex that converts pyruvate into acetyl-CoA, thus linking glycolysis with the citric acid cycle.

Pyruvate carboxylase is an enzyme that adds a carboxyl group to pyruvate in order to generate oxaloacetate. This reaction can be used either to generate oxaloacetate for use in the kreb's cycle, or in the gluconeogenesis pathway to synthesize glucose from a variety of substrates.

Example Question #1 : Citric Acid Cycle Enzymes

The enzyme aconitase is responsible for catalyzing which of the following reactions?

Possible Answers:

The conversion of citrate to isocitrate

The conversion of succinyl-CoA to succinate

The converstion of isocitrate to alpha-ketoglutarate

The conversion of malate to fumarate

Correct answer:

The conversion of citrate to isocitrate

Explanation:

The first step of the citric acid cycle involves the combination of acetyl-CoA with oxaloacetate, producing citrate. Next, aconitase catalyzes the isomerization of citrate to isocitrate, via the intermediate known as cis-aconitate. The conversion of isocitrate to alpha-ketoglutarate is catalyzed by isocitrate dehydrogenase. The citric acid cycle involves the conversion of fumarate to malate, not the reverse, although some bacteria do the reverse citric acid cycle to produce complex organic compounds. Succinyl-CoA is converted to succinate via succinyl-CoA synthetase (also known as succinate thiokinase).

Example Question #197 : Catabolic Pathways And Metabolism

Which enzyme of the citric acid cycle is membrane-bound?

Possible Answers:

Succinate dehydrogenase

Succinyl-CoA synthetase 

Isocitrate dehydrogenase

None of the Krebs cycle enzymes are membrane-bound

Aconitase 

Correct answer:

Succinate dehydrogenase

Explanation:

Succinate dehydrogenase (also known as succinate-coenzyme Q reductase or complex II) is bound to the inner mitochondrial membrane. It participates in both the citric acid cycle and the electron transport chain. Aconitase is an enzyme involved in glycolysis, not the citric acid cycle (Krebs cycle).

Example Question #198 : Catabolic Pathways And Metabolism

Which of the following enzymes would be inhibited by the addition of NADH?

Possible Answers:

Alpha-ketoglutarate dehydrogenase

Citrate synthase

Fumarase

Isocitrate dehydrogenase

More than one of these

Correct answer:

More than one of these

Explanation:

All enzymes that have NADH as a product would be inhibited by the addition of NADH. The correct answers are citrate synthase, isocitrate dehydrogenase, and alpha-ketoglutarate dehydrogenase. Pyruvate dehydrogenase, which synthesizes acetyl-CoA from pyruvate, is also inhibited by NADH.

Example Question #199 : Catabolic Pathways And Metabolism

Glucose is converted to pyruvate through glycolysis, and pyruvate must then be converted into acetyl-CoA in order to enter the citric acid cycle.

What is the name of the enzyme that catalyzes the conversion of pyruvate into acetyl-CoA?

Possible Answers:

Phosphofructokinase

Aconitase

Pyruvate synthase

Pyruvate kinase

Pyruvate dehydrogenase 

Correct answer:

Pyruvate dehydrogenase 

Explanation:

Each enzyme listed in the answer choices catalyzes a reaction in glycolysis or the citric acid cycle, but pyruvate dehydrogenase is the only one responsible for converting pyruvate to acetyl-CoA. Pyruvate is decarboxylated (removal of a ) and this is replaced with an S-CoA group, forming the necessary acetyl-CoA molecule to feed into the citric acid cycle. 

Example Question #200 : Catabolic Pathways And Metabolism

Which citric acid cycle enzyme uses ?

Possible Answers:

Alpha-ketoglutarate dehydrogenase

Malate dehydrogenase

Succinate dehydrogenase

Citrate synthase

Correct answer:

Succinate dehydrogenase

Explanation:

Succinate dehydrogenase is the only enzyme in the citric acid cycle to use . The other dehydrogenases use  while citrate synthase performs an unrelated reaction using acetyl-CoA.

Example Question #201 : Catabolic Pathways And Metabolism

Which of the following citric acid cycle enzymes carries out a reversible reaction?

Possible Answers:

Citrate synthase

Malate dehydrogenase

Isocitrate dehydrogenase 

Alpha-ketoglutarate dehydrogenase

Correct answer:

Malate dehydrogenase

Explanation:

Malate dehydrogenase catalyzes the conversion of malate to oxaloacetate and vice versa, while the other enzymes are the three irreversible steps in the citric acid cycle.

Example Question #202 : Catabolic Pathways And Metabolism

In the context of the citric acid cycle, the action of succinate dehydrogenase results in the formation of which of these?

Possible Answers:

 and Fumarate

 and Citrate

Fumarate and 

Oxaloacetate

Malate and 

Correct answer:

Fumarate and 

Explanation:

Succinate dehydrogenase catalyzes the reaction from succinate to fumarate.  A byproduct of this reaction is the formation of  from .  

Example Question #203 : Catabolic Pathways And Metabolism

Which of the following enzymes does not catalyze a reaction within the citric acid cycle?

Possible Answers:

Isocitrate dehydrogenase

Fumarase

Pyruvate kinase

Succinyl-CoA synthetase

Alpha-ketoglutarate dehydrogenase 

Correct answer:

Pyruvate kinase

Explanation:

The only enzyme listed in the answer choices that does not catalyze a reaction within the citric acid cycle is pyruvate kinase. Pyruvate kinase is an important enzyme in the final step of glycolysis, as it catalyzes the conversion of phosphoenolpyruvate and ADP to pyruvate and ATP. This reaction is not contained within the citric acid cycle though, and therefore pyruvate kinase does not catalyze any reactions in the citric acid cycle.

Each of the other enzymes listed catalyzes reactions within the citric acid cycle, as follows:

Isocitrate dehydrogenase catalyzes the conversion of isocitrate to alpha-ketoglutarate. 

Alpha-ketoglutarate dehydrogenase catalyzes the conversion of alpha-ketoglutarate to succinyl-CoA. 

Succinyl-CoA synthetase catalyzes the conversion of succinyl-CoA to succinate. 

Fumarase catalyzes the conversion of fumarate to malate. 

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