Biochemistry : Citric Acid Cycle

Study concepts, example questions & explanations for Biochemistry

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Example Questions

Example Question #2 : Reactants And Products Of The Citric Acid Cycle

Which of the following molecules involved in the citric acid cycle initiates the cycle by combining with acetyl-CoA?

Possible Answers:

Fumarate

Succinate

Malate

Citrate

Oxaloacetate

Correct answer:

Oxaloacetate

Explanation:

The first step in the citric acid cycle is for acetyl-CoA to react with oxaloacetate.  This forms citrate, which then continues through the cycle, ultimately reforming the oxaloacetate molecule to redo the cycle.

Example Question #231 : Catabolic Pathways And Metabolism

The pyruvate dehydrogenase complex creates acetyl-CoA from pyruvate. What other molecule is a product of this reaction?

Possible Answers:

Correct answer:

Explanation:

Pyruvate is converted to acetyl-CoA by the pyruvate dehydrogenase complex. Carbon dioxide is released during this reaction, and in addition to this,  is reduced to .

Example Question #232 : Catabolic Pathways And Metabolism

In what part of the cell does the pyruvate dehydrogenase complex function?

Possible Answers:

The mitochondrial matrix

Cytoplasm

The mitochondrial intermembrane

In the nucleus

Within the inner membrane of the mitochondria

Correct answer:

The mitochondrial matrix

Explanation:

The pyruvate dehydrogenase complex (PDC) is preparing pyruvate for the Krebs cycle by converting it to acetyl-CoA. Because the Krebs cycle functions within the mitochondrial matrix, the PDC is also taking place there.  This ensures quick and easy movement from the PDC into the Krebs cycle.

Example Question #233 : Catabolic Pathways And Metabolism

Which process involved in cellular respiration produces the largest quantity of high energy electron carriers?

Possible Answers:

Pyruvate dehydrogenase complex

ATP synthase

Glycolysis

Electron transport chain

Krebs cycle

Correct answer:

Krebs cycle

Explanation:

The Krebs cycle produces the most high energy electron carriers of any process involved in cellular respiration. Per glucose molecule, the Krebs cycle produces  and .

Example Question #41 : Citric Acid Cycle

Which of the following molecules enter the Krebs cycle directly, following glycolysis?

Possible Answers:

Acetyl-CoA

Pyruvate

Ubiquinone

Glucose

Pyruvate dehydrogenase

Correct answer:

Acetyl-CoA

Explanation:

Ubiquinone is a part of the electron transport chain, and has little to do with the Krebs cycle. Glucose is broken down during glycolysis, and does not enter the Krebs cycle directly. Many students make the mistake of thinking that pyruvate enters the Krebs cycle, since it is produced in glycolysis, and the Krebs cycle follows glycolysis. However, pyruvate is first converted to acetyl-CoA by the pyruvate dehydrogenase complex in the mitochondrial matrix, and acetyl-CoA enters the Krebs cycle.

Example Question #15 : Reactants And Products Of The Citric Acid Cycle

What is the role of isocitrate dehydrogenase in the citric acid cycle of the mitochondria?

Possible Answers:

Isocitrate dehydrogenase converts citrate to alpha-ketoglutarate and is inhibited by 

Isocitrate dehydrogenase converts alpha-ketoglutarate to isocitrate and is activated by 

Isocitrate dehydrogenase converts isocitrate to alpha-ketoglutarate and is inhibited by 

Isocitrate dehydrogenase converts citrate to isocitrate and is inhibited by 

Isocitrate dehydrogenase converts citrate to isocitrate and is activated by

Correct answer:

Isocitrate dehydrogenase converts isocitrate to alpha-ketoglutarate and is inhibited by 

Explanation:

Isocitrate dehydrogenase activation leads to oxidative decarboxylation of isocitrate in a two step process producing alpha-ketoglutarate and . In the mitochondria, the reaction produces also a charged electron carrier molecule, , from . Isocitrate dehydrogenase, inhibited by  and activated by , is a major regulator enzyme of the citric cycle.

Example Question #16 : Reactants And Products Of The Citric Acid Cycle

Which of the following steps within the citric acid cycle results in the production of a molecule of carbon dioxide ?

Possible Answers:

The conversion of malate to oxaloacetate

The conversion of succinyl-CoA to succinate

The conversion of isocitrate to alpha-ketoglutarate 

The conversion of succinate to fumarate

The conversion of fumarate to malate

Correct answer:

The conversion of isocitrate to alpha-ketoglutarate 

Explanation:

The only step of the citric acid cycle listed that results in the production of  as a side product is the conversion of isocitrate to alpha-ketoglutarate. In this step, the enzyme, isocitrate dehydrogenase catalyzes the conversion of isocitrate to alpha-ketoglutarate, while also converting  to  and  as side products, and generating a molecule of  in the process (i.e. reducing the carbon count from 5 in isocitrate to 4 in alpha-ketoglutarate). 

The conversion of alpha-ketoglutarate to succinyl-CoA also produces a molecule of  as a side product. However, this step is not listed as an answer choice.

None of the other answer choices listed produce  as side products. 

Example Question #241 : Catabolic Pathways And Metabolism

In which of the following cases would the citric acid cycle be downregulated?

Possible Answers:

Lower levels of NADH

High levels of ATP

High levels of ADP

Increased amounts of 

Correct answer:

High levels of ATP

Explanation:

The purpose of the citric acid cycle is to produce energy (both directly via GTP, and indirectly via NADH and . As such, energy can be though of to be on the products side of the sum of the reactions of the Krebs cycle. From Le Chatelier's principle, we know that if we want to inhibit a forward reaction, we can increase the concentration of the products. This will inhibit the forward reaction, and push the equilibrium to the left. Thus, in a high energy state, the ratio of ATP:ADP, like that of NADH: is high since both ATP and NADH are products of metabolism.

Example Question #242 : Catabolic Pathways And Metabolism

Why can't acetyl-CoA produced from beta-oxidation enter into the Krebs Cycle without carbohydrates present?

Possible Answers:

Acetyl-CoA can enter into the Krebs cycle without carbohydrates present

Acetyl-CoA combines with oxaloacetate to enter into the Krebs cycle which is a carbohydrate

Downstream of the Krebs cycle, the electron transport chain can not function without carbohydrates present, which in turn causes the Krebs cycle to stop functioning

None of these

Acetyl-CoA must first be activated by a carbohydrate before entering into the Krebs cycle

Correct answer:

Acetyl-CoA combines with oxaloacetate to enter into the Krebs cycle which is a carbohydrate

Explanation:

The entry point for acetyl CoA in the Krebs cycle is oxaloacetate. Acetyl-CoA and oxaloacetate combine to form citrate, which then continues through the cycle. Oxaloacetate is a carbohydrate, and so without carbs the acetyl-CoA can not enter into the Krebs cycle.

Example Question #243 : Catabolic Pathways And Metabolism

Which of the following is not a regulated step of the citric acid cycle?

Possible Answers:

Citrate synthase

Isocitrate dehydrogenase

Succinate dehydrogenase

None of these

Alpha-ketoglutarate dehydrogenase

Correct answer:

Succinate dehydrogenase

Explanation:

The regulated steps of the citric acid cycle are citrate synthase, isocitrate dehydrogenase, and alpha-ketoglutarate dehydrogenase. These steps are inhibited and stimulated by various products and reactants within the citric acid cycle. Succinate dehydrogenase is not regulated by products or reactants, and is therefore not rate limiting.

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