Biochemistry : Citric Acid Cycle Regulation

Study concepts, example questions & explanations for Biochemistry

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Example Questions

Example Question #1 : Citric Acid Cycle Regulation

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

Possible Answers:

High levels of ATP

Increased amounts of 

High levels of ADP

Lower levels of NADH

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 #2 : Citric Acid Cycle Regulation

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

Possible Answers:

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

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 can enter into the Krebs cycle without carbohydrates present

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

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 #3 : Citric Acid Cycle Regulation

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

Possible Answers:

Succinate dehydrogenase

None of these

Alpha-ketoglutarate dehydrogenase

Citrate synthase

Isocitrate 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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