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Questions and Answers
What is the net equation of the citric acid cycle?
What is the net equation of the citric acid cycle?
Acetyl-CoA + 3NAD+ + FAD + GDP + Pi + 2 H2O -> 2CO2 + 3NADH + FADH2 + GTP + CoA + 3H+
Explain the paradox of the citric acid cycle being part of aerobic respiration while not requiring O2.
Explain the paradox of the citric acid cycle being part of aerobic respiration while not requiring O2.
The citric acid cycle depends on a steady supply of NAD+ and FAD, which are generated from NADH and FADH2 by reaction with oxygen. Without oxygen to accept the electrons, the cycle will cease to operate.
What are the two stages of the citric acid cycle?
What are the two stages of the citric acid cycle?
1.) Oxidation of acetyl CoA 2.) The regeneration of oxaloacetate
Match each enzyme with its description.
Match each enzyme with its description.
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What is the energy source that drives the formation of citrate?
What is the energy source that drives the formation of citrate?
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How is the wasteful hydrolysis of acetyl CoA prevented by citrate synthase?
How is the wasteful hydrolysis of acetyl CoA prevented by citrate synthase?
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Explain how the citric acid cycle can be thought of as essentially a supramolecular enzyme.
Explain how the citric acid cycle can be thought of as essentially a supramolecular enzyme.
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How will the concentrations of citric acid cycle intermediates change immediately after the addition of malonate?
How will the concentrations of citric acid cycle intermediates change immediately after the addition of malonate?
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Why is malonate not a substrate for succinate dehydrogenase?
Why is malonate not a substrate for succinate dehydrogenase?
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How is succinate dehydrogenase unique compared with other enzymes in the citric acid cycle?
How is succinate dehydrogenase unique compared with other enzymes in the citric acid cycle?
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What reaction in the citric acid cycle results in the direct formation of a molecule of ATP?
What reaction in the citric acid cycle results in the direct formation of a molecule of ATP?
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What step in the citric acid cycle requires a molecule of inorganic phosphate?
What step in the citric acid cycle requires a molecule of inorganic phosphate?
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Which enzymes are the key regulatory enzymes of the citric acid cycle?
Which enzymes are the key regulatory enzymes of the citric acid cycle?
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Study Notes
Citric Acid Cycle Overview
- The net equation: Acetyl-CoA + 3NAD+ + FAD + GDP + Pi + 2 H2O -> 2CO2 + 3NADH + FADH2 + GTP + CoA + 3H+.
- Operates as part of aerobic respiration, relying on NAD+ and FAD generated from electron carriers and oxygen interaction.
Stages of the Citric Acid Cycle
- Consists of two stages:
- Oxidation of acetyl CoA.
- Regeneration of oxaloacetate.
Enzyme Functions
- Pyruvate dehydrogenase complex: Converts pyruvate into acetyl CoA.
- Citrate synthase: Condenses oxaloacetate and acetyl CoA to form citrate.
- Aconitase: Catalyzes the formation of isocitrate.
- Isocitrate dehydrogenase: Catalyzes the formation of alpha-ketoglutarate.
- Alpha-ketoglutarate dehydrogenase: Synthesizes succinyl CoA.
- Succinyl CoA synthetase: Generates ATP by converting succinyl CoA, Pi, and ADP into succinate, CoA, and ATP.
- Succinate dehydrogenase: Converts succinate into fumarate.
- Fumarase: Generates malate from fumarate.
- Malate dehydrogenase: Catalyzes the formation of oxaloacetate.
- Pyruvate carboxylase: Converts pyruvate into oxaloacetate.
Citrate Formation
- Formation of citrate from acetyl CoA and oxaloacetate is driven by the hydrolysis of a thioester, specifically from citryl CoA.
Regulation of Hydrolysis
- Citrate synthase prevents wasteful hydrolysis of acetyl CoA by:
- Binding oxaloacetate first, which enables proper condensation with acetyl CoA.
- Positioning catalytic residues to facilitate hydrolysis only after citryl CoA is formed.
Citrate Cycle as a Catalyst
- Oxaloacetate acts as a catalyst throughout the cycle by binding acetyl groups and undergoing regeneration, facilitating oxidative decarboxylation.
Effects of Malonate
- Malonate, a competitive inhibitor of succinate dehydrogenase, leads to:
- Increased concentrations of succinate and upstream intermediates, such as α-ketoglutarate.
- It is not a substrate for dehydrogenase because it lacks the necessary methylene groups present in succinate.
Unique Properties of Succinate Dehydrogenase
- Distinct from other citric acid cycle enzymes, succinate dehydrogenase is integrated into the mitochondrial membrane and associated with the electron-transport chain.
ATP Generation
- The reaction that directly generates ATP occurs through the action of succinyl CoA synthetase:
- Succinyl CoA + Pi + ADP → Succinate + CoA + ATP.
Inorganic Phosphate Requirement
- The only step in the citric acid cycle requiring inorganic phosphate is catalyzed by succinyl CoA synthetase.
Key Regulatory Enzymes
- The main regulatory enzymes in the citric acid cycle are isocitrate dehydrogenase and α-ketoglutarate dehydrogenase, controlling the cycle's rate and efficiency.
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Description
Explore the intricate processes of the Citric Acid Cycle, vital for aerobic respiration. This quiz covers the overall net equation, the stages of oxidation and regeneration, and the specific enzyme functions involved. Test your knowledge on how these elements contribute to cellular energy production.
