Citric Acid Cycle Overview and Stages

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Questions and Answers

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.

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?

1.) Oxidation of acetyl CoA 2.) The regeneration of oxaloacetate

Match each enzyme with its description.

Pyruvate dehydrogenase complex = Converts pyruvate into acetyl CoA Citrate synthase = Condenses oxaloacetate and acetyl CoA 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 Succinate dehydrogenase = Synthesizes fumarate Fumarase = Generates malate Malate dehydrogenase = Catalyzes the formation of oxaloacetate Pyruvate carboxylase = Converts pyruvate into oxaloacetate Signup and view all the answers

What is the energy source that drives the formation of citrate?

The hydrolysis of a thioester from acetyl CoA. Signup and view all the answers

How is the wasteful hydrolysis of acetyl CoA prevented by citrate synthase?

Citrate synthase does not allow acetyl CoA to bind until oxaloacetate is bound, ensuring that hydrolysis does not occur prematurely. Signup and view all the answers

Explain how the citric acid cycle can be thought of as essentially a supramolecular enzyme.

Enzymes or enzyme complexes facilitate reactions without being consumed, and oxaloacetate acts as a catalyst that is regenerated at the end of the cycle. Signup and view all the answers

How will the concentrations of citric acid cycle intermediates change immediately after the addition of malonate?

Succinate will increase in concentration, followed by alpha-ketoglutarate and the other intermediates upstream of the site of inhibition. Signup and view all the answers

Why is malonate not a substrate for succinate dehydrogenase?

Malonate has only one methylene group required for dehydrogenation, while succinate has two. Signup and view all the answers

How is succinate dehydrogenase unique compared with other enzymes in the citric acid cycle?

Succinate dehydrogenase is embedded in the mitochondrial membrane and is associated with the electron transport chain. Signup and view all the answers

What reaction in the citric acid cycle results in the direct formation of a molecule of ATP?

The reaction catalyzed by succinyl CoA synthetase. Signup and view all the answers

What step in the citric acid cycle requires a molecule of inorganic phosphate?

Succinyl CoA synthetase. Signup and view all the answers

Which enzymes are the key regulatory enzymes of the citric acid cycle?

Isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase. Signup and view all the answers

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.