Citric Acid Cycle Flashcards

Questions and Answers

What is the summary of the Citric Acid Cycle?

The citric acid cycle oxidizes the acetyl fragment of acetyl CoA to CO2, capturing high-energy electrons in the form of NADH and FADH2, to harvest high-energy electrons from carbon fuels.

What happens in the first stage of the citric acid cycle?

Two carbons are introduced by condensation of an acetyl group with oxaloacetate, forming citrate, which undergoes two oxidative decarboxylations to generate two molecules of CO2.

What are the two stages of the citric acid cycle?

Stage 1: Oxidative decarboxylations; Stage 2: Oxaloacetate regeneration.

Where does the citric acid cycle occur in the cell?

In the mitochondria. Signup and view all the answers

What is the overview of the CAC?

The citric acid cycle oxidizes two-carbon units, producing two molecules of CO2, one molecule of ATP, and high-energy electrons in the form of NADH and FADH2. Signup and view all the answers

What does citrate synthase do in Stage 1?

Citrate synthase catalyzes the condensation of acetyl CoA and oxaloacetate to form citrate. Signup and view all the answers

What occurs in the mechanism of citrate synthase?

Oxaloacetate binding induces structural changes forming an acetyl CoA binding site, resulting in the formation of citrate. Signup and view all the answers

What is the role of aconitase in Stage 1?

Aconitase catalyzes the formation of isocitrate from citrate. Signup and view all the answers

What does isocitrate dehydrogenase catalyze?

The oxidative decarboxylation of isocitrate, forming α-ketoglutarate and capturing high-energy electrons as NADH. Signup and view all the answers

What does α-ketoglutarate dehydrogenase complex produce?

It catalyzes the synthesis of succinyl CoA from α-ketoglutarate, generating NADH. Signup and view all the answers

What does succinyl CoA synthetase do?

It catalyzes the cleavage of a thioester linkage and concomitantly forms ATP. Signup and view all the answers

What is necessary for generating oxaloacetate in Stage 2?

Succinate dehydrogenase, fumarase, and malate dehydrogenase catalyze successive reactions. Signup and view all the answers

What is the net reaction of the CAC?

2 ATP from glucose. Signup and view all the answers

What regulates the citric acid cycle?

The concentrations of ATP and NADH, with key control points at isocitrate dehydrogenase and α-ketoglutarate dehydrogenase. Signup and view all the answers

What are the biosynthetic roles of the citric acid cycle?

Intermediates are drawn off for biosynthesis when energy needs are met, and replenished from pyruvate. Signup and view all the answers

What replenishes the CAC?

Pyruvate carboxylase replenishes the citric acid cycle. Signup and view all the answers

Study Notes

Overview of the Citric Acid Cycle

The citric acid cycle oxidizes the acetyl fragment of acetyl CoA to CO2, capturing high-energy electrons in the form of NADH and FADH2.
The cycle's function is to harvest high-energy electrons from carbon fuels.

Stages of the Citric Acid Cycle

Stage 1: Two carbons are introduced into the cycle by condensation of an acetyl group with oxaloacetate, forming citrate, which undergoes two oxidative decarboxylations, generating two CO2 molecules.
Stage 2: Oxaloacetate is regenerated, generating high-energy electrons used to power ATP synthesis in oxidative phosphorylation.

Location of the Citric Acid Cycle

The citric acid cycle occurs in the mitochondria.

Key Enzymes and Reactions

Citrate synthase: catalyzes the condensation of acetyl CoA and oxaloacetate to form citrate.
Aconitase: catalyzes the formation of isocitrate from citrate.
Isocitrate dehydrogenase: catalyzes the oxidative decarboxylation of isocitrate, forming α-ketoglutarate and capturing high-energy electrons as NADH.
α-Ketoglutarate dehydrogenase complex: catalyzes the synthesis of succinyl CoA from α-ketoglutarate, generating NADH.
Succinyl CoA synthetase: catalyzes the cleavage of a thioester linkage and concomitantly forms ATP.
Succinate dehydrogenase, fumarase, and malate dehydrogenase: catalyze successive reactions to regenerate oxaloacetate, generating FADH2 and NADH.

Regulation and Control

The citric acid cycle is regulated primarily by the concentrations of ATP and NADH.
Key control points are the enzymes isocitrate dehydrogenase and α-ketoglutarate dehydrogenase.

Biosynthetic Roles

Intermediates are drawn off for biosyntheses when the energy needs of the cell are met.
Intermediates are replenished by the formation of oxaloacetate from pyruvate.

Cellular Respiration

The citric acid cycle constitutes the first stage in cellular respiration, removing high-energy electrons from carbon fuels.
These electrons reduce O2 to generate a proton gradient, which is used to synthesize ATP in oxidative phosphorylation.

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Description

This quiz features flashcards summarizing the Citric Acid Cycle and its two stages. It is designed to help students understand the processes of oxidation and energy capture that occur during the cycle. Ideal for biology students looking to reinforce their knowledge of cellular respiration.