Citric Acid Cycle Reactions

Questions and Answers

What is the primary role of anaplerotic reactions in the citric acid cycle?

To regulate the flow of carbon atoms into the citric acid cycle

To catalyze the conversion of pyruvate to acetyl-CoA

To generate ATP during oxidative phosphorylation

To replenish intermediates that are siphoned off into other pathways (correct)

What is the primary function of the citric acid cycle in aerobic organisms?

To synthesize fatty acids and amino acids

To catalyze the breakdown of carbohydrates

To serve in both catabolic and anabolic processes (correct)

To generate ATP during oxidative phosphorylation

Which of the following reactions is NOT a site of regulation in the citric acid cycle?

Pyruvate dehydrogenase complex reaction

Citrate synthase reaction

Glycolysis (correct)

Isocitrate dehydrogenase reaction

What is the starting material for the citric acid cycle?

Acetyl-CoA Signup and view all the answers

What is the purpose of allosteric and covalent modification in the regulation of the citric acid cycle?

To ensure the production of intermediates at the rates required to keep the cell in a stable steady state Signup and view all the answers

Which of the following is an example of an anabolic process in the citric acid cycle?

The synthesis of cholesterol Signup and view all the answers

What is the result of the dynamic balance between anaplerotic reactions and the removal of intermediates for biosynthetic pathways?

The maintenance of almost constant concentrations of citric acid cycle intermediates Signup and view all the answers

What is the purpose of the citric acid cycle in the oxidation of fatty acids?

To catalyze the breakdown of fatty acids Signup and view all the answers

What is the role of the pyruvate dehydrogenase complex reaction in the regulation of the citric acid cycle?

To regulate the production of acetyl-CoA from pyruvate Signup and view all the answers

What is the significance of the citric acid cycle in aerobic organisms?

It plays a central role in both energy-yielding and biosynthetic pathways Signup and view all the answers

Study Notes

Reactions of the Citric Acid Cycle

The citric acid cycle is a cyclic pathway that takes place in the mitochondria and has eight steps.
Mitochondria contain all the enzymes and proteins necessary for the last stage of respiration, electron transfer, and ATP synthesis by oxidative phosphorylation.
The citric acid cycle is the site of most energy-yielding oxidative reactions and of the coupled synthesis of ATP in nonphotosynthetic eukaryotes.
In photosynthetic eukaryotes, mitochondria are the major site of ATP production in the dark, but in daylight, chloroplasts produce most of the organism's ATP.
In most bacteria, the enzymes of the citric acid cycle are in the cytosol, and the plasma membrane plays a role analogous to that of the inner mitochondrial membrane in ATP synthesis.

Steps of the Citric Acid Cycle

The cycle begins with acetyl-CoA donating its acetyl group to oxaloacetate to form citrate.
Citrate is transformed into isocitrate, which is dehydrogenated with loss of CO2 to yield a-ketoglutarate.
a-Ketoglutarate undergoes loss of a second molecule of CO2 and ultimately yields succinate.
Succinate is enzymatically converted into oxaloacetate, which is then ready to react with another molecule of acetyl-CoA.

Regulation of Glycolysis and Gluconeogenesis

Three reactions of glycolysis are so exergonic as to be essentially irreversible: those catalyzed by hexokinase, PFK-1, and pyruvate kinase.
Gluconeogenesis uses detours around each of these irreversible steps, with bypass reactions that also have a large, negative ∆G’.
Simultaneous operation of both pathways would consume ATP without accomplishing any chemical or biological work, leading to a futile cycle.

Hexokinase Isozymes

Hexokinase is a regulatory enzyme that catalyzes the entry of glucose into the glycolytic pathway.
Humans have four isozymes of hexokinase, encoded by four different genes, which are affected differently by their product, glucose-6-phosphate.

Citric Acid Cycle as an Amphibolic Pathway

The citric acid cycle is an amphibolic pathway, serving in both catabolic and anabolic processes.
The cycle provides precursors for many biosynthetic pathways.
Intermediates of the citric acid cycle are removed to serve as biosynthetic precursors and are replenished by anaplerotic reactions.

Regulation of the Citric Acid Cycle

The regulation of key enzymes in the citric acid cycle ensures the production of intermediates at the rates required to keep the cell in a stable steady state.
The flow of carbon atoms from pyruvate into and through the citric acid cycle is under tight regulation at two levels: the conversion of pyruvate to acetyl-CoA and the entry of acetyl-CoA into the cycle.
Acetyl-CoA is also produced by pathways other than the PDH complex reaction, and the availability of intermediates from these other pathways is important in the regulation of pyruvate oxidation and of the citric acid cycle.

Description

Understanding the citric acid cycle, a crucial process in cellular respiration, and its eight steps that take place in the mitochondria. Learn about the enzymes and proteins involved in electron transfer, ATP synthesis, and fatty acid oxidation.