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Questions and Answers
What is the energy change associated with the malate dehydrogenase reaction?
What is the energy change associated with the malate dehydrogenase reaction?
Which compound inhibits citrate synthase during the control of the Citric Acid Cycle?
Which compound inhibits citrate synthase during the control of the Citric Acid Cycle?
What type of reaction is the Citric Acid Cycle, based on its overall free energy change?
What type of reaction is the Citric Acid Cycle, based on its overall free energy change?
Which of the following is NOT an activator of isocitrate dehydrogenase?
Which of the following is NOT an activator of isocitrate dehydrogenase?
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How many ATP are generated from each pair of electrons from FADH2 in the electron-transport chain?
How many ATP are generated from each pair of electrons from FADH2 in the electron-transport chain?
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What role does the citrate synthase enzyme play in the citric acid cycle?
What role does the citrate synthase enzyme play in the citric acid cycle?
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Which of the following is a characteristic of the citrate synthase reaction?
Which of the following is a characteristic of the citrate synthase reaction?
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What effect do NADH, ATP, and succinyl-CoA have on citrate synthase?
What effect do NADH, ATP, and succinyl-CoA have on citrate synthase?
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Which aspect of the citric acid cycle is highlighted by its amphbolic nature?
Which aspect of the citric acid cycle is highlighted by its amphbolic nature?
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What is the primary fate of the carbon atoms in Acetyl-CoA during the TCA cycle?
What is the primary fate of the carbon atoms in Acetyl-CoA during the TCA cycle?
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Which of the following accurately describes an intermediate formed in the citrate synthase reaction?
Which of the following accurately describes an intermediate formed in the citrate synthase reaction?
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How does the structure of citrate synthase contribute to its function?
How does the structure of citrate synthase contribute to its function?
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Which of these is NOT a characteristic of the TCA cycle?
Which of these is NOT a characteristic of the TCA cycle?
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What is the metabolic consequence of the phosphorylation of acetyl CoA acetyltransferase in cancer cells?
What is the metabolic consequence of the phosphorylation of acetyl CoA acetyltransferase in cancer cells?
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Which nutrient deficiency is linked to beriberi and affects pyruvate metabolism?
Which nutrient deficiency is linked to beriberi and affects pyruvate metabolism?
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How does arsenite affect the pyruvate dehydrogenase complex?
How does arsenite affect the pyruvate dehydrogenase complex?
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What role does 2,3-Dimercaptopropanol play in arsenite poisoning?
What role does 2,3-Dimercaptopropanol play in arsenite poisoning?
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What is the likely impact of isocitrate dehydrogenase mutations on DNA?
What is the likely impact of isocitrate dehydrogenase mutations on DNA?
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What condition may arise from the disruption of the pyruvate dehydrogenase complex related to diabetes?
What condition may arise from the disruption of the pyruvate dehydrogenase complex related to diabetes?
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Which of the following statements about pyruvate dehydrogenase is incorrect?
Which of the following statements about pyruvate dehydrogenase is incorrect?
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What is the primary energy source that certain cancers switch to due to enzymatic changes?
What is the primary energy source that certain cancers switch to due to enzymatic changes?
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Which statement best describes the role of flavin coenzymes in biological systems?
Which statement best describes the role of flavin coenzymes in biological systems?
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What primary functions does Coenzyme A (CoA) serve in biochemical reactions?
What primary functions does Coenzyme A (CoA) serve in biochemical reactions?
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What is the significance of the sulfhydryl group in Coenzyme A?
What is the significance of the sulfhydryl group in Coenzyme A?
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How does lipoic acid contribute to metabolic processes?
How does lipoic acid contribute to metabolic processes?
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In which enzymes is lipoic acid commonly found?
In which enzymes is lipoic acid commonly found?
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What is the primary function of thiamine pyrophosphate (TPP) in metabolic reactions?
What is the primary function of thiamine pyrophosphate (TPP) in metabolic reactions?
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Which enzymes surround the core of the Pyruvate Dehydrogenase complex?
Which enzymes surround the core of the Pyruvate Dehydrogenase complex?
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What is the role of pyruvate dehydrogenase kinase (PDK) in the regulation of PDH?
What is the role of pyruvate dehydrogenase kinase (PDK) in the regulation of PDH?
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Which of the following statements best describes the interaction of transacetylase domains?
Which of the following statements best describes the interaction of transacetylase domains?
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What type of reactions do NAD+/NADH and NADP+/NADPH primarily facilitate?
What type of reactions do NAD+/NADH and NADP+/NADPH primarily facilitate?
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How many lipoamide arms are represented in the structure of the Pyruvate Dehydrogenase complex?
How many lipoamide arms are represented in the structure of the Pyruvate Dehydrogenase complex?
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Which domain interacts with the E3 enzyme in the structure of transacetylase?
Which domain interacts with the E3 enzyme in the structure of transacetylase?
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Which enzyme is responsible for the dephosphorylation of PDH?
Which enzyme is responsible for the dephosphorylation of PDH?
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What is the primary function of acetyl CoA in the citric acid cycle?
What is the primary function of acetyl CoA in the citric acid cycle?
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Which enzyme complex is primarily responsible for the oxidative decarboxylation of pyruvate?
Which enzyme complex is primarily responsible for the oxidative decarboxylation of pyruvate?
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Which of the following components is NOT part of the pyruvate dehydrogenase complex?
Which of the following components is NOT part of the pyruvate dehydrogenase complex?
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What role do the regulatory enzymes PDK and PDP play in the pyruvate dehydrogenase complex?
What role do the regulatory enzymes PDK and PDP play in the pyruvate dehydrogenase complex?
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In the first step of oxidative decarboxylation, which molecule is directly modified?
In the first step of oxidative decarboxylation, which molecule is directly modified?
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How many coenzymes are involved in the pyruvate dehydrogenase complex's functioning?
How many coenzymes are involved in the pyruvate dehydrogenase complex's functioning?
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What occurs during the decarboxylation step catalyzed by E1 in the pyruvate dehydrogenase complex?
What occurs during the decarboxylation step catalyzed by E1 in the pyruvate dehydrogenase complex?
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Where does the citric acid cycle take place within the cell?
Where does the citric acid cycle take place within the cell?
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Study Notes
The Citric Acid Cycle
- The citric acid cycle, also known as the tricarboxylic acid cycle (TCA) or Krebs cycle, is a central metabolic pathway in aerobic respiration.
- It processes the two-carbon acetyl unit of acetyl CoA to two molecules of CO2, while also harvesting high-energy electrons for ATP production.
- The cycle takes place within the mitochondrial matrix.
Oxidative Decarboxylation
- This is the first part of the citric acid cycle.
- It involves three steps: decarboxylation, oxidation, and transfer to CoA.
- It is catalyzed by the pyruvate dehydrogenase complex (PDH).
- The complex has three enzymes (E1, E2, E3) and five coenzymes, including TPP, CoA, lipoamide, FAD, and NAD+.
- The PDH regulatory enzymes regulate the activity of the complex.
- Pyruvate+CoA+NAD+→Acetyl CoA+CO2+NADH+H+
Citric Acid Cycle Steps
- Acetyl-CoA combines with oxaloacetate to form citrate.
- Citrate is isomerized to isocitrate.
- Isocitrate is oxidized and decarboxylated to form α-ketoglutarate, releasing CO2 and producing NADH.
- α-ketoglutarate is oxidized and decarboxylated to succinyl CoA, releasing CO2 and producing NADH.
- Succinyl CoA is converted to succinate, producing GTP or ATP.
- Succinate is oxidized to fumarate, producing FADH2.
- Fumarate is hydrated to malate.
- Malate is oxidized to oxaloacetate, producing NADH.
Regulation of Citric Acid Cycle
- The cycle is regulated by controlling the activity of its enzymes.
- Citrate synthase activity is inhibited by ATP, NADH, and succinyl CoA.
- Isocitrate dehydrogenase is activated by ADP and NAD+ and inhibited by ATP and NADH.
- α-ketoglutarate dehydrogenase complex is inhibited by ATP, NADH, and succinyl CoA and activated by ADP and NAD+.
- Pyruvate dehydrogenase is inhibited by ATP and NADH, and also by product inhibition by acetyl-CoA
Coenzymes of the Citric Acid Cycle
- TPP (thiamine pyrophosphate): Important for decarboxylation reactions.
- Coenzyme A (CoA): Important for acyl group transfer.
- Lipoic acid: Acts as a swinging arm in the pyruvate dehydrogenase complex.
- FAD (flavin adenine dinucleotide): Participates in redox reactions, accepting electrons.
- NAD+ (nicotinamide adenine dinucleotide): Participates in redox reactions, accepting electrons.
Anaplerotic Reactions
- These reactions replenish the intermediates of the citric acid cycle.
- The most important anaplerotic reaction is the conversion of pyruvate to oxaloacetate by pyruvate carboxylase.
- Other anaplerotic reactions include PEP carboxylase and malic enzyme.
- replenishing TCA cycle intermediates.
The Carbon Atoms of Acetyl-CoA
- In the second turn of the cycle, the carboxyl C from acetyl CoA becomes CO2.
- The methyl C in acetyl CoA survives two full cycles.
- Half of the remaining methyl C exits the cycle in each turn as acetate after the previous turn.
Diseases associated with the citric acid cycle
- The deficiency of pyruvate dehydrogenase phosphatase can lead to an accumulation of lactate and affect different systems.
- Defects in succinate dehydrogenase, fumarase, pyruvate dehydrogenase kinase, and isocitrate dehydrogenase contribute to cancer growth.
- Mutations in isocitrate dehydrogenase produce 2-hydroxyglutarate, altering gene expression and promoting cell growth.
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Description
Explore the essential steps and significance of the citric acid cycle, also known as the Krebs cycle. This quiz covers the process of oxidative decarboxylation and the reactions involved in generating energy through aerobic respiration. Test your knowledge on the enzymes and coenzymes crucial to this metabolic pathway.
