The Citric Acid Cycle and Cellular Respiration
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The Citric Acid Cycle and Cellular Respiration

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Questions and Answers

What is the final electron acceptor in the cellular respiration process?

  • FADH2
  • O2 (correct)
  • NADH
  • CO2

    • What is the primary function of the PDH complex?

  • To facilitate the breakdown of fatty acids
  • To convert glucose into amino acids
  • To convert pyruvate into acetyl Co-A, yielding CO2 and NADH (correct)
  • To synthesize ATP from ADP

    • Which of the following is NOT a stage of cellular respiration?

  • Conversion of acetylCoA to CO2
  • Oxidation of electron carriers with O2
  • Glycolysis of glucose (correct)
  • Oxidation of organic fuels to acetylCoA

    • Which vitamin is involved in the formation of Coenzyme A?

    <p>Pantothenic acid (B5)</p> Signup and view all the answers

    What type of reaction does the pyruvate dehydrogenase complex catalyze?

    <p>Oxidative decarboxylation</p> Signup and view all the answers

    Where does cellular respiration primarily occur in eukaryotic cells?

    <p>Mitochondria</p> Signup and view all the answers

    Which cofactor is key for redox reactions in the PDH complex?

    <p>Lipoic acid</p> Signup and view all the answers

    What effect does phosphorylation have on PDH complex activity?

    <p>Decreases its activity</p> Signup and view all the answers

    Which of the following statements is true regarding the conversion of acetylCoA to CO2?

    <p>It involves a series of enzymatic reactions.</p> Signup and view all the answers

    What condition results from inherited decreased activity of the PDH complex?

    <p>Chronic Lactic Acidosis (CLA)</p> Signup and view all the answers

    Which compound serves as the ultimate electron acceptor in vivo?

    <p>Oxygen</p> Signup and view all the answers

    What characterizes the citric acid cycle as amphibolic?

    <p>It simultaneously involves both anabolic and catabolic processes.</p> Signup and view all the answers

    Which vitamins are essential for energy metabolism?

    <p>Thiamine, Riboflavin, Niacin, Pantothenic acid</p> Signup and view all the answers

    Which statement best describes anaplerotic reactions?

    <p>They replenish intermediates in metabolic pathways.</p> Signup and view all the answers

    Which of the following diseases is associated with a deficiency of Niacin (Vitamin B3)?

    <p>Pellagra</p> Signup and view all the answers

    What is the role of thiamine pyrophosphate (TPP) in the decarboxylation of pyruvate?

    <p>It serves as a coenzyme transporting the product acetaldehyde.</p> Signup and view all the answers

    Which enzyme is responsible for the decarboxylation step of pyruvate to acetaldehyde?

    <p>Pyruvate dehydrogenase (E1)</p> Signup and view all the answers

    What is the structure of the PDH complex primarily composed of?

    <p>Supramolecular enzymatic complex with multiple enzymes</p> Signup and view all the answers

    What happens to the acetaldehyde after it is formed in the PDH complex?

    <p>It undergoes an internal redox reaction on the lipoyl moiety of E2.</p> Signup and view all the answers

    What molecular modification occurs to acetyl CoA during the reaction sequence?

    <p>It is released free to the reaction environment.</p> Signup and view all the answers

    Which component is directly involved in the oxidation of activated acetaldehyde?

    <p>Lipoyl moiety of E2</p> Signup and view all the answers

    What is the significance of the swinging lipoyl arm of E2 in the PDH complex?

    <p>It allows E2 to interact with E1 and E3 during the process.</p> Signup and view all the answers

    What is used as an 'electron sink' during the decarboxylation of pyruvate?

    <p>Hydroxyethyl TPP</p> Signup and view all the answers

    Which type of reaction is involved when acetaldehyde is transferred to the lipoyl moiety?

    <p>Redox reaction</p> Signup and view all the answers

    What is the primary function of the PDH complex in cellular metabolism?

    <p>To catalyze the conversion of pyruvate into acetyl CoA.</p> Signup and view all the answers

    What occurs during the reoxidation of the reduced lipoyl group?

    <p>Dihydrolipoyl dehydrogenase converts FADH2 to NADH</p> Signup and view all the answers

    What is the role of CoA-SH in the citric acid cycle?

    <p>To recycle back after oxidative decarboxylation</p> Signup and view all the answers

    Which compound is formed when acetyl-CoA donates its acetyl group to oxaloacetate?

    <p>Citrate</p> Signup and view all the answers

    How many ATP are produced for each NADH generated in the citric acid cycle?

    <p>3 ATP</p> Signup and view all the answers

    What type of reactions are considered anaplerotic?

    <p>Reactions that fill up citric acid cycle intermediates</p> Signup and view all the answers

    What is the end product when fumarate is hydrated?

    <p>Malate</p> Signup and view all the answers

    Which of the following statements about citrate is true?

    <p>It is converted to isocitrate by aconitase.</p> Signup and view all the answers

    What is the fate of succinate in the citric acid cycle?

    <p>It is oxidized to fumarate.</p> Signup and view all the answers

    Which component is not an intermediate of the citric acid cycle?

    <p>Pyruvate</p> Signup and view all the answers

    What does the term amphibolic refer to in the context of the citric acid cycle?

    <p>Reactions that are both anabolic and catabolic</p> Signup and view all the answers

    Which intermediate is produced during the oxidative decarboxylation of isocitrate?

    <p>α-ketoglutarate</p> Signup and view all the answers

    What is the overall reaction for the citric acid cycle?

    <p>CH3CO-S-CoA + GDP + P + H2O + 3 NAD+ + FAD -&gt; 2 CO2 + HS-CoA + GTP + 3 NADH + FADH2</p> Signup and view all the answers

    What is the final acceptor of electrons harvested from pyruvate?

    <p>NAD+</p> Signup and view all the answers

    Which component directly transfers energy stored in succinyl-CoA?

    <p>GTP</p> Signup and view all the answers

    Study Notes

    The Citric Acid Cycle

    • A cyclical metabolic pathway that plays a crucial role in cellular respiration
    • It's a key process for energy production and an important intermediary in various metabolic pathways
    • The citric acid cycle is a central metabolic hub, converting acetyl CoA to CO2 with concomitant electron transfer
    • Converging catabolism generates intermediates for energy or biosynthesis
    • Diverging anabolism provides the starting materials for biosynthesis

    Cellular Respiration and Energy Production

    • Aerobic catabolism involving three key stages
    • Stage 1: Organic fuels (e.g., glucose) are oxidized to acetyl CoA
    • Stage 2: Acetyl CoA is oxidized to CO2 and reduces electron carriers (NADH and FADH2)
    • Stage 3: Electron carriers are oxidized by O2, producing ATP through oxidative phosphorylation

    Cellular Respiration in Mitochondria

    • Mitochondria are the sites of cellular respiration
    • The outer membrane is freely permeable to small molecules
    • The inner membrane is impermeable and contains respiratory electron carriers, ATP synthase, and other transporters
    • The matrix contains the enzymes for pyruvate oxidation, the citric acid cycle, and fatty acid oxidation

    Oxidation of Pyruvate to Acetyl CoA

    • Catalyzed by the pyruvate dehydrogenase complex (PDH)
    • An irreversible oxidative decarboxylation process
    • Converts pyruvate (3 carbons) to acetyl CoA (2 carbons) and releases CO2
    • The complex includes three enzymes and five cofactors (TPP, lipoic acid, NAD+, FAD, and CoA)

    Coenzyme A and Acetyl CoA

    • Coenzyme A (CoA) is a crucial component of acetyl CoA, a key intermediate in metabolism
    • Acetyl CoA has a higher energy content than ATP

    Lipoic Acid

    • Lipoic acid is a key cofactor in the PDH complex
    • It undergoes reversible redox reactions, facilitating the transfer of acetyl groups

    Oxidation of Pyruvate to Acetyl CoA (Further Details)

    • A five-step process catalyzed by the PDH complex
    • Involves the participation of enzymes E1, E2, and E3, and several cofactors
    • The PDH complex can be regulated by phosphorylation/dephosphorylation

    Oxidation of Pyruvate to Acetyl CoA (Complex Structure)

    • The PDH complex is a supramolecular complex with three distinct enzymes (E1, E2, and E3) arranged to facilitate efficient catalysis
    • E2 includes a swinging lipoyl arm that transfers intermediates between the active sites of E1 and E3. The enzymes are mechanically coupled.

    Step 1: Decarboxylation of Pyruvate

    • Thiamine pyrophosphate (TPP) plays a crucial role in this decarboxylation step

    Step 2: Oxidation of Activated Acetaldehyde to Acetate

    • The acetaldehyde on TPP is transferred to the lipoyl group of the E2 enzyme
    • Acetaldehyde is oxidized, and CoA is attached, forming acetyl CoA

    Steps 3, 4: Reoxidation of Reduced Lipoyl Group; Electron Harvesting

    • The reduced lipoyl group (from the E2 enzyme) is reoxidized by E3, generating reduced electron carriers (FADH2)
    • FADH2 is then reoxidized, generating NADH (a key electron carrier in cellular respiration)

    Citric Acid Cycle: Steps

    • A series of enzyme-catalyzed reactions completing the oxidation of acetyl CoA
    • Intermediates are critical in anabolic pathways and maintaining steady state

    Citric Acid Cycle: Steps (Further Details)

    • The cycle involves the generation of ATP, NADH, and FADH2 and forms a cycle
    • Citrate is isomerized to isocitrate
    • Isocitrate is oxidatively decarboxylated to α-ketoglutarate
    • a-ketoglutarate is oxidatively decarboxylated to succinyl CoA
    • Succinyl CoA is converted to succinate with GTP production
    • Succinate is oxidized to fumarate
    • Fumarate is hydrated to malate
    • Malate is oxidized to oxaloacetate

    Citric Acid Cycle: Summary

    • The citric acid cycle is a central metabolic pathway, crucial for energy production and the formation of biosynthetic precursors.
    • In this cycle, two-carbon acetyl CoA molecules are completely oxidized to CO2, generating reduced electron carriers (NADH and FADH2) and GTP (which can be converted to ATP).
    • The cycle is also significant for the generation of intermediates used in other metabolic pathways.

    Citric Acid Cycle: Regulation

    • The regulation of the citric acid cycle occurs at the level of the various highly exothermic steps

    Fate of Citrate

    • Citrate has metabolic fates beyond energy generation.
    • Citrate can be exported to the cytoplasm, providing reducing equivalents for biosynthetic processes.

    Sources and Fate of Succinate

    • Succinate arises from the citric acid cycle's operations and plays a role in various metabolic pathways,including heme synthesis.

    Goals and Objectives

    • Topics covered include cellular respiration, mitochondrial structure, pyruvate oxidation, the citric acid cycle, and metabolic regulation, including anaplerotic reactions.

    Drugs and Diseases

    • Several vitamins (e.g., thiamine, riboflavin, niacin) and lipoic acid are critical for the reactions in the citric acid cycle. Deficiencies can lead to diseases like Beri-Beri.

    Anaplerotic Reactions

    • These reactions replenish the citric acid cycle's intermediates, ensuring constant intermediate concentrations

    Biological Tethers

    • Lipoic acid, biotin, and CoA act to shuttle groups between enzymes during metabolic pathways, allowing for transfer of intermediates.

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    Description

    Explore the intricate processes of the citric acid cycle and cellular respiration. This quiz covers key stages of aerobic catabolism, mitochondrial function, and energy production mechanisms. Test your knowledge on how these metabolic pathways interconnect to sustain life.

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