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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