Biochemistry: Citric Acid Cycle Quiz
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Questions and Answers

What is the primary role of the Citric Acid Cycle in cellular metabolism?

  • Breaking down proteins into amino acids.
  • Oxidation of acetyl-CoA to carbon dioxide and saving the released energy as NADH, FADH2, and GTP. (correct)
  • Synthesizing glucose from carbon dioxide.
  • Directly consuming oxygen to produce ATP.

    • Why does the Citric Acid Cycle not occur anaerobically, even though oxygen is not directly involved in the cycle's reactions?

  • NADH and FADH2 accumulate if oxygen is not available for the electron transport chain. (correct)
  • The cycle produces toxic byproducts in the absence of oxygen.
  • The enzymes in the cycle are directly inhibited by the absence of oxygen.
  • Acetyl-CoA production is dependent on oxygen.

    • Which of the following fuel types can be oxidized by the Citric Acid Cycle after conversion to Acetyl-CoA?

  • Only ketone bodies.
  • Glucose, fatty acids, and ketone bodies. (correct)
  • Only fatty acids.
  • Only glucose.

    • What primarily controls the rate or activity of the Citric Acid Cycle?

    <p>The energy status of the cell. (B)</p> Signup and view all the answers

    Which enzyme of the TCA cycle is the major control enzyme?

    <p>Isocitrate dehydrogenase (A)</p> Signup and view all the answers

    What inhibits isocitrate dehydrogenase?

    <p>ATP (B)</p> Signup and view all the answers

    α-Ketoglutarate dehydrogenase requires which of the following?

    <p>Thiamine, lipoic acid, CoA, FAD, and NAD (A)</p> Signup and view all the answers

    Approximately what percentage of available energy is lost as heat during catabolism?

    <p>60% (D)</p> Signup and view all the answers

    Which enzyme catalyzes the substrate-level phosphorylation of GDP to GTP within the citric acid cycle?

    <p>3-Succinyl-CoA synthetase (D)</p> Signup and view all the answers

    Besides its role in the citric acid cycle, succinate dehydrogenase also functions as what?

    <p>Complex II of the electron transport chain (B)</p> Signup and view all the answers

    What two substrates are condensed by citrate synthase to form citrate?

    <p>Acetyl group and oxaloacetate (D)</p> Signup and view all the answers

    Which of the following is the primary source of electrons that feed into the electron transport chain?

    <p>NADH (C)</p> Signup and view all the answers

    What accepts electrons directly from NADH in the electron transport chain?

    <p>NADH dehydrogenase (complex I) (D)</p> Signup and view all the answers

    What is the final electron acceptor in the electron transport chain?

    <p>Oxygen (C)</p> Signup and view all the answers

    Which of the following is the correct order of electron carriers in the electron transport chain?

    <p>NADH dehydrogenase → Coenzyme Q → Cytochrome b/c1 → Cytochrome c → Cytochrome a/a3 (C)</p> Signup and view all the answers

    What is the approximate free energy change ($\Delta G$) when FADH2 is oxidized in the electron transport chain?

    <p>$\Delta G = -42$ kcal/mol (B)</p> Signup and view all the answers

    What is the primary role of the proton gradient generated by the electron transport chain (ETC)?

    <p>Providing the energy for ATP synthesis by oxidative phosphorylation. (D)</p> Signup and view all the answers

    Which complexes within the electron transport chain directly contribute to the translocation of protons across the inner mitochondrial membrane?

    <p>Complex I, Complex III, and Complex IV (D)</p> Signup and view all the answers

    How many ATP molecules are typically generated by F1F0 ATP synthase when an FADH2 molecule is oxidized in the electron transport chain?

    <p>Approximately 2 ATP (D)</p> Signup and view all the answers

    What is the primary mechanism by which cyanide inhibits cellular respiration and oxidative phosphorylation?

    <p>By irreversibly binding to cytochrome a/a3, preventing electron transfer to oxygen. (A)</p> Signup and view all the answers

    Inhibition of the electron transport chain leads to several metabolic changes. Which of the following is a direct consequence of this inhibition?

    <p>Decreased Oxygen Consumption (A)</p> Signup and view all the answers

    Carbon monoxide (CO) poisoning shares some similarities with cyanide poisoning in its effects on cellular respiration, but also has distinct characteristics. Which of the following is a unique feature of carbon monoxide's mechanism of toxicity?

    <p>It binds to hemoglobin, displacing oxygen. (B)</p> Signup and view all the answers

    What is the effect of opening proton channels in the mitochondrial inner membrane?

    <p>It decreases the proton gradient. (C)</p> Signup and view all the answers

    Which of the following is the correct order of the molecules and complexes involved in the proton gradient creation?

    <p>NADH dehydrogenase -&gt; Cytochrome b/c1 -&gt; Cytochrome a/a3 (A)</p> Signup and view all the answers

    What is the primary effect of uncouplers on ATP synthesis and oxygen consumption?

    <p>Decrease ATP synthesis and increase oxygen consumption. (C)</p> Signup and view all the answers

    How does brown adipose tissue contribute to thermogenesis in newborns?

    <p>By using a natural uncoupling protein (UCP) to dissipate energy as heat. (C)</p> Signup and view all the answers

    Which of the following is NOT a described effect of bacterial toxins acting as indirect uncouplers?

    <p>Enhancing ATP synthesis by increasing proton flow through ATP synthase. (C)</p> Signup and view all the answers

    What is the function of the NADH shuttle system related to glycolysis?

    <p>To transport the substrate for oxidative metabolism from the cytosol to the mitochondrial electron transport chain. (A)</p> Signup and view all the answers

    What is the consequence of pore-forming toxins on mitochondrial function?

    <p>Reduced efficiency of ATP production due to proton leakage. (A)</p> Signup and view all the answers

    Why are bacterial toxins described in the content as indirect uncouplers of the respiratory chain?

    <p>They increase membrane permeability to protons, disrupting the proton gradient. (D)</p> Signup and view all the answers

    Which of the following is a physiological response in adults when exposed to cold temperatures, which is different from the response in babies?

    <p>Shivering to generate heat through muscle activity. (C)</p> Signup and view all the answers

    Which of the following is true regarding the location of NADH production?

    <p>NADH molecules produced by glycolysis are in the cytosol. (D)</p> Signup and view all the answers

    Flashcards

    Citric Acid Cycle

    A key metabolic pathway for energy extraction from food molecules, also known as Krebs cycle or TCA cycle.

    Primary Role of Citric Acid Cycle

    To oxidize acetyl-CoA to carbon dioxide, generating energy carriers.

    Energy Carriers

    Molecules like NADH, FADH2, and GTP produced in the cycle, used to fuel ATP production.

    Isocitrate Dehydrogenase

    A major control enzyme in the TCA cycle, inhibited by NADH and ATP, activated by ADP.

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    α-Ketoglutarate Dehydrogenase

    An enzyme similar to pyruvate dehydrogenase, requires thiamine and other cofactors for function.

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    Anaerobic Conditions

    The citric acid cycle does not operate without oxygen due to accumulation of NADH and FADH2.

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    Control of the Cycle

    Activity is regulated by the energy status of the cell, without hormonal control.

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    Efficiency of Energy Use

    Only about 40% of energy from glucose oxidation is used for ATP, while 60% is lost as heat.

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    Proton Gradient

    A difference in proton concentration across the mitochondrial inner membrane.

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    Electrochemical Gradient

    A gradient that combines the concentration gradient and voltage across a membrane.

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    NADH and FADH2

    Electron carriers that donate electrons to the electron transport chain.

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    F1-F0-ATP Synthase

    A complex that synthesizes ATP using the energy from the proton gradient.

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    Oxidative Phosphorylation

    Process of ATP production using energy from electron transport and proton gradient.

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    Cyanide

    A poison that inhibits electron transport by binding to cytochrome a/a3.

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    Carbon Monoxide

    A gas that inhibits electron transport by binding to cytochrome a/a3, but less strongly than cyanide.

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    ATP Yield from NADH and FADH2

    Oxidation of NADH generates about 3 ATP; FADH2 generates about 2 ATP.

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    3-Succinyl-CoA synthetase

    Enzyme that catalyzes the phosphorylation of GDP to GTP.

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    Succinate dehydrogenase

    Enzyme located on the inner mitochondrial membrane, part of the electron transport chain.

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    Citrate synthase

    Enzyme that condenses acetyl CoA with oxaloacetate to form citrate.

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    Sources of NADH

    NADH is produced by enzymes, especially in the citric acid cycle and pyruvate dehydrogenase.

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    Function of O2 in ETC

    O2 accepts electrons at the end of the electron transport chain, producing water.

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    Complex I of ETC

    NADH dehydrogenase that oxidizes NADH and passes electrons into the chain.

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    Coenzyme Q

    A lipid in the electron transport chain that carries electrons from complex I to complex III.

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    Cytochromes

    Heme-containing proteins that transfer electrons through the electron transport chain.

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    Uncouplers

    Substances that reduce the proton gradient in mitochondria, affecting ATP synthesis.

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    Effects of Uncouplers

    They lead to decreased ATP synthesis and increased oxygen consumption and NADH oxidation.

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    Brown adipose tissue

    A type of fat that generates heat in newborns through uncoupling proteins.

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    2,4-DNP

    A known chemical uncoupler that increases metabolic rate and heat production.

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    Bacterial Toxins

    Substances produced by bacteria that can act as indirect uncouplers by forming pores.

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    NADH shuttle system

    A mechanism that transports NADH from the cytosol to mitochondria for energy metabolism.

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    Streptococcus pneumoniae toxin

    Pneumolysin, a toxin that forms pores in host cells, affecting energy production.

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    Proton Motive Force

    The force generated by the proton gradient across the mitochondrial membrane, driving ATP synthesis.

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    Study Notes

    Citric Acid Cycle - Objectives

    • The cycle's primary role is in cellular metabolism.
    • This lecture introduces how the cycle functions as a key step in extracting energy from food molecules.
    • Students will gain a comprehensive understanding of the significant steps, including the participating molecules and enzymes.
    • The lecture will explore the reactions within the cycle, identifying how it regenerates a crucial intermediate for further fuel breakdown.
    • Students will learn how the cycle produces energy carriers (NADH and FADH2), which fuel ATP production.
    • The connection between the Citric Acid Cycle and the Electron Transport Chain, highlighting their roles in cellular respiration, will be explored.

    Citric Acid Cycle (Krebs Cycle / TCA cycle)

    • The citric acid cycle, also known as the Krebs cycle or the tricarboxylic acid cycle (TCA), occurs within the mitochondria.
    • While oxygen isn't directly required in the cycle, it can't proceed anaerobically due to NADH and FADH2 accumulation if oxygen isn't present for the electron transport chain.
    • Its primary function is the oxidation of acetyl-CoA to carbon dioxide.
    • The energy released from this process is stored as NADH, FADH2, and GTP.
    • Oxidation of glucose yields about 40% useable energy for ATP synthesis. The remaining 60% is lost as heat.

    Key Points of TCA Cycle

    • Isocitrate dehydrogenase is a major control enzyme. It's inhibited by NADH and ATP, activated by ADP.
    • α-Ketoglutarate dehydrogenase is similar to pyruvate dehydrogenase. Requires thiamine, lipoic acid, CoA, FAD, and NAD. Deficiency in thiamine slows acetyl-CoA oxidation in the citric acid cycle.

    Enzymes of the Citric Acid Cycle

    • Citrate synthase: Condenses acetyl group and oxaloacetate to form citrate.
    • Aconitase: Catalyzes citrate to isocitrate.
    • Isocitrate dehydrogenase: Oxidizes isocitrate to α-ketoglutarate, releasing CO2 and producing NADH.
    • α-Ketoglutarate dehydrogenase: Oxidizes α-ketoglutarate to succinyl-CoA, releasing CO2 and producing NADH.
    • Succinyl-CoA synthetase: Converts succinyl-CoA to succinate, generating GTP (or ATP).
    • Succinate dehydrogenase: Oxidizes succinate to fumarate, producing FADH2.
    • Fumarase: Converts fumarate to malate.
    • Malate dehydrogenase: Oxidizes malate to oxaloacetate, producing NADH.

    Electron Transport Chain (ETC) Reactions

    • NADH + O2 → NAD+ + H2O ΔG = -56 kcal/mol
    • FADH2 + O2 → FAD + H2O ΔG = -42 kcal/mol

    Proton Gradient

    • The ETC generates electricity that powers proton pumps.
    • Protons are moved from the matrix to the intermembrane space, creating a proton gradient (similar to pumping Na+ across a membrane).
    • The three major complexes (I, III, and IV) translocate protons as electricity passes through them. The gradient is maintained across the mitochondrial inner membrane.

    Oxidative Phosphorylation

    • Proton flow into the mitochondria through the F0 component powers the F1 component (ATP synthase).
    • Phosphorylates ADP using Pi to produce ATP.
    • NADH oxidation typically yields 3 ATP molecules.
    • FADH2 oxidation usually yields 2 ATP molecules.

    Inhibitors of the Electron Transport Chain (ETC)

    • Cyanide: Irreversibly binds to cytochrome a/a3, preventing electron transfer to oxygen, causing tissue hypoxia.
    • Carbon monoxide: Binds to cytochrome a/a3, though less tightly than cyanide. Also binds to hemoglobin, displacing oxygen.
    • Other inhibitors: Metformin, phenformin, rotenone, malonate, antimycin A, doxorubicin, oligomycin, GBNOXIN.

    Uncouplers

    • Uncouplers disrupt the proton gradient, leading to:
      • Decreased ATP synthesis
      • Increased oxygen consumption
      • Increased NADH oxidation
    • Important uncouplers include 2,4-dinitrophenol (2,4-DNP), aspirin, and other salicylates.
    • Brown adipose tissue contains the protein UCP (thermogenin) enabling energy loss as heat to regulate temperature in newborns.

    Bacterial Toxins as Uncouplers

    • Some bacterial toxins (e.g., pneumolysin, staphylococcal α-toxin, listeriolysin O) create pores in membranes.
    • This leads to proton leakage, decreasing ATP production efficiency — similar to uncouplers.

    Fate of NADH

    • Most NADH is generated inside mitochondria by the citric acid cycle.
    • Glycolysis-derived NADH is in the cytosol.
    • Mitochondrial inner membrane lacks direct NADH transport.
    • Shuttle systems (e.g., glycerol-phosphate or malate-aspartate) carry reducing equivalents of cytosolic NADH into mitochondria.

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    Related Documents

    Citric Acid Cycle PDF

    Description

    Test your knowledge on the Citric Acid Cycle, a crucial metabolic pathway in cellular respiration. This quiz covers its function, regulation, and the enzymes involved in the process. Ideal for students studying biochemistry or related fields.

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