Harper's Biochemistry Chapter 16 - The Citric Acid Cycle
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Questions and Answers

What regulates the activity of citrate synthase?

  • Long-chain fatty acyl-CoA (correct)
  • NADH
  • ADP
  • NADPH
  • Which enzyme catalyzes the conversion of pyruvate to oxaloacetate?

  • Pyruvate carboxylase (correct)
  • Citrate synthase
  • Succinate dehydrogenase
  • Malate dehydrogenase
  • What is the fate of oxaloacetate released by citrate lyase?

  • It is reduced to malate. (correct)
  • It directly reenters the mitochondrion.
  • It is converted to citrate.
  • It is phosphorylated.
  • Which molecule provides half of the NADPH required for fatty acid synthesis?

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

    Which vitamin coenzyme is utilized by pyruvate carboxylase?

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

    What is the primary role of the citric acid cycle in metabolism?

    <p>To oxidize acetyl-CoA and produce ATP</p> Signup and view all the answers

    Which substance is primarily depleted in hyperammonemia, causing impaired function of the citric acid cycle?

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

    Which enzyme is primarily responsible for converting pyruvate to acetyl-CoA?

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

    What is an anaplerotic reaction in the context of the citric acid cycle?

    <p>A reaction that replenishes cycle intermediates</p> Signup and view all the answers

    Which of the following vitamins is known to function as a coenzyme in the citric acid cycle?

    <p>Vitamin B1 (Thiamine)</p> Signup and view all the answers

    What is the main function of the citric acid cycle besides ATP formation?

    <p>Provision of carbon skeletons for gluconeogenesis</p> Signup and view all the answers

    Which metabolic pathway involves both anabolic and catabolic processes?

    <p>Citric acid cycle</p> Signup and view all the answers

    What type of phosphorylation occurs during the reactions of the citric acid cycle?

    <p>Substrate-level phosphorylation</p> Signup and view all the answers

    The citric acid cycle is often described as amphibolic. What does this mean?

    <p>It is involved in both catabolic and anabolic pathways</p> Signup and view all the answers

    What activates the dehydrogenases in the citric acid cycle?

    <p>Calcium ions (Ca2+)</p> Signup and view all the answers

    Which of the following ratios reflects the energy status of the cell during the citric acid cycle?

    <p>[ATP]/[ADP] and [NADH]/[NAD+]</p> Signup and view all the answers

    How does the availability of oxidized cofactors influence the activity of the citric acid cycle?

    <p>Increased oxidized cofactors enhance the cycle's activity</p> Signup and view all the answers

    Which of the following substances is a key product of the citric acid cycle that can be used for fatty acid synthesis?

    <p>Acetyl-CoA</p> Signup and view all the answers

    Which metabolic pathways can produce intermediates for the citric acid cycle?

    <p>Glycolysis and the pentose phosphate pathway</p> Signup and view all the answers

    Which vitamin coenzyme is involved in the decarboxylation processes of the citric acid cycle?

    <p>Vitamin B1 (Thiamine)</p> Signup and view all the answers

    What term refers to the balance of precursor and product molecules in the citric acid cycle?

    <p>Anaplerosis and cataplerosis</p> Signup and view all the answers

    Which enzyme of the citric acid cycle is most directly linked to the generation of NADH?

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

    During high energy demand in muscle contraction, what process occurs to finally produce ATP?

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

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

    <p>It combines with oxaloacetate to form citrate.</p> Signup and view all the answers

    Which process is responsible for the generation of one ATP directly in the citric acid cycle?

    <p>Cataplerosis during the conversion of succinyl-CoA to succinate.</p> Signup and view all the answers

    What is anaplerosis in the context of the citric acid cycle?

    <p>The addition of carbon to replenish cycle intermediates.</p> Signup and view all the answers

    Which coenzyme is involved in the conversion of pyruvate to Acetyl-CoA?

    <p>Coenzyme A</p> Signup and view all the answers

    Which of the following statements best describes the amphibolic nature of the citric acid cycle?

    <p>It can both catabolize and anabolize carbon compounds.</p> Signup and view all the answers

    Which enzyme is responsible for the isomerization of citrate to isocitrate?

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

    The reduction of coenzymes in the respiratory chain leads to what process in the citric acid cycle?

    <p>Phosphorylation of ADP to ATP.</p> Signup and view all the answers

    Which metabolic pathway exemplifies cataplerosis from the citric acid cycle?

    <p>Conversion of oxaloacetate to phosphoenolpyruvate.</p> Signup and view all the answers

    What key feature characterizes the enzymes involved in the citric acid cycle?

    <p>They are located in the mitochondrial matrix and attached to membranes.</p> Signup and view all the answers

    Which vitamin coenzyme is primarily involved in the oxidative reactions of the citric acid cycle?

    <p>Vitamin B3 (Niacin)</p> Signup and view all the answers

    What role does succinyl-CoA–acetoacetate-CoA transferase play in metabolism?

    <p>It catalyzes the transfer of CoA to acetoacetate.</p> Signup and view all the answers

    Which vitamin is primarily linked with the coenzyme for the decarboxylation reaction in α-ketoglutarate dehydrogenase?

    <p>Vitamin B1</p> Signup and view all the answers

    What is a characteristic feature of the citric acid cycle regarding metabolic processes?

    <p>It is amphibolic, serving both anabolic and catabolic functions.</p> Signup and view all the answers

    What does the term 'cataplerosis' refer to in the context of metabolism?

    <p>The withdrawal of intermediates from the citric acid cycle.</p> Signup and view all the answers

    In the context of the citric acid cycle, what is succinate oxidized to during its metabolism?

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

    Which of the following is involved in the synthesis of acetyl-CoA?

    <p>Pantothenic acid as part of coenzyme A</p> Signup and view all the answers

    What kind of reactions are primarily involved in the conversion of amino acids during the citric acid cycle?

    <p>Transamination and deamination</p> Signup and view all the answers

    Which compound serves as the substrate for the phosphorylation of ADP as per the citric acid cycle?

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

    Which key metabolic process is linked to the buildup of oxaloacetate?

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

    What is the result of the dehydrogenation reaction catalyzed by succinate dehydrogenase?

    <p>Conversion of succinate to fumarate</p> Signup and view all the answers

    What role do dehydrogenases play in the citric acid cycle?

    <p>They catalyze the oxidation of reduced coenzymes.</p> Signup and view all the answers

    Which of the following best describes anaplerosis in the context of the citric acid cycle?

    <p>The addition of intermediates to replenish the cycle.</p> Signup and view all the answers

    Which factor influences the regulation of the citric acid cycle through energy status?

    <p>The levels of oxidized and reduced cofactors.</p> Signup and view all the answers

    Which statement best describes the citric acid cycle's role in metabolic pathways?

    <p>It provides energy and building blocks for various biosynthetic pathways.</p> Signup and view all the answers

    What is the primary consequence of a change in calcium ion concentration during muscle contraction in relation to the citric acid cycle?

    <p>Activation of dehydrogenases in the citric acid cycle.</p> Signup and view all the answers

    What is the harmful effect of fluoroacetate in the metabolic pathway?

    <p>It inhibits the enzyme aconitase.</p> Signup and view all the answers

    Which cofactor is necessary for the decarboxylation reaction of isocitrate?

    <p>Mn2+</p> Signup and view all the answers

    What type of enzyme reduces ubiquinone in the electron transport chain?

    <p>Succinate dehydrogenase</p> Signup and view all the answers

    What is a primary function of oxaloacetate in metabolic pathways?

    <p>Substrate for gluconeogenesis.</p> Signup and view all the answers

    Which reaction results from the oxidation of malate?

    <p>Generation of oxaloacetate.</p> Signup and view all the answers

    What does the accumulation of isocitrate indicate in the citric acid cycle?

    <p>Reduced activity of aconitase.</p> Signup and view all the answers

    How does the ratio of NADH to NAD+ influence the citric acid cycle?

    <p>Affects the net flux towards oxaloacetate.</p> Signup and view all the answers

    What is an essential feature of isocitrate dehydrogenase isoenzymes found in the mitochondria?

    <p>They are dependent on NAD+ for the oxidation process.</p> Signup and view all the answers

    How does hyperammonemia affect the citric acid cycle?

    <p>It depletes citric acid cycle intermediates and inhibits enzymatic activity.</p> Signup and view all the answers

    Which of the following reactions in the citric acid cycle generates reducing equivalents?

    <p>Oxidation of isocitrate to α-ketoglutarate.</p> Signup and view all the answers

    Which aspect of the citric acid cycle is primarily affected by the availability of oxidized cofactors?

    <p>The formation of ATP.</p> Signup and view all the answers

    What is the primary consequence of impaired ATP formation from the citric acid cycle in the central nervous system?

    <p>Causes severe neurological damage.</p> Signup and view all the answers

    Which function in metabolism is NOT associated with the citric acid cycle?

    <p>Producing glucose from acetyl-CoA.</p> Signup and view all the answers

    In what way does the citric acid cycle contribute to fatty acid synthesis?

    <p>By providing intermediates that are precursors for fatty acids.</p> Signup and view all the answers

    Which process is directly linked to the regeneration of citric acid cycle intermediates?

    <p>Anaplerotic reactions.</p> Signup and view all the answers

    What does an increase in ATP levels signify for the activity of citrate synthase?

    <p>It causes inhibition of the enzyme.</p> Signup and view all the answers

    How is malate formed from oxaloacetate in the mitochondrion?

    <p>By reduction using NADH.</p> Signup and view all the answers

    What is a key characteristic of the citric acid cycle that supports both catabolism and anabolism?

    <p>Its involvement in both ATP production and biosynthetic pathways.</p> Signup and view all the answers

    What is a consequence of hyperammonemia in metabolic processes?

    <p>Loss of consciousness and convulsions.</p> Signup and view all the answers

    What role does the NADH/NAD+ ratio play in the conversion of malate to pyruvate?

    <p>It drives the reaction forwards, favoring formation of pyruvate.</p> Signup and view all the answers

    What happens to citrate when both ATP and long-chain fatty acyl-CoA levels are high?

    <p>Its synthesis is inhibited.</p> Signup and view all the answers

    Which component is crucial in determining the rate of citrate formation?

    <p>Concentration of oxaloacetate.</p> Signup and view all the answers

    How does citrate synthase activity change with varying availability of oxaloacetate?

    <p>Activity decreases if oxaloacetate levels drop significantly.</p> Signup and view all the answers

    What metabolic functions does the pyruvate carboxylase enzyme perform?

    <p>It carboxylates pyruvate to produce oxaloacetate using ATP.</p> Signup and view all the answers

    What is the direct entry point for propionate into gluconeogenesis?

    <p>Succinyl-CoA</p> Signup and view all the answers

    Which enzyme is inhibited by its product, thereby influencing the transport of citrate?

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

    Which metabolic cycle is responsible for the production of cytosolic acetyl-CoA for fatty acid synthesis?

    <p>Citric acid cycle</p> Signup and view all the answers

    What reaction must occur for citrate to be available for transport out of the mitochondrion?

    <p>Cleavage by citrate lyase</p> Signup and view all the answers

    Which of the following substances is formed as a result of the reaction catalyzed by pyruvate carboxylase?

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

    Which pathway uses both the products from glycolysis and citric acid cycle for energy metabolism?

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

    What limits the availability of citrate for exporting from the mitochondrion?

    <p>Inhibition of aconitase</p> Signup and view all the answers

    Which product of the citric acid cycle plays a role in fatty acid synthesis?

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

    During which reaction is acetyl-CoA specifically utilized in the citric acid cycle?

    <p>Formation of citrate from oxaloacetate</p> Signup and view all the answers

    Which enzyme facilitates the conversion of malate to oxaloacetate in the cytosol?

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

    Match the following components of the citric acid cycle with their respective functions:

    <p>Acetyl-CoA = Enters the cycle and forms citrate Citrate = Converted to isocitrate Isocitrate = Oxidized to produce CO2 Succinyl-CoA = Converted to succinate, generating ATP</p> Signup and view all the answers

    Match the following terms related to the citric acid cycle with their definitions:

    <p>Anaplerosis = The process of adding carbon to the cycle Cataplerosis = The process of removing carbon from the cycle Oxidative phosphorylation = Generation of ATP via electron transport Substrate-level phosphorylation = Direct conversion of ADP to ATP within the cycle</p> Signup and view all the answers

    Match the following enzymes with their reactions in the citric acid cycle:

    <p>Citrate synthase = Condenses Acetyl-CoA and oxaloacetate Aconitase = Isomerizes citrate to isocitrate Isocitrate dehydrogenase = Oxidizes isocitrate to alpha-ketoglutarate Succinate dehydrogenase = Oxidizes succinate to fumarate</p> Signup and view all the answers

    Match the following metabolic processes to their descriptions:

    <p>Gluconeogenesis = Conversion of pyruvate to oxaloacetate Lipogenesis = Utilizes NADPH for fatty acid synthesis Beta-oxidation = Breakdown of fatty acids to Acetyl-CoA Respiratory chain = Reoxidation of coenzymes to generate ATP</p> Signup and view all the answers

    Match the following coenzymes with their related functions in the citric acid cycle:

    <p>NAD+ = Accepts electrons during dehydrogenation FAD = Participates in the oxidation of succinate Coenzyme A = Involved in the formation of Acetyl-CoA GDP = Couples with a substrate-level phosphorylation to form GTP</p> Signup and view all the answers

    Match the following substances with their roles in metabolism:

    <p>Citrate = Source of acetyl-CoA for fatty acid synthesis Fluoroacetate = Inhibitor of aconitase Oxaloacetate = Substrate for gluconeogenesis Isocitrate = Intermediate in the citric acid cycle</p> Signup and view all the answers

    Match the following enzymes with their respective reactions:

    <p>Aconitase = Catalyzes the isomerization of citrate to isocitrate Isocitrate dehydrogenase = Converts isocitrate to α-ketoglutarate Fumarase = Catalyzes the hydration of fumarate to malate Malate dehydrogenase = Oxidizes malate to oxaloacetate</p> Signup and view all the answers

    Match the following compounds with their metabolic functions:

    <p>NAD+ = Electron acceptor in oxidation reactions NADH = Produced during isocitrate dehydrogenation Acetyl-CoA = Entry point for fatty acid synthesis Fumarate = Intermediary in the citric acid cycle</p> Signup and view all the answers

    Match the following ions with their roles in the citric acid cycle:

    <p>Mg2+ = Required for decarboxylation of isocitrate Mn2+ = Co-factor for isocitrate dehydrogenase Iron-sulfur proteins = Part of the electron transport chain Calcium ions = Influence regulation of the citric acid cycle</p> Signup and view all the answers

    Match the following reactions with their corresponding products:

    <p>Citrate synthesis = Formation of citrate from acetyl-CoA Isocitrate dehydrogenation = Conversion of isocitrate to α-ketoglutarate Fumarate hydration = Conversion of fumarate to malate Malate oxidation = Formation of oxaloacetate from malate</p> Signup and view all the answers

    Study Notes

    The Citric Acid Cycle: A Metabolic Hub

    • The citric acid cycle, also known as the Krebs cycle or the tricarboxylic acid cycle, is a vital metabolic pathway occurring within mitochondria.
    • This cycle is the final common pathway for the oxidation of carbohydrates, lipids, and proteins.
    • The cycle oxidizes the acetyl moiety of acetyl-CoA to CO2, coupling this oxidation to the reduction of coenzymes such as NAD+ and FAD.
    • These reduced coenzymes (NADH and FADH2) are then reoxidized in the mitochondrial electron transport chain, ultimately leading to the generation of ATP.
    • The cycle also provides essential intermediates for various metabolic processes, including gluconeogenesis, amino acid synthesis, and fatty acid synthesis.

    Citric Acid Cycle Reactions:

    • The citric acid cycle begins with the condensation of acetyl-CoA with oxaloacetate, forming citrate.
    • Citrate is then isomerized to isocitrate.
    • Isocitrate undergoes oxidative decarboxylation to α-ketoglutarate, generating the first NADH.
    • α-ketoglutarate is also oxidatively decarboxylated to succinyl-CoA, producing the second NADH.
    • Succinyl-CoA is converted to succinate, generating GTP through substrate-level phosphorylation.
    • Succinate is oxidized to fumarate, generating FADH2.
    • Fumarate is hydrated to malate.
    • Finally, malate is oxidized to oxaloacetate, producing the third NADH.
    • The cycle completes a full turn with the regeneration of oxaloacetate.

    The Citric Acid Cycle and Energy Production:

    • Each turn of the citric acid cycle produces nine molecules of ATP via oxidative phosphorylation, and one ATP (or GTP) through substrate-level phosphorylation.

    Regulation:

    • The citric acid cycle is tightly regulated to ensure efficient energy production and cellular function.
    • Key regulatory enzymes include citrate synthase, isocitrate dehydrogenase, and α-ketoglutarate dehydrogenase.
    • ATP, NADH, and long-chain fatty acyl-CoA act as allosteric inhibitors of citrate synthase and isocitrate dehydrogenase, reflecting an energy-rich state.
    • Conversely, ADP and Ca2+ act as allosteric activators, indicating an energy-demanding state.

    Hyperammonemia and Implications:

    • Hyperammonemia, an excess of ammonia in the blood, can severely impair citric acid cycle function.
    • Hyperammonemia can occur due to liver disease or genetic defects in amino acid metabolism.
    • High ammonia levels deplete citric acid cycle intermediates, such as α-ketoglutarate, by diverting them towards glutamate and glutamine synthesis.
    • Ammonia also inhibits the oxidative decarboxylation of α-ketoglutarate.
    • These effects lead to a decrease in ATP production and can cause severe neurologic deficits, including coma, convulsions, and even death.

    Citric Acid Cycle

    • The citric acid cycle is a key metabolic pathway located in the mitochondria
    • It oxidizes the acetyl group of Acetyl-CoA to carbon dioxide
    • This process is coupled to the reduction of coenzymes, which are then reoxidized in the electron transport chain
    • This process is linked to the production of ATP
    • The citric acid cycle is the final common pathway for the oxidation of carbohydrates, lipids, and proteins.
    • The citric acid cycle can be regulated by the availability of certain vitamins and the energy status of a cell
    • Hyperammonemia, a condition associated with advanced liver disease, can impair the citric acid cycle
    • Hyperammonemia can be fatal due to impaired ATP production.

    Citric Acid Cycle Reactions

    • Acetyl-CoA joins with oxaloacetate to form citrate: Catalyzed by citrate synthase.
    • Citrate is isomerized to isocitrate: Catalyzed by aconitase.
    • Isocitrate is oxidized and decarboxylated to α-ketoglutarate: Catalyzed by isocitrate dehydrogenase.
    • α-ketoglutarate is oxidized and decarboxylated to succinyl-CoA: Catalyzed by α-ketoglutarate dehydrogenase complex
    • Succinyl-CoA is converted to succinate: Catalyzed by succinyl-CoA synthetase.
    • Succinate is oxidized to fumarate: Catalyzed by succinate dehydrogenase.
    • Fumarate is hydrated to malate: Catalyzed by fumarase.
    • Malate is oxidized to oxaloacetate: Catalyzed by malate dehydrogenase.

    Citric Acid Cycle - Regulation

    • Citrate synthase is inhibited by ATP and long-chain fatty acyl-CoA.
    • Isocitrate dehydrogenase is activated by ADP and inhibited by ATP and NADH.
    • α-ketoglutarate dehydrogenase complex is regulated in the same way as pyruvate dehydrogenase.
    • Succinate dehydrogenase is inhibited by oxaloacetate.
    • The availability of oxaloacetate is controlled by malate dehydrogenase and depends on the [NADH]/[NAD+] ratio.

    Citric Acid Cycle - Importance

    • The citric acid cycle provides a pathway for the catabolism of amino acids.
    • The citric acid cycle provides a pathway for the synthesis of certain amino acids.
    • It is essential for gluconeogenesis.
    • It is the major pathway for the formation of ATP.
    • It is required for fatty acid synthesis.

    Anaplerosis and Cataplerosis

    • Anaplerosis refers to the replenishment of citric acid cycle intermediates.
    • Cataplerosis refers to the removal of citric acid cycle intermediates.
    • These processes are critical for maintaining the balance of the cycle.

    Medical Importance

    • Fluoroacetate poisoning: This poison is found in some plants and can be fatal to animals.
    • Hyperammonemia: A condition associated with advanced liver disease, can impair the citric acid cycle leading to neurological damage.
    • Impaired ATP Formation: Hyperammonemia leads to reduced formation of ATP, which is essential for energy production in the brain and other tissues.

    Fatty Acid Synthesis

    • The citric acid cycle provides citrate for the synthesis of fatty acids.
    • Citrate is transported from the mitochondria to the cytosol where it is used for fatty acid synthesis.
    • This pathway is essential for providing acetyl-CoA for fatty acid synthesis.

    Vitamin Requirements

    • Biotin: Required as a coenzyme for pyruvate carboxylase.
    • Niacin: Required for NAD+-dependent isocitrate dehydrogenase.
    • Riboflavin: Required for succinate dehydrogenase.
    • Thiamine: Required for α-ketoglutarate dehydrogenase complex.

    Citric Acid Cycle Location

    • Enzymes of the citric acid cycle are found in the mitochondrial matrix, either free or attached to the inner mitochondrial membrane and the crista membrane.

    Citric Acid Cycle Function

    • It serves as a central pathway for carbohydrate, lipid, and protein catabolism.
    • The cycle must maintain a balance in the number of carbons entering and leaving.
    • Acetyl-CoA (two carbon molecule) combines with oxaloacetate (four carbon molecule) to form citrate (six carbon molecule).
    • During each turn of the cycle, citrate is converted back to oxaloacetate, and two carbons are released as CO2.
    • Anaplerosis refers to the addition of carbon to the cycle, like pyruvate to oxaloacetate for gluconeogenesis.
    • Cataplerosis is the removal of carbon from the cycle, such as oxaloacetate to phosphoenolpyruvate.
    • Anaplerosis must equal cataplerosis to sustain the citric acid cycle.

    Citric Acid Cycle Reactions

    • Citrate Synthesis: Acetyl-CoA reacts with oxaloacetate catalyzed by citrate synthase to form citryl-CoA, which is hydrolyzed, releasing citrate and CoASH.
    • Isomerization: Citrate is isomerized to isocitrate by aconitase through dehydration to cis-aconitate and rehydration to isocitrate.
    • Oxidative Decarboxylation of Isocitrate: Isocitrate is dehydrogenated to oxalosuccinate, which is enzyme-bound and undergoes decarboxylation to α-ketoglutarate, catalyzed by isocitrate dehydrogenase.
    • Oxidative Decarboxylation of α-ketoglutarate: α-ketoglutarate is converted to succinyl-CoA in a reaction catalyzed by α-ketoglutarate dehydrogenase complex, which requires cofactors like thiamin diphosphate, lipoate, NAD+, FAD, and CoA.
    • Succinyl-CoA to Succinate: Succinyl-CoA is converted to succinate by succinate thiokinase, generating ATP or GTP through substrate-level phosphorylation.
    • Succinate to Fumarate: Succinate is oxidized to fumarate by succinate dehydrogenase, reducing FAD to FADH2.
    • Fumarate to Malate: Fumarate is hydrated to malate by fumarase.
    • Malate to Oxaloacetate: Malate is oxidized to oxaloacetate by malate dehydrogenase, reducing NAD+ to NADH.

    ATP Production

    • One turn of the citric acid cycle generates a total of 10 ATPs.
    • The cycle produces three NADH and one FADH2 molecules per acetyl-CoA oxidized.
    • During oxidative phosphorylation, reoxidation of NADH yields ~2.5 ATP, and reoxidation of FADH2 yields ~1.5 ATPs.
    • Substrate-level phosphorylation in the conversion of succinyl-CoA to succinate generates 1 ATP.

    Regulation of the Citric Acid Cycle

    • The citric acid cycle is primarily regulated by the availability of oxidized cofactors (NAD+).
    • Regulation of individual enzymes:
      • Citrate synthase is allosterically inhibited by ATP and long-chain fatty acyl-CoA.
      • Isocitrate dehydrogenase is allosterically activated by ADP, counteracted by ATP and NADH.
      • α-ketoglutarate dehydrogenase is regulated similarly to pyruvate dehydrogenase.
      • Succinate dehydrogenase is inhibited by oxaloacetate.

    Vitamins and the Citric Acid Cycle

    • Four B vitamins are essential for the citric acid cycle:
      • Riboflavin (as FAD) is a cofactor for succinate dehydrogenase.
      • Niacin (as NAD+) is an electron acceptor for isocitrate dehydrogenase, α-ketoglutarate dehydrogenase, and malate dehydrogenase.
      • Thiamin (as thiamin diphosphate) is a cofactor for α-ketoglutarate dehydrogenase.
      • Biotin is a coenzyme for pyruvate carboxylase, which carboxylates pyruvate to oxaloacetate.

    Citric Acid Cycle and Fatty Acid Synthesis

    • Citrate, as a source of acetyl-CoA, can be transported to the cytosol for fatty acid synthesis.
    • The accumulation of isocitrate inhibits aconitase, making citrate "free" to be transported to the cytosol.
    • Malic enzyme, which converts malate to pyruvate, utilizes NADPH, a co-substrate in fatty acid synthesis.

    Hyperammonemia

    • Hyperammonemia, elevated ammonia levels in the blood, can occur in liver disease or genetic diseases of amino acid metabolism.
    • Hyperammonemia depletes citric acid cycle intermediates by removing α-ketoglutarate for the synthesis of glutamate and glutamine.
    • This depletion further decreases citric acid cycle flux and ATP generation.
    • Ammonia inhibits the α-ketoglutarate dehydrogenase complex, further reducing the citric acid cycle flux.

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    Test your knowledge on the Citric Acid Cycle, a crucial metabolic pathway that plays a key role in cellular respiration. Learn about its reactions, the conversion of acetyl-CoA, and its importance in energy production and metabolic processes. This quiz covers essential concepts and steps within the cycle.

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