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Gluconeogenesis Overview and Enzymes
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Gluconeogenesis Overview and Enzymes

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What is the primary function of gluconeogenesis in the body?

  • To convert glucose into glycogen for storage.
  • To maintain glucose levels during fasting. (correct)
  • To break down fatty acids for energy.
  • To enhance the uptake of glucose in muscle cells.

    • Which pathway requires energy input in the form of ATP?

  • Glycolysis.
  • Cellular respiration.
  • Lipolysis.
  • Gluconeogenesis. (correct)

    • Which enzyme catalyzes the conversion of glucose-6-phosphate to glucose?

  • Hexokinase.
  • Pyruvate kinase.
  • Glucose-6-phosphatase. (correct)
  • Phospho-fructokinase.

    • Which of the following reactions is considered irreversible in glycolysis?

    <p>Phosphoenolpyruvate to pyruvate.</p> Signup and view all the answers

    In which organ does gluconeogenesis primarily occur?

    <p>Liver.</p> Signup and view all the answers

    Which reaction represents a bypass in gluconeogenesis?

    <p>Glucose-6-phosphate to glucose.</p> Signup and view all the answers

    What is the role of gluconeogenesis in fast-twitch muscle fibers?

    <p>To provide a continual glucose supply.</p> Signup and view all the answers

    Which organ contributes to gluconeogenesis under specific conditions, although to a lesser extent than the liver?

    <p>Kidneys.</p> Signup and view all the answers

    What does Pyruvate Carboxylase catalyze in the process of gluconeogenesis?

    <p>Pyruvate + HCO3- + ATP à oxalo-acetate + ADP + Pi</p> Signup and view all the answers

    What contributes to the spontaneity of the second reaction in the bypass of Pyruvate Kinase?

    <p>The cleavage of a ~P bond from GTP</p> Signup and view all the answers

    What role does Acetyl CoA play in the activity of Pyruvate Carboxylase?

    <p>It allosterically activates the enzyme</p> Signup and view all the answers

    What is the immediate product of the reaction catalyzed by PEP Carboxykinase?

    <p>Phosphoenolpyruvate (PEP)</p> Signup and view all the answers

    How does an increase in oxaloacetate concentration affect the Krebs cycle?

    <p>It enhances acetyl CoA entry into the cycle</p> Signup and view all the answers

    What is a consequence of oxaloacetate depletion in the liver?

    <p>Reduced acetyl CoA entry into the Krebs cycle</p> Signup and view all the answers

    What activates Pyruvate Carboxylase during gluconeogenesis?

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

    In which metabolic pathway is oxaloacetate primarily utilized?

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

    What is produced as a byproduct of the conversion of oxaloacetate to PEP?

    <p>CO2</p> Signup and view all the answers

    Which statement best describes the energy contribution of ATP in gluconeogenesis?

    <p>ATP is hydrolyzed to release energy for oxaloacetate production</p> Signup and view all the answers

    What role does the subunit functioning as a translocase serve in relation to G-6-Pase?

    <p>It provides access of substrate G-6-P to the active site.</p> Signup and view all the answers

    Which of the following is the primary function of phosphofructokinase?

    <p>Catalyzes the phosphorylation of fructose-6-P using ATP.</p> Signup and view all the answers

    What is required for bypassing the Pyruvate Kinase reaction?

    <p>Cleavage of two phosphate bonds.</p> Signup and view all the answers

    What is the primary product of the reaction catalyzed by Fructose-1,6-bisphosphatase?

    <p>Fructose-6-P and inorganic phosphate (Pi).</p> Signup and view all the answers

    Which statement correctly describes the relationship between PEP and ATP?

    <p>PEP has a higher negative DG of phosphate hydrolysis than ATP.</p> Signup and view all the answers

    In glycolysis, what is the substrate that phosphofructokinase acts on?

    <p>Fructose-6-phosphate.</p> Signup and view all the answers

    What is the process of gluconeogenesis primarily aimed at producing?

    <p>Glucose from pyruvate.</p> Signup and view all the answers

    Which of the following statements about ATP is true in the context of glycolysis?

    <p>ATP is utilized to phosphorylate fructose-6-P.</p> Signup and view all the answers

    What is the result of hydrolyzing one phosphate bond from ATP during the conversion of PEP?

    <p>It is not sufficient to drive the synthesis of PEP.</p> Signup and view all the answers

    What is the end product of the reaction catalyzed by pyruvate kinase in glycolysis?

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

    What is the primary role of fructose-2,6-bisphosphate in the metabolic pathways discussed?

    <p>It stimulates glycolysis by allosterically activating phosphofructokinase.</p> Signup and view all the answers

    How does ATP influence the regulation of glycolysis and gluconeogenesis?

    <p>Low ATP levels stimulate glycolysis and inhibit gluconeogenesis.</p> Signup and view all the answers

    What is the significance of the reciprocal allosteric regulation by adenine nucleotides?

    <p>It prevents the futile cycling of metabolic pathways.</p> Signup and view all the answers

    Which of the following best describes the action of Protein Kinase A (PKA) on metabolic pathways?

    <p>PKA phosphorylates and inhibits glycolysis while stimulating gluconeogenesis.</p> Signup and view all the answers

    What is the effect of glucagon on liver cells in terms of metabolic control?

    <p>Glucagon stimulates gluconeogenesis while inhibiting glycolysis through a cyclic AMP cascade.</p> Signup and view all the answers

    What is the primary source of pyruvate and oxaloacetate for gluconeogenesis during fasting?

    <p>Amino acid catabolism</p> Signup and view all the answers

    Which of the following enzymes converts oxaloacetate to phosphoenolpyruvate in gluconeogenesis?

    <p>PEP Carboxykinase</p> Signup and view all the answers

    How many ATP equivalents are expended in the process of gluconeogenesis?

    <p>6</p> Signup and view all the answers

    What molecule derived from fat cells contributes significantly to gluconeogenesis?

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

    Which of the following statements about glycolysis and gluconeogenesis is TRUE?

    <p>Both glycolysis and gluconeogenesis are spontaneous processes.</p> Signup and view all the answers

    What is the end product of gluconeogenesis?

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

    Which enzyme is responsible for the conversion of fructose-1,6-bisphosphate to fructose-6-phosphate?

    <p>Fructose-1,6-bisphosphatase</p> Signup and view all the answers

    What would occur if glycolysis and gluconeogenesis were both active in a cell?

    <p>A waste of energy in a futile cycle</p> Signup and view all the answers

    Which of the following is NOT a precursor for gluconeogenesis?

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

    What is the primary function of pyruvate carboxylase in gluconeogenesis?

    <p>Converts pyruvate to oxaloacetate</p> Signup and view all the answers

    Study Notes

    Gluconeogenesis Overview

    • Gluconeogenesis converts lactate to glucose, utilizing some glycolytic reactions in reverse and unique reactions specific to this pathway.
    • Requires ATP input to maintain blood glucose levels during fasting and provides glucose to fast-twitch muscle fibers.
    • Primarily occurs in the liver, with limited activity in the kidneys and small intestine under certain conditions.

    Enzyme Pathways

    • Many gluconeogenesis enzymes overlap with glycolysis, except three critical glycolytic enzymes:
      • Hexokinase/Glucokinase
      • Phosphofructokinase
      • Pyruvate Kinase
    • These enzymes facilitate irreversible reactions that must be bypassed in gluconeogenesis.

    Bypass Reactions

    • Hexokinase/Glycolysis Reaction:
      • Catalyzes glucose + ATP to glucose-6-phosphate + ADP.
    • Fructose-1,6-bisphosphatase/Gluconeogenesis Reaction:
      • Converts fructose-1,6-bisphosphate + H2O to fructose-6-phosphate + Pi.
    • Bypassing Pyruvate Kinase involves two steps:
      • Pyruvate Carboxylase: Pyruvate + HCO3- + ATP → Oxaloacetate + ADP + Pi.
      • PEP Carboxykinase: Oxaloacetate + GTP → PEP + GDP + CO2.

    Role of Acetyl CoA

    • Pyruvate Carboxylase is allosterically activated by acetyl CoA.
    • The demand for oxaloacetate increases during gluconeogenesis, potentially limiting the Krebs cycle.

    Substrate Sources for Gluconeogenesis

    • Predominantly derived from amino acid catabolism during fasting or starvation.
    • Muscle protein degradation supplies amino acids for conversion to gluconeogenesis substrates.
    • Glycerol, from fat breakdown, contributes significantly to gluconeogenesis.

    Reciprocal Regulation of Pathways

    • Glycolysis and gluconeogenesis are reciprocally regulated to prevent futile cycles:
      • Phosphofructokinase is inhibited by ATP, stimulated by AMP.
      • Fructose-1,6-bisphosphatase is inhibited by AMP.
    • High ATP promotes glucose storage as glycogen, while low ATP levels stimulate gluconeogenesis.

    Global Control in Liver Cells

    • Low blood glucose levels trigger glucagon release, activating a cyclic AMP cascade.
    • Protein Kinase A is responsible for phosphorylating and inhibiting glycolysis while stimulating gluconeogenesis.
    • Fructose-2,6-bisphosphate regulates both pathways, enhancing glycolysis while inhibiting gluconeogenesis.

    Key Energy Costs

    • Glycolysis generally yields 2 ATP per glucose molecule processed.
    • Gluconeogenesis expends 6 ATP equivalents to synthesize glucose.
    • A futile cycle of simultaneous glycolysis and gluconeogenesis would waste 4 ATP.

    Summary of Glycolysis and Gluconeogenesis

    • Glycolysis Reaction:
      • Glucose + 2 NAD+ + 2 ADP + 2 Pi → 2 Pyruvate + 2 NADH + 2 ATP.
    • Gluconeogenesis Reaction:
      • 2 Pyruvate + 2 NADH + 4 ATP + 2 GTP → Glucose + 2 NAD+ + 4 ADP + 2 GDP + 6 Pi.

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    Description

    This quiz covers the essential aspects of gluconeogenesis, including its conversion of lactate to glucose and the key enzymes involved. Explore the distinct bypass reactions that differentiate gluconeogenesis from glycolysis and understand the role of this pathway during fasting. Ideal for students studying biochemistry or metabolism.

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