Gluconeogenesis: Conversion of Pyruvate to PEP
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Questions and Answers

Which enzyme is responsible for the conversion of pyruvate to oxaloacetate in the first step of gluconeogenesis?

  • Phosphoenolpyruvate carboxykinase
  • Pyruvate carboxylase (correct)
  • Malate dehydrogenase
  • Pyruvate kinase

    • What is the role of malate dehydrogenase in gluconeogenesis?

  • To convert oxaloacetate to pyruvate
  • To transport oxaloacetate across the mitochondrial membrane
  • To phosphorylate oxaloacetate to phosphoenolpyruvate
  • To convert oxaloacetate to malate for transport out of the mitochondria (correct)

    • Which molecule serves as the phosphate group donor in the conversion of oxaloacetate to phosphoenolpyruvate by phosphoenolpyruvate carboxykinase?

  • ADP
  • GDP
  • ATP
  • GTP (correct)

    • Why is the conversion of pyruvate to phosphoenolpyruvate in gluconeogenesis not simply the reverse of the pyruvate kinase reaction in glycolysis?

    <p>All of the above</p> Signup and view all the answers

    What is the role of biotin in the conversion of pyruvate to oxaloacetate?

    <p>It acts as a coenzyme for pyruvate carboxylase</p> Signup and view all the answers

    Which of the following is NOT a glucogenic precursor?

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

    Why is the conversion of pyruvate to phosphoenolpyruvate considered a bypass reaction in gluconeogenesis?

    <p>Because it bypasses a step in glycolysis that is irreversible</p> Signup and view all the answers

    What is the primary function of phosphoenolpyruvate carboxykinase in gluconeogenesis?

    <p>To decarboxylate and phosphorylate oxaloacetate to phosphoenolpyruvate</p> Signup and view all the answers

    Which of the following is NOT a precursor used for gluconeogenesis in animals?

    <p>Fatty acids</p> Signup and view all the answers

    What is the role of pyruvate carboxylase in gluconeogenesis?

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

    Which enzyme is responsible for converting glycerol to glycerol 3-phosphate in gluconeogenesis?

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

    Which bypass reaction occurs in gluconeogenesis to circumvent pyruvate kinase?

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

    Which compound can be directly converted into dihydroxyacetone phosphate in gluconeogenesis?

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

    In which tissue is the enzyme responsible for the final step of gluconeogenesis absent?

    <p>Skeletal muscle</p> Signup and view all the answers

    What is the primary function of phosphoenolpyruvate carboxykinase in gluconeogenesis?

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

    Which condition primarily leads to an increase in glycerol use for gluconeogenesis?

    <p>Prolonged fasting</p> Signup and view all the answers

    What is the primary role of the oxaloacetate-malate shuttling system in gluconeogenesis?

    <p>To transport NADH into the cytosol for use in gluconeogenesis.</p> Signup and view all the answers

    Why is the conversion of pyruvate to oxaloacetate necessary in gluconeogenesis when lactate is the precursor?

    <p>To provide a substrate for the mitochondrial isozyme of PEP carboxykinase.</p> Signup and view all the answers

    Which enzyme catalyzes the conversion of fructose 1,6-bisphosphate to fructose 6-phosphate in gluconeogenesis?

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

    Where is the enzyme glucose 6-phosphatase located?

    <p>Lumen of the endoplasmic reticulum</p> Signup and view all the answers

    Which of the following statements correctly describes the role of pyruvate carboxylase in gluconeogenesis?

    <p>Pyruvate carboxylase catalyzes the conversion of pyruvate to oxaloacetate in the cytosol.</p> Signup and view all the answers

    What is the primary role of the mitochondrial isozyme of PEP carboxykinase in gluconeogenesis?

    <p>To transport PEP out of the mitochondrion.</p> Signup and view all the answers

    Which of the following statements accurately describes the relationship between glycolysis and gluconeogenesis?

    <p>Gluconeogenesis bypasses some of the irreversible steps in glycolysis using different enzymes.</p> Signup and view all the answers

    Which of the following is NOT a glucogenic precursor that can be used in gluconeogenesis?

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

    Study Notes

    Conversion of Pyruvate to Phosphoenolpyruvate

    • Pyruvate is converted to phosphoenolpyruvate (PEP) during gluconeogenesis through a two-step process.
    • The first step involves pyruvate carboxylase converting pyruvate to oxaloacetate in the mitochondrial matrix, requiring biotin.
    • Due to the lack of efficient transporters for oxaloacetate, it is reduced to malate to facilitate its export from mitochondria.
    • Malate is then re-oxidized to oxaloacetate in the cytosol by malate dehydrogenase, producing NADH from NAD+.
    • Phosphoenolpyruvate carboxykinase catalyzes the conversion of oxaloacetate to PEP, using GTP as a phosphate donor and requiring Mg2+.
    • The pathway utilized may vary depending on the glucogenic precursor; if pyruvate is derived from alanine or transported from the cytosol, similar steps occur.
    • In cases where lactate is the precursor, it is first converted to pyruvate, then directly to PEP by the mitochondrial isozyme of PEP carboxykinase.

    Conversion of Fructose 1,6-Bisphosphate to Fructose 6-Phosphate

    • The second step of gluconeogenesis bypasses phosphofructokinase-1 (PFK-1) in glycolysis.
    • Fructose 1,6-bisphosphate is dephosphorylated to fructose 6-phosphate by fructose 1,6-bisphosphatase, which requires Mg2+.
    • This reaction occurs in the cytosol without ATP production.

    Conversion of Glucose 6-Phosphate to Glucose

    • The third step bypasses hexokinase from glycolysis.
    • Glucose 6-phosphate is dephosphorylated to glucose by glucose 6-phosphatase, which resides in the endoplasmic reticulum (ER).
    • Glucose 6-phosphate must be transported into the ER for dephosphorylation before glucose and phosphate are exported.

    Substrates for Gluconeogenesis

    • Non-carbohydrate precursors utilized in gluconeogenesis include most amino acids, glycerol, and TCA cycle intermediates.
    • Lactate, produced by skeletal muscles and erythrocytes, is converted to pyruvate in the liver by lactate dehydrogenase for gluconeogenesis.
    • Amino acids enter gluconeogenesis via transamination or deamination, producing pyruvate or TCA cycle intermediates.
    • Glycerol, derived from the breakdown of triacylglycerols in adipose tissues, enters gluconeogenesis as dihydroxyacetone phosphate (DHAP).
    • Glycerol is first phosphorylated to glycerol 3-phosphate using ATP, then oxidized to DHAP, producing NADH from NAD+.
    • Prolonged fasting increases glycerol's contribution to gluconeogenesis, accounting for approximately 20% of glucose production.

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    Related Documents

    Gluconeogenesis PDF

    Description

    Learn about the first bypass reaction of gluconeogenesis, where pyruvate is converted to phosphoenolpyruvate via pyruvate carboxylase and other enzymes.

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