Fructose Metabolism and Enzymes
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Questions and Answers

What is the effect of increased glucagon secretion on fructose-2,6-bisphosphate levels?

  • It causes a fluctuating effect on fructose-2,6-bisphosphate levels.
  • It increases fructose-2,6-bisphosphate levels.
  • It has no effect on fructose-2,6-bisphosphate levels.
  • It decreases fructose-2,6-bisphosphate levels. (correct)

    • How does low blood glucose affect glycolysis and gluconeogenesis?

  • Glycolysis and gluconeogenesis both remain unchanged.
  • Both glycolysis and gluconeogenesis are inhibited.
  • Glycolysis is accelerated while gluconeogenesis is inhibited.
  • Glycolysis slows down and gluconeogenesis is accelerated. (correct)

    • What is the role of protein kinase A in response to low blood glucose?

  • It phosphorylates enzymes involved in glycolysis and gluconeogenesis. (correct)
  • It inactivates all metabolic processes.
  • It decreases cAMP levels, reducing enzyme activity.
  • It enhances glycolysis by activating PFK.

    • What happens to glycogen metabolism when blood glucose levels are low?

    <p>Glycogen breakdown is activated and glycogen synthesis is inactivated.</p> Signup and view all the answers

    In muscle tissue, what differentiates its control mechanism for fructose-2,6-bisphosphate?

    <p>It lacks a phosphorylation site.</p> Signup and view all the answers

    What is the first step in the bypass of the pyruvate kinase reaction during gluconeogenesis?

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

    Which enzyme is responsible for converting fructose-1,6-bisphosphate to fructose-6-phosphate?

    <p>Fructose bisphosphatase</p> Signup and view all the answers

    Which reaction is not directly reversible during gluconeogenesis?

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

    What role does biotin play in gluconeogenesis?

    <p>It serves as a carrier for CO2 in pyruvate carboxylation</p> Signup and view all the answers

    During low blood glucose levels, which hormone's increased secretion affects gluconeogenesis?

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

    Which tissue is primarily responsible for gluconeogenesis?

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

    What is the role of fructose-2,6-bisphosphate during gluconeogenesis?

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

    Which step in gluconeogenesis specifically requires GTP?

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

    What is the effect of increased insulin secretion during high blood glucose?

    <p>Enzyme dephosphorylation</p> Signup and view all the answers

    How does fructose-2,6-bisphosphate affect gluconeogenesis?

    <p>It inhibits FBPase, thereby inhibiting gluconeogenesis</p> Signup and view all the answers

    Which enzyme is inhibited by ATP and citrate as allosteric inhibitors?

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

    What role do glucagon and adrenaline play in the regulation of glycogen metabolism?

    <p>Promote phosphorylation of glycogen phosphorylase and glycogen synthase</p> Signup and view all the answers

    Which statement correctly describes the effect of AMP on PFK and FBPase activities?

    <p>AMP activates PFK while inhibiting FBPase.</p> Signup and view all the answers

    What is one of the key products of the pentose phosphate pathway when using 3x glucose-6-phosphate?

    <p>2 x Fructose-6-phosphate</p> Signup and view all the answers

    Which phase is NOT part of the pentose phosphate pathway?

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

    Which type of tissues primarily utilize the pentose phosphate pathway?

    <p>Tissues synthesizing fatty acids</p> Signup and view all the answers

    What substance does NADPH help reduce to eliminate Reactive Oxygen Species (ROS)?

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

    What common misconception is associated with the pentose phosphate pathway?

    <p>It only produces energy.</p> Signup and view all the answers

    During the oxidative phase of the pentose phosphate pathway, how many carbon atoms are released as CO2?

    <p>3 carbons</p> Signup and view all the answers

    What is the primary importance of producing NADPH in the pentose phosphate pathway?

    <p>To maintain reduced forms of cellular components</p> Signup and view all the answers

    In the non-oxidative stage of the pentose phosphate pathway, which compound is produced along with two molecules of fructose-6-phosphate?

    <p>Glyceraldehyde 3-phosphate</p> Signup and view all the answers

    What enzyme catalyzes the phosphorylation of fructose in the major metabolic pathway in the liver?

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

    What is the product of the cleavage of fructose 1-phosphate by aldolase B?

    <p>Glyceraldehyde and dihydroxyacetone phosphate</p> Signup and view all the answers

    What metabolic defect leads to fructose (sucrose) intolerance in newborns?

    <p>Fructose-1-phosphate aldolase B deficiency</p> Signup and view all the answers

    What condition is caused by the accumulation of fructose-1-phosphate in the liver?

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

    Which metabolic pathway do red blood cells primarily rely on for ATP generation?

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

    What metabolite produced in red blood cells modulates hemoglobin's oxygen affinity?

    <p>2,3-Bisphosphoglycerate</p> Signup and view all the answers

    Which enzyme is inhibited by high concentrations of fructose-1-phosphate affecting ATP generation?

    <p>Aldolase of the glycolytic pathway</p> Signup and view all the answers

    Approximately what percentage of glucose in red blood cells is used to produce 2,3-bisphosphoglycerate?

    <p>25%</p> Signup and view all the answers

    What is a key function of NADPH generated from the oxidative branch of the pentose phosphate pathway in red blood cells?

    <p>To preserve glutathione homeostasis</p> Signup and view all the answers

    Which enzyme directly converts 1,3-bisphosphoglycerate (1,3-BPG) into 2,3-bisphosphoglycerate (2,3-BPG)?

    <p>2,3-bisphosphoglycerate mutase</p> Signup and view all the answers

    Study Notes

    Fructose Metabolism

    • Enzyme hexokinase shows low affinity for fructose, making this a minor metabolic pathway.
    • In the liver, fructokinase phosphorylates fructose to fructose 1-phosphate; this enzyme is also present in the kidney and intestine.
    • Fructose 1-phosphate is split by aldolase B into glyceraldehyde and dihydroxyacetone phosphate (DHAP).

    Fructose Intolerance

    • Fructose intolerance arises from a defect involving fructose-1-phosphate aldolase B, leading to accumulation of fructose-1-phosphate in the liver.
    • Accumulation causes liver damage and jaundice.
    • High fructose-1-phosphate levels inhibit aldolase in glycolysis and glycogen phosphorylase, resulting in reduced ATP generation and hypoglycemia.

    Glycolysis and Red Blood Cells (RBCs)

    • RBCs exclusively depend on glycolysis for ATP production.
    • Around 25% of glucose utilized in RBCs generates 2,3-bisphosphoglycerate (2,3-BPG), crucial for modulating hemoglobin's oxygen affinity.
    • RBCs utilize the oxidative branch of the pentose phosphate pathway (PPP) to synthesize NADPH, maintaining glutathione homeostasis against oxidative stress.

    Importance of Glycolysis in RBCs

    • Glycolysis produces 2,3-BPG, an allosteric effector that facilitates oxygen release from hemoglobin.

    Pentose Phosphate Pathway (PPP)

    • Comprises three phases: Oxidation, Isomerization, and Sugar rearrangement.
    • Utilizing three glucose-6-phosphate yields:
      • 3 CO2
      • 6 NADPH
      • 2 Fructose-6-phosphate
      • Glyceraldehyde
    • Active in tissues synthesizing fatty acids or sterols where NADPH is required; lower activity observed in muscle and brain.

    Carbon Pathway in PPP

    • In oxidative phase, three six-carbon molecules convert to three five-carbon sugars, releasing three CO2.
    • In non-oxidative phase, three five-carbon sugars rearrange to form two six-carbon sugars and one three-carbon compound.

    Importance of NADPH

    • NADPH maintains protein thiol groups in a reduced state and reduces oxidized glutathione.
    • Reduced glutathione detoxifies reactive oxygen species (ROS), preventing membrane damage and hemoglobin alteration from peroxides.

    Glycogenolysis

    • Glycogen phosphorylase cleaves α(1-6) glycosidic bonds during degeneration of glycogen.

    Gluconeogenesis

    • Gluconeogenesis synthesizes glucose from precursors like pyruvate, lactate, and amino acids; it bypasses irreversible glycolytic reactions.
    • Bypass steps involve:
      • Carboxylation of pyruvate to oxaloacetate (catalyzed by pyruvate carboxylase).
      • Conversion of oxaloacetate to phosphoenolpyruvate by phosphoenolpyruvate carboxykinase (PEPCK).
      • Fructose-1,6-bisphosphate being converted to fructose-6-phosphate by fructose bisphosphatase.
      • Glucose-6-phosphate converted to glucose by glucose-6-phosphatase, unique to liver and kidney.

    Regulation of Gluconeogenesis

    • Low blood glucose triggers glucagon secretion, enhancing enzyme phosphorylation.
    • Increases concentration of fructose-2,6-bisphosphate leads to:
      • Inhibition of glycolytic phosphofructokinase-1 (PFK).
      • Activation of fructose-1,6-bisphosphatase, promoting gluconeogenesis.
    • High blood glucose enhances insulin secretion, leading to enzyme dephosphorylation, activating PFK and inhibiting fructose-2,6-bisphosphatase, stimulating glycolysis.

    Control of Glycogenesis and Glycogenolysis

    • Reciprocal regulation of glycogen synthesis and breakdown occurs via phosphorylation and dephosphorylation.
    • Hormonal regulation by glucagon and adrenalin promotes phosphorylation, while insulin promotes dephosphorylation of glycogen synthase and phosphorylase.

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    Description

    This quiz explores the metabolism of fructose, focusing on the role of the enzyme fructokinase in the liver, kidney, and intestine. It also highlights the differences in phosphorylating fructose versus glucose. Test your knowledge on these metabolic pathways and enzyme affinities.

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