Carbohydrates Metabolism in Skeletal Muscle and Liver
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Questions and Answers

What is the major fuel for most tissues?

  • Nucleic acids
  • Proteins
  • Glucose (correct)
  • Lipids

    • Which process converts glucose to pyruvate?

  • Glycolysis (correct)
  • Glycogenesis
  • Gluconeogenesis
  • Krebs cycle

    • What is the end product of anaerobic glycolysis?

  • Lactate (correct)
  • Acetyl-CoA
  • Pyruvate
  • ATP

    • Which pathway does glucose participate in for the synthesis of glycogen?

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

    What is the precursor of fatty acids and cholesterol in metabolism?

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

    How does skeletal muscle utilize glucose as a fuel?

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

    What is the role of insulin in controlling glucose availability for glycolysis in skeletal muscle?

    <p>Regulating transport of glucose into the cell</p> Signup and view all the answers

    Which enzyme in the liver has a high affinity for glucose and acts at a constant rate to provide glucose-6-phosphate under normal conditions?

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

    What is the function of glucokinase in the liver?

    <p>Regulate glucose concentration in hepatic portal blood</p> Signup and view all the answers

    Which cells contain glucokinase to detect high concentrations of glucose?

    <p>Pancreatic β-islet cells</p> Signup and view all the answers

    What happens as more glucose is phosphorylated by glucokinase in the liver?

    <p>Increased glycolysis and ATP formation</p> Signup and view all the answers

    How does increased ATP formation due to glucokinase activity affect membrane potential in pancreatic β-islet cells?

    <p>Causes membrane depolarization</p> Signup and view all the answers

    What is the main end product of glycolysis in fast-growing cancer cells?

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

    Which enzyme catalyzes the phosphorylation of glucose to glucose-6-phosphate in glycolysis?

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

    What causes lactic acidosis in some cases?

    <p>Impaired activity of pyruvate dehydrogenase</p> Signup and view all the answers

    Which molecule is used for gluconeogenesis in the liver, contributing to hypermetabolism in cancer cachexia?

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

    What inhibits hexokinase allosterically in the process of glycolysis?

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

    What happens when there is a shortage of oxygen during glycolysis?

    <p>Impairment of mitochondrial re-oxidation of NADH</p> Signup and view all the answers

    What leads to the fusion of insulin secretory granules with the cell membrane?

    <p>Influx of calcium ions</p> Signup and view all the answers

    Which compound is important at the junction of several metabolic pathways including glycolysis, gluconeogenesis, and glycogenolysis?

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

    How many molecules of ATP are formed in glycolysis per molecule of glucose undergoing the process?

    <p>2 molecules</p> Signup and view all the answers

    Which enzyme catalyzes the conversion of fructose to fructose 1-phosphate in glycolysis?

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

    What is a consequence of fructose bypassing the main regulatory steps in glycolysis?

    <p>Increased lipogenesis in the liver and adipose tissue</p> Signup and view all the answers

    Which step in glycolysis involves a dehydration process, forming phosphoenolpyruvate?

    <p><strong>Enolase</strong></p> Signup and view all the answers

    What inhibits enolase, an enzyme involved in glycolysis?

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

    Why is the reaction catalyzed by pyruvate kinase considered irreversible under physiological conditions?

    <p>Large free energy change involved</p> Signup and view all the answers

    What occurs to pyruvate under anaerobic conditions?

    <p>Reduced to lactate</p> Signup and view all the answers

    Which enzyme catalyzes the conversion of pyruvate to acetyl-CoA under aerobic conditions?

    <p>Not specified in the text</p> Signup and view all the answers

    What happens to the reducing equivalents from NADH formed in glycolysis?

    <p>Taken up into mitochondria for oxidation</p> Signup and view all the answers

    Why is glycolysis inhibited when blood samples are taken for glucose measurement?

    <p>Fluoride inhibits enolase</p> Signup and view all the answers

    What is the main function of the liver in maintaining blood glucose concentration between meals?

    <p>Breaking down glycogen to release glucose</p> Signup and view all the answers

    In gluconeogenesis, substrates like lactate and pyruvate from the cytosol enter the mitochondrion to produce which precursor for glucose synthesis?

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

    Which tissue has a substantial requirement for glucose, even in prolonged fasting?

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

    Which pathway occurs in the cytosol of skeletal muscle for synthesizing fatty acids?

    <p>Fatty acid synthesis</p> Signup and view all the answers

    Where does glycolysis primarily take place within the cell?

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

    What effect does phosphorylation of fructose to fructose 1-phosphate have on glycolysis?

    <p>Bypasses main regulatory steps</p> Signup and view all the answers

    Which enzyme catalyzes the formation of fructose 6-phosphate from glucose 6-phosphate?

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

    Why is phosphoenolpyruvate formed during glycolysis a critical compound?

    <p>It is involved in gluconeogenesis</p> Signup and view all the answers

    Which enzyme catalyzes the conversion of fructose to fructose 1-phosphate in glycolysis?

    <p>Phosphohexose isomerase</p> Signup and view all the answers

    What is the primary outcome of the fusion of insulin secretory granules with the cell membrane?

    <p>Release of insulin</p> Signup and view all the answers

    What is the main function of glucokinase in the liver?

    <p>To regulate the concentration of glucose available to peripheral tissues</p> Signup and view all the answers

    Why is glucokinase present in pancreatic β-islet cells?

    <p>To detect high concentrations of glucose</p> Signup and view all the answers

    What is the result of increased ATP formation due to glucokinase activity in the liver?

    <p>Membrane depolarization and opening of a calcium channel</p> Signup and view all the answers

    What distinguishes glucokinase from hexokinase in terms of affinity for glucose?

    <p>Glucokinase has a high affinity (low Km) for glucose</p> Signup and view all the answers

    How does hexokinase function in the liver under normal conditions?

    <p>Acts at a constant rate to provide glucose-6-phosphate for glycogen synthesis</p> Signup and view all the answers

    Why is pyruvate kinase reaction irreversible under physiological conditions?

    <p>The large free energy change involved</p> Signup and view all the answers

    What happens to NADH formed in glycolysis under anaerobic conditions?

    <p>It is reduced to lactate by lactate dehydrogenase</p> Signup and view all the answers

    What permits the oxidization of NADH and allows another molecule of glucose to undergo glycolysis?

    <p>Formation of lactate from pyruvate</p> Signup and view all the answers

    What happens to pyruvate under aerobic conditions?

    <p>It is transported into mitochondria for oxidation to acetyl-CoA</p> Signup and view all the answers

    Why does enolase require the presence of Mg2+ or Mn2+ ions?

    <p>To act as cofactors for its enzymatic activity</p> Signup and view all the answers

    What is the main fuel used by most tissues?

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

    In glycolysis, what is the end product when it occurs anaerobically?

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

    Which pathway involves glucose in the synthesis of glycogen?

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

    What is the source of reducing equivalents (NADPH) for fatty acid synthesis?

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

    Which compound provides the carbon skeletons for the synthesis of nonessential amino acids?

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

    What does skeletal muscle form aerobically when utilizing glucose as a fuel?

    <p>$CO_2$</p> Signup and view all the answers

    In skeletal muscle, what regulates the availability of glucose for glycolysis?

    <p>Glucose transport into the cell</p> Signup and view all the answers

    What is the function of glucokinase in the liver following a meal?

    <p>Regulate glucose concentration in peripheral tissues</p> Signup and view all the answers

    Which enzyme in the liver has a Km much higher than the normal intracellular concentration of glucose?

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

    What leads to closure of an ATP-potassium channel in pancreatic β-islet cells?

    <p>Increased glycolysis</p> Signup and view all the answers

    Which enzyme provides more glucose 6-phosphate than needed for glycolysis in the liver?

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

    What happens as a result of increased ATP formation due to glucokinase activity in the liver?

    <p>Membrane depolarization</p> Signup and view all the answers

    What is the end product of glycolysis in fast-growing cancer cells?

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

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

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

    What inhibits hexokinase allosterically during glycolysis?

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

    What is the main fuel for most tissues during energy production?

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

    Why does lactic acidosis occur under certain conditions?

    <p>Deficiency of vitamin B1 (thiamin)</p> Signup and view all the answers

    Where does gluconeogenesis take place for converting lactate to glucose in the body?

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

    Why is enolase inhibited by fluoride in blood samples taken for glucose measurement?

    <p>To prevent glycolysis from occurring and altering glucose levels.</p> Signup and view all the answers

    What makes the reaction catalyzed by pyruvate kinase irreversible under physiological conditions?

    <p>The formation of enol-pyruvate as an immediate product.</p> Signup and view all the answers

    What happens to NADH under anaerobic conditions during glycolysis?

    <p>It is used to reduce lactate catalyzed by lactate dehydrogenase.</p> Signup and view all the answers

    Why does pyruvate undergo oxidative decarboxylation to acetyl-CoA under aerobic conditions?

    <p>To permit the oxidization of NADH.</p> Signup and view all the answers

    What is the primary role of lactate dehydrogenase in glycolysis under anaerobic conditions?

    <p>To reduce NADH to lactate.</p> Signup and view all the answers

    Why is the reaction catalyzed by pyruvate kinase considered irreversible?

    <p>Since its immediate product undergoes spontaneous isomerization.</p> Signup and view all the answers

    What is the primary function of skeletal muscle in terms of carbohydrates metabolism?

    <p>Synthesizes muscle protein</p> Signup and view all the answers

    During gluconeogenesis, where do substrates such as lactate and pyruvate enter to yield oxaloacetate as a precursor for glucose synthesis?

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

    What is the significant role of the liver between meals in terms of blood glucose concentration maintenance?

    <p>Maintains blood glucose concentration by breaking down glycogen</p> Signup and view all the answers

    Which tissue has a substantial requirement for glucose even during prolonged fasting?

    <p>Brain tissue</p> Signup and view all the answers

    What is the significance of glycolysis, the pentose phosphate pathway, and fatty acid synthesis occurring in the cytosol?

    <p>Facilitates various metabolic processes within the cell</p> Signup and view all the answers

    What is essential for tissues like the brain and erythrocytes due to their reliance on glucose for fuel?

    <p>Adequate blood concentration of glucose</p> Signup and view all the answers

    What is the role of the pyruvate dehydrogenase complex in glucose metabolism beyond glycolysis?

    <p>Facilitating the conversion of pyruvate to acetyl-CoA for entry into the citric acid cycle</p> Signup and view all the answers

    Why are erythrocytes completely reliant on glucose as their metabolic fuel?

    <p>Due to the absence of mitochondria in erythrocytes</p> Signup and view all the answers

    What effect does anoxic conditions have on glycolysis in skeletal muscle?

    <p>Promotes the rapid production of ATP via glycolysis</p> Signup and view all the answers

    What molecular mechanism is responsible for the inhibition of GAPDH by methylglyoxal in cancer cells?

    <p>Formation of covalent adducts with enzymes</p> Signup and view all the answers

    How does the absence of oxygen influence the function of glycolysis in cancer cells?

    <p>Promotes high rates of glycolysis for ATP production</p> Signup and view all the answers

    Study Notes

    Carbohydrate Metabolism

    • Glucose is the major fuel for most tissues, metabolized to pyruvate through glycolysis
    • Aerobic tissues metabolize pyruvate to acetyl-CoA, which enters the citric acid cycle for complete oxidation to CO2 and H2O, linked to ATP formation
    • Glycolysis can also occur anaerobically, with the end product being lactate

    Glucose Metabolic Role

    • Glucose and its metabolites participate in other processes, such as:
      • Synthesis of glycogen and the pentose phosphate pathway
      • Providing reducing equivalents (NADPH) for fatty acid synthesis
      • Triose phosphate intermediates giving rise to the glycerol moiety of triacylglycerols
      • Pyruvate and citric acid cycle intermediates providing carbon skeletons for nonessential amino acid synthesis
      • Acetyl-CoA being the precursor of fatty acids and cholesterol

    Skeletal Muscle

    • Utilizes glucose as a fuel, both aerobically and anaerobically
    • Stores glycogen as a fuel for muscle contraction and synthesizes muscle protein
    • Accounts for approximately 50% of body mass, storing protein that can be drawn upon to supply amino acids for gluconeogenesis in starvation

    Liver

    • Hexokinase has a high affinity for glucose, providing glucose-6-phosphate to meet the liver's needs
    • Glucokinase has a higher Km than hexokinase and is involved in:
      • Removing glucose from the hepatic portal blood following a meal
      • Regulating the concentration of glucose available to peripheral tissues
      • Providing more glucose 6-phosphate than required for glycolysis, which is used for glycogen synthesis and lipogenesis

    Glycolysis

    • Occurs in the cytosol, with all enzymes being cytosolic
    • Glucose enters glycolysis by phosphorylation to glucose-6-phosphate, catalyzed by hexokinase
    • Hexokinase is inhibited allosterically by its product, glucose-6-phosphate
    • In tissues other than the liver and pancreatic β-islet cells, glucose availability for glycolysis is controlled by transport into the cell, regulated by insulin
    • Glucokinase is also found in pancreatic β-islet cells to detect high concentrations of glucose
    • As more glucose is phosphorylated by glucokinase, there is increased glycolysis, leading to increased ATP formation, which leads to:
      • Closure of an ATP-potassium channel
      • Membrane depolarization
      • Opening of a voltage-gated calcium channel

    Glycolysis Regulation

    • Major sites of regulation are:
      • Hexokinase (and glucokinase)
      • Phosphofructokinase
      • Pyruvate kinase
    • Reactions catalyzed by these enzymes are irreversible
    • Most reactions of glycolysis are freely reversible

    Fructose Metabolism

    • Fructose enters glycolysis by phosphorylation to fructose 1-phosphate, bypassing the main regulatory steps

    • This leads to increased formation of pyruvate and acetyl-CoA, and can contribute to the development of obesity

    • In the liver and adipose tissue, this leads to increased lipogenesis### Enolase Inhibition

    • Enolase is inhibited by fluoride

    • Fluoride is used in blood sample tubes to inhibit glycolysis when measuring glucose levels

    Enolase Dependence

    • Enolase requires the presence of either Mg2+ or Mn2+ ions

    Pyruvate Kinase Reaction

    • Pyruvate kinase catalyzes the transfer of the phosphate from phosphoenolpyruvate to ADP, producing two molecules of ATP per glucose molecule
    • The reaction is irreversible under physiological conditions due to:
      • Large free energy change
      • Spontaneous isomerization of enol-pyruvate to pyruvate, making the product unavailable for the reverse reaction

    Anaerobic Conditions

    • Under anaerobic conditions, NADH cannot be reoxidized through the respiratory chain
    • Pyruvate is reduced to lactate by lactate dehydrogenase, permitting the oxidation of NADH
    • This allows another glucose molecule to undergo glycolysis

    Aerobic Conditions

    • Under aerobic conditions, pyruvate is transported into mitochondria
    • Pyruvate undergoes oxidative decarboxylation to acetyl-CoA and then oxidation to CO2 in the citric acid cycle
    • Reducing equivalents from NADH formed in glycolysis are taken up into mitochondria for oxidation

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    Carbohydrates Metabolism PDF

    Description

    This quiz covers the metabolism of carbohydrates in tissues such as skeletal muscle and liver. Topics include glucose transport into cells, regulation by insulin, glycogen synthesis, and the role of enzymes like hexokinase in the liver.

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