Biochem: Pathways
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Which organ is primarily responsible for gluconeogenesis?

  • Muscle
  • Kidney
  • Liver (correct)
  • Adipose Tissue
  • What inhibits the fructose bisphosphatase reaction in gluconeogenesis?

  • AMP (correct)
  • Glycogen
  • Acetyl CoA
  • Glucagon
  • Which of the following substrates can be converted into glucose through gluconeogenesis?

  • Lactate (correct)
  • Fatty Acids
  • Sodium bicarbonate
  • Triglycerides
  • What is the primary effect of glucagon during fasting?

    <p>Activating gluconeogenesis</p> Signup and view all the answers

    Which statement about the pyruvate carboxylase reaction is true?

    <p>It is inactive without acetyl CoA</p> Signup and view all the answers

    Which type of glycogen storage disorder is most commonly associated with a deficiency in glucose-6-phosphatase?

    <p>Von Gierke’s disease</p> Signup and view all the answers

    Which of the following clinical manifestations is NOT commonly associated with glycogen storage disorders?

    <p>Acute pancreatitis</p> Signup and view all the answers

    What is the estimated incidence of glycogen storage disorders in live births?

    <p>1 case per 20,000 to 43,000 live births</p> Signup and view all the answers

    Which type of glycogen storage disease is specifically noted to have a higher incidence among Jews in northern Africa?

    <p>McArdle disease</p> Signup and view all the answers

    Which enzyme deficiency is associated with Pompe's disease?

    <p>Lysosomal acid α-1,4 glucosidase</p> Signup and view all the answers

    What type of glycosidic bonds are formed at the branch points of glycogen?

    <p>α-(1→6) glycosidic bonds</p> Signup and view all the answers

    Which enzyme phosphorylates glucose to initiate glycolysis?

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

    What is the main hormone active during the synthesis of glycogen?

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

    Which step in the glycogenesis pathway is considered the rate-limiting step?

    <p>Action of glycogen synthase</p> Signup and view all the answers

    What is produced at the end of the glycogenesis pathway?

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

    Which enzyme is responsible for the conversion of isocitrate to α-ketoglutarate in the Kreb's cycle?

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

    In the Kreb's cycle, which end product is generated alongside 3 NADH and 1 FADH2?

    <p>2 CO2 and 1 GTP</p> Signup and view all the answers

    Which of the following statements about citrate in the Kreb's cycle is NOT true?

    <p>Citrate stimulates the enzyme PFK-1.</p> Signup and view all the answers

    Which factor does NOT activate isocitrate dehydrogenase in the Kreb's cycle?

    <p>High ATP levels</p> Signup and view all the answers

    What is the primary location of the Kreb's cycle in eukaryotic cells?

    <p>Mitochondrial matrix</p> Signup and view all the answers

    What is the primary reaction catalyzed by Isocitrate Dehydrogenase in the TCA cycle?

    <p>Oxidative decarboxylation of Isocitrate to α-ketoglutarate</p> Signup and view all the answers

    Which of the following statements about Succinyl-CoA Synthetase is correct?

    <p>It performs substrate-level phosphorylation to produce GTP or ATP.</p> Signup and view all the answers

    What inhibits the α-ketoglutarate Dehydrogenase Complex?

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

    Which enzyme is responsible for the hydration of Fumarate to Malate?

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

    What is a characteristic result of the reaction catalyzed by Malate Dehydrogenase?

    <p>Elevation of NADH levels</p> Signup and view all the answers

    What is a primary characteristic of the Krebs cycle?

    <p>It serves both anabolic and catabolic functions.</p> Signup and view all the answers

    Which compound is identified as the central molecule of biochemistry in the context of the Krebs cycle?

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

    During fasting, from where does acetyl CoA primarily originate?

    <p>Beta oxidation of fatty acids</p> Signup and view all the answers

    What factors regulate the rate of the Krebs cycle?

    <p>Energy requirement of the cell and necessity for carbon precursors</p> Signup and view all the answers

    Which of the following accurately describes transamination and deamination in relation to the Krebs cycle?

    <p>They produce pyruvate and other intermediates from amino acids.</p> Signup and view all the answers

    What is the role of glucokinases in glucose metabolism?

    <p>They allow for liver accumulation of glucose stores despite G6P levels.</p> Signup and view all the answers

    Which step of glycolysis is identified as both the rate-limiting and the second irreversible step?

    <p>Phosphorylation of fructose-6-phosphate.</p> Signup and view all the answers

    What intermediate is formed during glycolysis that affects oxygen dissociation from hemoglobin?

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

    Which molecule is produced during substrate-level phosphorylation in glycolysis?

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

    During the oxidation of glyceraldehyde phosphate, what is reduced?

    <p>NAD+.</p> Signup and view all the answers

    What is the primary function of glycolysis in glucose metabolism?

    <p>Converts glucose into 3-C compounds for energy</p> Signup and view all the answers

    Which enzyme is responsible for the first irreversible step of glycolysis?

    <p>Phosphofructokinase-1</p> Signup and view all the answers

    Under which condition do tissues like the brain continue to utilize glucose despite low blood glucose levels?

    <p>Low Km hexokinase activity</p> Signup and view all the answers

    What occurs to glucose-6-phosphate in the first step of glycolysis?

    <p>It undergoes phosphorylation to form fructose-6-phosphate</p> Signup and view all the answers

    How does the activity of glucokinase change in response to elevated postprandial blood glucose levels?

    <p>It becomes significantly active</p> Signup and view all the answers

    What is the main product of glycolysis under aerobic conditions?

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

    What is required for the transport of NADH across the inner mitochondrial membrane?

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

    When is glycolysis activated?

    <p>During the well-fed state after a meal</p> Signup and view all the answers

    Which of the following is NOT a step in glycolysis?

    <p>Formation of acetyl-CoA</p> Signup and view all the answers

    Under which condition does anaerobic glycolysis occur?

    <p>In the absence of mitochondria or oxygen</p> Signup and view all the answers

    Study Notes

    Gluconeogenesis

    • Converts non-carbohydrates to glucose/glycogen to prevent hypoglycemia during fasting.
    • Predominantly occurs in liver (90%) and kidneys (10%), not in muscle or adipose tissue.
    • Reversed by glucose-6-phosphate; hexokinase is the first irreversible step in glycolysis but not committed.
    • Substrates include glucogenic amino acids, lactate (via the Cori cycle), and glycerol.

    Regulation of Gluconeogenesis

    • Pyruvate carboxylase is inactive without acetyl CoA.
    • Fructose bisphosphatase is inhibited by AMP.
    • Glucagon is the hormone active during fasting.

    Glycogen Structure and Function

    • Glycogen exists as granules in the cytosol and serves as an important fuel reserve.
    • Functions to buffer blood-glucose levels, especially via glucose-6-phosphate.
    • More glycogen is stored in muscle compared to liver, correlating with tissue mass.

    Glycogenesis

    • Occurs in energy-excess states, primarily in liver and muscles.
    • Involves the conversion of glucose to glycogen:
      • Glycogen synthase forms α-(1→4) glycosidic bonds.
      • Activated glucose is in the form of UDP-glucose.
    • Rate-limiting step may involve enzyme deficiency or dysfunction affecting glucose-to-glycogen conversion.

    Glycogen Storage Disorders (GSD)

    • Incidence of GSDs estimated at 1 in every 20,000-43,000 live births; no significant ethnic differences.
    • Von Gierke’s disease (GSD Type I) - Glucose-6-phosphatase deficiency, causing fasting hypoglycemia and lactic acidosis. Affects liver, intestinal mucosa, and kidneys.
    • Pompe’s disease (GSD Type II) - Deficiency of lysosomal acid α-1,4 glucosidase leads to glycogen accumulation in lysosomes.

    Other Glycogen Storage Disorders

    • McArdle’s disease (GSD Type V) - Phosphorylase deficiency in skeletal muscle causing myoglobinuria, fatigue, and exercise intolerance.
    • Cori’s disease (GSD Type III) - Glycogen debranching enzyme deficiency, leading to abnormally structured glycogen.
    • Andersen’s disease (GSD Type IV) - Branching enzyme deficiency results in abnormal glycogen with fewer branch points.
    • Hers disease (GSD Type VI) - Liver glycogen phosphorylase deficiency leading to enlarged liver.
    • Tarui’s disease (GSD Type VII) - Muscle phosphofructokinase deficiency causing exercise-related symptoms.

    Clinical Manifestations and Treatment

    • Symptoms of GSDs vary by type; common issues include fatigue, exercise intolerance, and lactic acidosis.
    • Treatment approaches may involve high-protein diets and controlled exercise.

    Kreb's Cycle Overview

    • Also known as the tricarboxylic acid (TCA) cycle, it is the final common pathway for aerobic oxidation of carbohydrates, lipids, and proteins.
    • Known as an amphibolic pathway, functioning in both catabolic and anabolic processes.

    Function and Importance

    • Major source for ATP production.
    • Provides key substrates for gluconeogenesis, amino acid synthesis, and fatty acid synthesis.

    Location

    • Occurs within the mitochondrial matrix of all cells containing mitochondria.
    • Exception: succinate dehydrogenase is located on the inner mitochondrial membrane.

    Substrate and End Products

    • Main substrate is Acetyl-CoA.
    • Produces: 2 CO2, 1 GTP, 3 NADH, and 1 FADH2 per cycle.

    Rate-Limiting Step

    • Isocitrate to α-ketoglutarate is the rate-limiting step, catalyzed by isocitrate dehydrogenase.

    Key Enzymes and Reactions

    • Citrate Synthase: Condenses Acetyl-CoA with oxaloacetate to form citrate.
    • Aconitase: Converts citrate to isocitrate; inhibited by fluoroacetate.
    • Isocitrate Dehydrogenase: Catalyzes the oxidative decarboxylation of isocitrate to α-ketoglutarate.
    • α-Ketoglutarate Dehydrogenase Complex: Converts α-ketoglutarate to succinyl-CoA, yielding CO2 and NADH.
    • Succinyl-CoA Synthetase: Associated with substrate-level phosphorylation producing GTP (or ATP).
    • Succinate Dehydrogenase: Oxidizes succinate to fumarate, yielding FADH2.
    • Fumarase: Hydrates fumarate to malate.
    • Malate Dehydrogenase: Oxidizes malate to oxaloacetate, yielding NADH.

    CO2 and Energy Production

    • Key steps that produce CO2: oxidative decarboxylation reactions.
    • Key steps that yield NADH and FADH2 accompany the generation of energy through electron transport.

    Inhibition and Regulation

    • Inhibitors include arsenite, fluoroacetate, and malonate, affecting different enzymes in the cycle.
    • Regulation depends on energy requirements and the need for carbon precursors.

    Additional Pathways

    • Kreb’s cycle intermediates support other pathways:
      • Gluconeogenesis: Key for producing glucose precursors.
      • Fatty Acid Synthesis: Acetyl-CoA is a major building block.
      • Heme Synthesis: Succinyl-CoA combines with glycine in heme formation.

    ATP Yield

    • Complete oxidation from one Acetyl-CoA molecule results in approximately 10 ATP.
    • Complete oxidation of glucose yields around 30 to 32 ATP, depending on shuttle systems.

    Summary of Key Facts

    • Kreb’s cycle plays a crucial role in metabolism and energy production.
    • Central molecule is Acetyl-CoA, with various pathways determined by the nutritional state (well-fed vs fasting).
    • Takes place in mitochondrial matrix, barring the succinate dehydrogenase step in the inner mitochondrial membrane.

    Glycolysis Overview

    • Major pathway for glucose metabolism, converting glucose to 3-carbon compounds for energy.
    • Occurs in the cytosol, primarily through the Embden-Meyerhof-Parnas pathway.
    • Substrate is glucose; end products are two molecules of either pyruvate or lactate.
    • Key regulation point is Step 1: Phosphorylation of glucose by phosphofructokinase-1.

    ATP Yield in Glycolysis

    • ATP produced through substrate-level phosphorylation, generating 2 ATP molecules per glucose molecule during glycolysis.
    • Formation of pyruvate is the primary outcome of glycolysis, while lactate is produced under anaerobic conditions.

    Aerobic vs. Anaerobic Glycolysis

    • Aerobic glycolysis occurs in cells with mitochondria and adequate O2; yields pyruvate.
    • Anaerobic glycolysis occurs in cells without mitochondria or with insufficient O2; reduces pyruvate to lactate.
    • NADH is recycled to NAD+ via lactate formation under anaerobic conditions, crucial for maintaining glycolytic flux.

    Key Steps of Glycolysis

    • Step 1: Phosphorylation of glucose to glucose-6-phosphate (irreversible).
    • Step 2: Isomerization of glucose-6-phosphate to fructose-6-phosphate.
    • Step 3: Phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate (ratelimit step).
    • Step 4-5: Cleavage of fructose-1,6-bisphosphate into two 3-carbon units.
    • Step 6: Oxidation of glyceraldehyde phosphate to 1,3-bisphosphoglycerate, involves reduction of NAD+.
    • Step 10: Formation of pyruvate from phosphoenolpyruvate (PEP), also a substrate-level phosphorylation step.

    Regulation of Glycolysis

    • Glucokinase operates in the liver, actively trapping glucose during hyperglycemia, and is not inhibited by G6P.
    • Hexokinases (I, II, III) are allosterically inhibited by G6P, regulating glucose usage in peripheral tissues.
    • Fructose-1,6-bisphosphate activates pyruvate kinase, while glucagon inhibits glucose metabolism.

    Pyruvate Dehydrogenase Complex

    • Pyruvate is oxidized to acetyl-CoA in the mitochondria if oxygen is present.
    • Key cofactors for the complex include NAD+ and coenzyme A; NADH generated enters the electron transport chain.

    Luebering-Rapoport Pathway (RBCs)

    • Metabolic bypass in red blood cells that produces 2,3-bisphosphoglycerate (2,3-BPG).
    • 2,3-BPG lowers hemoglobin's affinity for oxygen, facilitating oxygen release in tissues.
    • Involves two enzymes: Bisphosphoglycerate mutase and 2,3-BPG phosphatase.

    Importance of Regulation

    • Ensures efficient management of glucose based on energy demands and availability.
    • Blood glucose levels influence liver and peripheral tissue usage through glucokinase and hexokinase activity.

    Clinical Relevance

    • Lactic acidosis can result from intense exercise, septic shock, and impaired oxygen delivery.
    • Anaerobic metabolism becomes prominent in conditions where oxygen is scarce, leading to lactate accumulation.

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    Description

    Explore the key concepts and mechanisms involved in gluconeogenesis, including the conversion of non-carbohydrates into glucose or glycogen. This quiz covers the roles of enzymes such as glucose-6-phosphate and emphasizes the physiological importance of gluconeogenesis in the liver and kidneys.

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