Major metabolic pathways

Learning Objectives & Outcomes

  • After studying this chapter, you should be able to:

    • Recall the pathways of glycolysis (including reaction sequence, major intermediates, generation of ATP and regulation)

    • Recall the fates of pyruvate

    • Recall the ‘link pathway’ aka. pyruvate dehydrogenase (substrate, enzymes, carriers, products)

    • Recall the TCA cycle (reactions, yield, function, and regulation)

Oxidation and Reduction

  • Key concepts for this chapter:

    • Reduction:

      1. Addition of H

      2. Removal of O

      3. Addition of electrons

    • Oxidation:

      1. Removal of H

      2. Addition of O

      3. Removal of electrons

    • Forms of Coenzymes:

      • Reduced Form: NADH, FADH2

      • Oxidized Form: NAD+, FAD+

Oxidation and Reduction in Glycolysis

  • Example Reactions:

    • Pyruvate + NADH + H+ ↔ Lactate + NAD+ (reduction of pyruvate to lactate)

    • NAD+ is reduced; dehydrogenase catalyzes oxidation

Overview of Metabolism Pathways

  1. Glycolysis

  2. TCA cycle

  3. Fructose & Galactose metabolism

  4. Hexose monophosphate shunt

  5. Lipid metabolism

  6. Urea cycle

  7. Gluconeogenesis

  8. Glycogen metabolism

Glycolysis: Universal Energy Source

  • Glucose: The universal fuel for human cells

  • Location: Glycolysis occurs in the cytosol

  • Process:

    • Glucose → Pyruvate + NADH + ATP

    • NADH generates more ATP via the electron transport chain

    • Pyruvate can enter TCA cycle (aerobic oxidation) or be converted to lactate (anaerobic)

Glycolysis Phases

  • Phase I (Preparative phase):

    • ATP investment phase - uses ATP to phosphorylate glucose

    • Major products: Fructose 1,6-bisphosphate

  • Phase II (Harvesting phase):

    • ATP-generating; produces 4 ATP and 2 NADH from 2 pyruvate

    • 2 ATP are considered net gain after investment

Enzymes in Glycolysis Investment Phase

  • Hexokinase/Glucokinase:

    • Enzyme responsible for phosphorylation of glucose.

    • Commits glucose to metabolic processes aside from glycolysis.

  • Phosphoglucose Isomerase:

    • Converts glucose 6-phosphate to fructose 6-phosphate (aldose to ketose).

  • Phosphofructokinase (PFK-1):

    • Second phosphorylation step, considered rate-limiting in glycolysis.

    • Irreversible step; heavily regulated.

  • Aldolase:

    • Splits fructose 1,6-bisphosphate into dihydroxyacetone-1,6-bisphosphate, DHAP and glyceraldehyde-3-phosphate, GAP

    • Only GAP continues.

  • Triose-phosphate Isomerase:

    • Converts DHAP into GAP to proceed in glycolysis.

Differences in Hexokinase vs. Glucokinase

  • Hexokinase:

    • Found in most tissues; low Km, high affinity.

    • Phosphorylates glucose even at low concentrations.

  • Glucokinase:

    • Found in liver and pancreas; high Km, responds only at high glucose levels.

    • Acts as a glucose sensor for homeostasis.

Returns Phase in Glycolysis

  • Key Enzymes:

    • GAP-dehydrogenase: Converts GAP to 1,3-Bisphosphoglycerate, 1,3-BPG, yielding NADH.

    • Phosphoglycerate Kinase: Produces first ATP from one phosphate of 1,3-BPG to produce 3-Phosphoglycerate, continuing the energy payoff phase of glycolysis.

    • Phosphoglycero-mutase: Converts 3-Phosphoglycerate to 2-Phosphoglycerate, facilitating the rearrangement necessary for subsequent reactions.

    • Enolase: Removes water to form Phospho-enol-pyruvate, PEP.

    • Pyruvate Kinase: Generates second ATP and produces pyruvate.

Regulation of Glycolysis

  • Essential to control energy generation to prevent surplus.

  • Key Enzymes:

    • Hexokinase: Inhibited by its product Glucose-6-phosphate.

    • Phosphofructokinase (PFK-1): Inhibited by ATP; activated by AMP.

    • Pyruvate Kinase: Activated by fructose-1,6-bisphosphate; inhibited by ATP.

    • Pyruvate Dehydrogenase: Activated by ADP and Calcium ion; inhibited by NADH and acetyl-CoA.

    • Hormonal Regulation: Insulin promotes glycolysis; glucagon inhibits it.

Sequential Chemical Pathway Mechanism for Hormonal Regulation in Glycolysis

1. Insulin Signal Transduction:

  • Receptor Binding: Insulin attaches to its specific receptor on target cells, starting a signaling cascade.

  • Activation of PI3K: The binding activates an enzyme called phosphoinositide 3-kinase (PI3K).

  • Production of PIP3: PI3K converts phosphatidylinositol 4,5-bisphosphate (PIP2) into phosphatidylinositol 3,4,5-trisphosphate (PIP3).

  • Activation of PKB/Akt: PIP3 recruits and activates protein kinase B (PKB/Akt), enhancing its signaling pathways.

  • Increased GLUT4 Translocation: This leads to the movement of glucose transporter type 4 (GLUT4) to the cell membrane, allowing more glucose to enter the cell.

  • Activation of Glycolytic Enzymes: Insulin signaling promotes the activity of enzymes like phosphofructokinase-2 (PFK-2). Higher fructose-2,6-bisphosphate levels activate phosphofructokinase-1 (PFK-1), driving glycolysis forward.

2. Glucagon Signal Transduction:

  • Receptor Binding: Glucagon binds to its receptor on liver cells.

  • Activation of Adenylate Cyclase: This event activates adenylate cyclase, leading to the production of cyclic AMP (cAMP).

  • Activation of PKA: cAMP triggers the activation of protein kinase A (PKA).

  • Inhibition of PFK-2: PKA phosphorylates PFK-2, decreasing fructose-2,6-bisphosphate levels.

  • Decreased Glycolysis: As fructose-2,6-bisphosphate levels drop, so does the activation of PFK-1, resulting in reduced glycolytic activity.

3. Feedback Regulation: These sequential mechanisms precisely regulate glycolysis, ensuring it operates effectively under varying energy availability determined by insulin and glucagon levels.

Enzyme Reactions of Adenylyl Cyclase and Phosphodiesterase

1. Adenylyl Cyclase:

  • Function: Converts ATP into cyclic AMP (cAMP), a secondary messenger.

  • Activation: Stimulated by G-proteins in response to hormones like glucagon.

  • Role in Glycolysis: Increased cAMP activates protein kinase A (PKA), leading to reduced levels of fructose-2,6-bisphosphate and inhibiting glycolysis.

2. Phosphodiesterase:

  • Function: Degrades cAMP and cyclic GMP into AMP and GMP, respectively.

  • Role in Signal Termination: Regulates signaling duration by reducing cyclic nucleotide levels.

  • Impact on Glycolysis: Lowers cAMP levels, thus decreasing PKA activity and potentially increasing glycolysis when glucagon signaling is downregulated.

Fate of Pyruvate

  • Depends on conditions:

    • Oxidative (aerobic): Pyruvate → Acetyl-CoA → TCA cycle.

    • Non-oxidative (anaerobic): Pyruvate → Lactate or ethanol (in yeast).

  • Importance of NAD+: drives glycolysis, regeneration is crucial.

Summary of Pyruvate Fates

  • Aerobic Conditions: Pyruvate → Acetyl-CoA + CO2 + NADH

  • Anaerobic Conditions:

    • Animals: Pyruvate → Lactate + NAD+

    • Yeast: Pyruvate → Ethanol + CO2 + NAD+

Glycolysis Under Different Conditions

  • Anaerobic: Red blood cells convert pyruvate to lactate to regenerate NAD+. Less ATP is produced.

  • Aerobic: Pyruvate enters TCA cycle, yielding high ATP.

2,3-BPG in Oxygen Regulation

  • Function: 2,3-bisphosphoglycerate (2,3-BPG) is a crucial metabolite in red blood cells during glycolysis, modulating hemoglobin's oxygen-binding affinity.

  • Mechanism of Action: Binds to deoxygenated hemoglobin's beta chains, stabilizing the T (tense) state for lower oxygen affinity, facilitating oxygen release during high demand (exercise, altitude).

  • Physiological Importance: Ensures adequate oxygen delivery; elevated 2,3-BPG occurs in chronic hypoxia to enhance oxygen release.

  • Regulation of Synthesis: Influenced by pH, temperature, and substrate concentration; increased acidity raises 2,3-BPG levels for oxygen release.

  • Clinical Relevance: Changes in 2,3-BPG levels affect oxygen delivery in conditions like anemia and COPD.

  • Shift Direction: Causes a rightward shift in the oxygen-hemoglobin dissociation curve, indicating decreased oxygen affinity and increased oxygen release to tissues.

Metabolic Regulation Overview

  • Glycolysis reaction regulated by hormonal signals depending on nutritional status (fed vs. fasting states).

  • Insulin: Lowers blood glucose; promotes glycolysis.

  • Glucagon: Increases blood glucose; inhibits glycolysis.

SDL Questions

• Describe an overview of the reactions of glycolysis.

• How do the names of the first three enzymes of the glycolysis relate to the reactions that they catalyze?

• What is 2,3-BPG? What is its role for those who are adjusting to higher altitude?

• How is glycolysis regulated? What are the key players?

• Why in grey-colored blood sampling test tubes has sodium fluoride in them?

• State and describe the fates of pyruvate in metabolism.

• Why red blood cells generate lactate from pyruvate?