Storage Mechanisms and Control in Carbohydrate Metabolism

Chapter 18: Storage Mechanisms and Control in Carbohydrate Metabolism

Chapter Outline
  • Glycogen Degradation and Production
  • Gluconeogenesis and its Role
  • Control of Carbohydrate Metabolism
  • Alternative Pathways
Glycogen

  • Storage Form of Glucose:

    • Glycogen is stored in response to excess glucose from digestion.
    • Similar to starch but features more branching.
    • Structure allows for multiple glucose molecules to be released simultaneously, enhancing availability for energy production.

  • Energy Source Utilization:

    • Glycogen is crucial for quick energy release, especially during high intensity exercise.
    • Muscle can mobilize glycogen without oxygen, unlike fat oxidation which occurs in mitochondria.
    • Duration and intensity of exercise dictate the proportion of glycogen vs. fat used as fuel, with glycogen being more readily available.
Glycogen Production

  • Energy Requirement:

    • Synthesis of glycogen requires energy from UTP hydrolysis.

  • Stage 1 - Formation of UDPG:

    • Reaction of glucose-1-phosphate with UTP forms uridine diphosphate glucose (UDPG) and pyrophosphate (PPi).
    • Catalyzed by UDP-glucose pyrophosphorylase.

  • Stage 2 - Chain Elongation:

    • UDP-glucose is added to glycogen chain, forming new α(1→4) glycosidic bonds via glycogen synthase.
    • Glycogenin serves as a primer and provides the hydroxyl group for chain initiation.

  • Stage 3 - Branching:

    • Branching enzyme introduces branches by transferring segments from existing chains to form α(1→6) linkages.
Glycogen Breakdown

  • Initiation:

    • Triggered by low blood glucose, glycogen breakdown primarily occurs in liver and muscle.
    • Liver glycogen contributes to blood glucose levels, while muscle glycogen is metabolized on-site for energy.

  • Key Reactions in Breakdown:

    • First Reaction: Glycogen phosphorylase catalyzes the release of glucose-1-phosphate from glycogen via phosphorolysis (not hydrolysis).
    • Second Reaction: Debranching enzymes break α(1→6) linkages, allowing further glucose release.
    • Third Reaction: Glucose-1-phosphate converts to glucose-6-phosphate via phosphoglucomutase.
Control of Glycogen Metabolism
  • Enzymatic Regulation:
    • Glycogen phosphorylase and glycogen synthase serve as primary regulatory sites and are subject to allosteric control and covalent modification.
  • Antagonistic Actions:
    • Enzymes regulating glycogen breakdown often inversely affect those regulating synthesis, maintaining homeostasis in glucose levels.
Gluconeogenesis

  • Conversion of Pyruvate to Glucose:

    • Gluconeogenesis is essential for generating glucose from pyruvate, particularly during starvation or intense exercise.
    • Not merely a reversal of glycolysis due to three irreversible reactions (Steps 1, 3, and 10).

  • Steps Involved:

    • Step 1: Pyruvate is converted to oxaloacetate by pyruvate carboxylase (requires ATP, activated by acetyl-CoA).
    • Step 2: Oxaloacetate is transformed into phosphoenolpyruvate (PEP) by phosphoenolpyruvate carboxykinase (requires GTP).

  • Energy Cost:

    • Gluconeogenesis consumes 4 ATP, 2 GTP, and 2 NADH for each molecule of glucose synthesized.
Reciprocal Regulation in Glucose Metabolism
  • Opposing Pathways:
    • Different enzymes cascade decisions that determine the flow of glucose through glycolysis and gluconeogenesis, often influenced by substrate availability and energy states (AMP, ATP, and citrate).
Pentose Phosphate Pathway
  • Alternative Pathway for Glucose-6-Phosphate:
    • Glucose-6-phosphate can diverge from glycolysis to provide NADPH and five-carbon sugars (e.g., ribose).
  • Oxidative Reactions:
    • First reaction produces NADPH; second reaction decarboxylates 6-phosphogluconate to form ribulose-5-phosphate.
  • Integration with Glycolysis:
    • The pathway connects to glycolysis through the production and utilization of key sugar phosphate intermediates.