Gluconeogenesis and Glycogen Metabolism Study Notes

Overview of Gluconeogenesis and Glycogen Metabolism

Highlights from the Last Lecture

  • Gluconeogenesis: Overall reaction and energy sources

  • Role in maintaining blood glucose levels

  • Discussion of the Cori cycle

  • Glycogen degradation processes

  • Key enzymes involved in glucose metabolism

  • Differences in glycogen metabolism between muscle and liver

  • Roles of hormones: epinephrine and glucagon

Hormonal Control of Glycogen Breakdown

  • Glucagon and Epinephrine:

    • These hormones stimulate the breakdown of glycogen.

    • Muscular activity leads to increased epinephrine release:

    • Epinephrine stimulates glycogen breakdown primarily in muscle, and to a lesser extent in the liver.

    • Glucagon:

    • More responsive in the liver, indicating a starved state where glucose must be released into the bloodstream.

Glycogen Synthesis

General Pathways

  • Glycogen synthesis and degradation occur via distinct pathways.

  • Glycogen degradation yields glucose 1-phosphate via glycogen phosphorylase.

  • UDP-glucose is utilized as the monomer for extending glycogen chains.

Synthesis of UDP-Glucose

  • Reactive Conversion:

    • UDP-glucose is synthesized from glucose 1-phosphate and uridine triphosphate (UTP) in a reaction catalyzed by UDP-glucose pyrophosphorylase:

    • Reaction: Glucose 1-phosphate+UTP→UDP-glucose+PPi\text{Glucose 1-phosphate} + \text{UTP} \rightarrow \text{UDP-glucose} + \text{PPi}

      • Here, pyrophosphate (PPi) is released as a byproduct.

    • This reaction is readily reversible.

Role of Glycogen Synthase

  • Catalysis of Glucose Transfer:

    • Glycogen synthase catalyzes the transfer of glucose from UDP-glucose to nonreducing terminal residues of existing glycogen chains.

    • Forms an α-1,4-glycosidic linkage:

    • Reaction description:

      • The terminal hydroxyl group of glycogen displaces UDP when connected.

    • Glycogen Synthase Regulation:

    • Key regulatory enzyme for glycogen synthesis.

    • Exists in two isozymic forms:

      1. Liver variant

      2. Muscle and other tissues variant

    • Function: Adds glucosyl residues only to a polysaccharide chain with more than four residues, necessitating a primer.

Involvement of Glycogenin

  • Glycogenin's Role:

    • Serves as the primer for glycogen synthase.

    • Each subunit synthesizes α-1,4-glucose polymer chains on its partner subunit with lengths of 10 to 20 glucosyl units.

Formation of α-1,6 Linkages by Branching Enzyme

  • Importance of Branching:

    • Glycogen synthase forms only α-1,4 linkages; additional enzyme activity is required for α-1,6 linkages.

    • Benefits include:

    • Increased solubility of glycogen.

    • Create more terminal residues for action sites of enzymes like glycogen phosphorylase and synthase.

    • Enhances the rate of both glycogen synthesis and degradation.

Mechanism of Branching

  • Branching Process:

    • Occurs after several glucosyl residues are linked via α-1,4 linkages.

    • An α-1,6 branch is formed by breaking an α-1,4 link and creating an α-1,6 link.

    • A block of 7 residues is transferred to a more interior site, needing to include the nonreducing terminus of chains ≥ 11 residues long.

    • New branch must be ≥ 4 residues away from an existing one.

Regulatory Mechanisms

  • Forms of Glycogen Synthase:

    • 1. Active nonphosphorylated form (a)

    • 2. Inactive phosphorylated form (b)

  • Activator: Glucose 6-phosphate is a potent activator of glycogen synthase.

Glycogen Efficiency and Storage

  • Energy Considerations:

    • Only 2 ATP molecules are required to convert dietary glucose to glycogen.

    • Complete oxidation of glucose from glycogen yields 31 ATP molecules.

Reciprocal Regulation of Glycogen Metabolism

  • Glycogen synthesis is inhibited through pathways activated by glucagon and epinephrine that stimulate glycogen breakdown.

Insulin's Role in Glycogen Synthesis

  • Stimulation of Glycogen Synthesis:

    • Insulin activates a signal transduction pathway that promotes glycogen synthesis.

    • Increases availability of glucose transporters (GLUT4) to facilitate glucose uptake.

Regulation of Blood-Glucose Concentration by the Liver

  • Impact of High Blood Glucose Levels:

    • Inhibits glycogen degradation (glycogen phosphorylase) while promoting glycogen synthesis (glycogen synthase).

Clinical Insight: Diabetes Mellitus

  • Diabetes Characteristics:

    • Result of insulin insufficiency and glucagon excess.

    • Leads to high glucose levels and underutilization of glucose as fuel, causing it to appear in urine.

    • Type 1 Diabetes: Insulin is not produced.

    • Type 2 Diabetes: Insulin is produced but not effectively utilized due to insulin resistance.

    • Historical Note: The term diabetes was used in the second century AD by Aretaeus, comparing the condition to “an excess passage of water like a siphon," while 'Mellitus' is derived from Latin meaning “sweetened with honey."

Next Lecture Information and Upcoming Exam

  • Next Lecture Topic: Dr. Brown will discuss glycogen synthesis, after the midterm.

  • Midterm #1 Date: January 28, 2026; covers all material excluding glycogen synthesis.