18/3 Gluconeogenesis, Glycogen & Glucagon Signalling

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  • gluconeogenesis - glucose production from non-carbs

  • glycogen stores glucose

  • gluconeogenesis controlled by glucagon

  • key steps of gluconeogensis is controlled by phosphorylation

  • signal transduction

Physiological Context & Importance of Glucose

  • Humans can survive ~1 month without ingesting calories provided water is available.

  • Maintaining blood glucose at ≈4–5 mM is critical.

    • Mild hypoglycaemia (≈3–4 mM): lethargy, hunger.

    • Severe hypoglycaemia (<3 mM): poor judgement ➝ disorientation ➝ coma ➝ death.

  • Two complementary strategies keep plasma glucose constant when dietary carbohydrate is absent:

    • Glycogenolysis – mobilises stored glycogen (rapid, short-term).

    • Gluconeogenesis (GNG) – de-novo synthesis of glucose from non-carbohydrate precursors (slower, long-term).

Major Pathways – Direction & Location

  • Glycolysis (red in slides): Glucose ⟶ Pyruvate (energy-yielding, generating ATP). Predominantly cytosolic, universal.

  • Gluconeogenesis (blue/green arrows): Pyruvate/PEP ⟶ Glucose (energy-consuming). Mainly in liver; minor contribution from kidney cortex (especially from glutamine).

  • Key organs

    • Liver: central to both glycogen turnover & gluconeogenesis; exports glucose to blood.

    • Kidney: backup GNG (≈10 % during prolonged fast).

    • Skeletal muscle: large glycogen store for local use; lacks glucagon receptors, therefore not a source of blood glucose.

      red not reversible

Non-Carbohydrate Precursors Feeding GNG

  • Lactate – generated during anaerobic glycolysis in muscle/RBCs; enters liver via Cori cycle.

  • Amino acids (esp. alanine & glutamine) – released from muscle proteolysis.

  • Glycerol – liberated from adipose triglyceride breakdown; enters pathway at dihydroxy-acetone-phosphate (DHAP).

    • Fatty-acid carbon cannot yield net glucose, but glycerol backbone can.

Irreversible Steps & Bypass Reactions

Certain glycolytic steps have large negative free energies ( \Delta G^{\circ '} \ll 0 ) and must be circumvented in GNG:

  1. Pyruvate kinase step

    • Glycolysis: PEP→pyruvate

    • GNG bypass: Pyruvate →oxaloacetate→PEP (pyruvate carboxylase)

  2. PFK-1 step

    • Glycolysis: Fructose-6-P ⟶ Fructose-1,6-bis-P.

    • GNG bypass: Fructose-1,6-bis-P ⟶ Fructose-6-P (Fructose-1,6-bisphosphatase-1).

  3. Hexokinase/Glucokinase step

    • Glycolysis: Glucose ⟶ Glucose-6-P.

    • GNG bypass (liver, kidney): Glucose-6-P ⟶ Glucose + P_i (Glucose-6-phosphatase, ER-membrane).

Hormonal Control – Insulin vs Glucagon

insulin - stops sugar production

glucagon - encourages sugar production

  • Fed state (high glucose):

    • ↑ Insulin; ↓ Glucagon.

    • Promotes glycogen synthesis & glycolysis; suppresses GNG.

  • Fasting state (low glucose):

    • ↓ Insulin; ↑ Glucagon.

    • Activates glycogenolysis, GNG, lipolysis.

  • Glucagon target tissues

    • Liver & adipose tissue: YES.

    • Skeletal muscle: NO (critical exam fact).

  • Adrenaline (epinephrine) fulfils glucagon-like role in muscle during exercise.

Glucagon Signal-Transduction Cascade (Classical G_s Pathway)

In essence, signal transduction allows glucagon's message to quickly and efficiently turn off glucose-consuming processes and turn on glucose-producing processes like gluconeogenesis in the liver, helping to maintain stable blood sugar levels

  • 1° messenger: Glucagon (peptide) binds GPCR (7-TM receptor) on hepatocyte membrane.

  • Receptor undergoes conformational change ➝ activates heterotrimeric G_s protein (GDP → GTP exchange on G_\alpha).

  • G_\alpha(GTP) dissociates and stimulates Adenylyl cyclase (AC).

  • AC converts ATP → cAMP (2° messenger). 1 receptor → many G_proteins → many cAMP (AMPLIFICATION).

  • Protein kinase A (PKA) binds 4 cAMP → releases catalytic subunits → broad phosphorylation program.

  • Termination: cAMP → AMP by phosphodiesterase; intrinsic GTPase activity resets G_\alpha.

Enzyme Regulation by Phosphorylation

  • Residues modified: Ser, Thr, Tyr (hydroxyl-containing).

  • Phosphorylation can either activate OR inhibit a target enzyme (conformational/charge effects).

    • Added negative charge alters electrostatic & H-bond network.

  • Kinase ⇆ phosphatase opponent pairs allow rapid reversibility.

  • protein kinase: adds phosphate to amino acid side chain

  • protein phosphatase: removes phosphate from amino acid side chain

Key PKA Targets in Carbohydrate Metabolism

1. Pyruvate kinase (liver isoform)

  • PKA phosphorylation → inactive form.

  • Consequence: blocks final glycolytic step, diverts PEP toward GNG.

2. Bifunctional Enzyme PFK-2 / FBPase-2

  • Single polypeptide with kinase domain (makes Fructose-2,6-bis-P) & phosphatase domain (hydrolyses it).

  • Fructose-2,6-bis-phosphate (F2,6BP)

    • Potent allosteric activator of PFK-1 (↑ glycolysis).

    • Potent allosteric inhibitor of FBPase-1 (↓ GNG).

  • Phosphorylation by PKA switches OFF the kinase domain & ON the phosphatase domain → ↓ F2,6BP.

    • ↓ PFK-1 activity + ↑ FBPase-1 activity → glycolysis slows, GNG accelerates.

3. Glycogen Metabolic Enzymes (covered briefly)

  • PKA cascade (via phosphorylase kinase) activates glycogen phosphorylase (breakdown) & inactivates glycogen synthase (synthesis).

Glycogen Metabolism

Absolutely, Raine — glycogen metabolism is like your body’s glucose savings account, with deposits and withdrawals happening based on energy needs. Let’s break it down into two main processes:

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### 🏗 Glycogenesis (Glycogen Synthesis)

This is how your body stores excess glucose after a meal:

1. Glucose → Glucose-6-phosphate (via hexokinase/glucokinase)

2. G6P → Glucose-1-phosphate (via phosphoglucomutase)

3. G1P + UTP → UDP-glucose (activated form)

4. UDP-glucose → Glycogen (via glycogen synthase)

5. Branching enzyme adds α-1,6 linkages for compact storage

🧠 Triggered by: Insulin, high blood glucose, and low energy demand.

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### 🔓 Glycogenolysis (Glycogen Breakdown)

This is how your body releases glucose when energy is needed:

1. Glycogen → Glucose-1-phosphate (via glycogen phosphorylase)

2. G1P → G6P (via phosphoglucomutase)

3. In liver: G6P → Glucose (via glucose-6-phosphatase) → enters bloodstream

In muscle: G6P enters glycolysis for local ATP production

🧠 Triggered by: Glucagon (liver), epinephrine (muscle), and low blood glucose.

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### 🔄 Regulation: The Balancing Act

| Hormone | Effect |

|-------------|------------|

| Insulin | Activates glycogenesis, inhibits glycogenolysis |

| Glucagon | Activates glycogenolysis in liver |

| Epinephrine | Activates glycogenolysis in muscle and liver |

| AMP/ATP | Allosterically regulate enzymes based on energy status |

Phosphorylation plays a key role:

- Glycogen synthase is inactivated by phosphorylation

- Glycogen phosphorylase is activated by phosphorylation

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Would you like a visual map of these pathways or a quick quiz to test your understanding?

Representative Equations (contextual)

  • Overall hepatic gluconeogenesis (from 2 pyruvate):
    2pyruvate+ 4ATP + 2GTP + 2NADH + 6H2O →glucose+ 4ADP + 2GDP + 6Pi + 2NAD + 2H

  • UDP-glucose formation:
    \text{Glucose-1-P} + UTP \rightarrow UDP\text{–Glucose} + PP_i

  • Adenylyl cyclase reaction:
    ATP \xrightarrow{AC} cAMP + PP_i

Ethical / Practical Considerations

  • Understanding hormone-driven glucose homeostasis underpins treatment of diabetes & hypoglycaemia.

  • Pharmacological manipulation (e.g., cAMP-phosphodiesterase inhibitors, PKA modulators) can affect metabolism & are active areas of drug discovery.

Key Points for Exam Study

  1. Physiological Context & Glucose Homeostasis

    • Maintaining blood glucose at "\approx4–5\,mM" is critically important.

    • Understand the consequences of severe hypoglycaemia (<3 mM): poor judgement, disorientation, coma, death.

  2. Two Core Strategies for Glucose Maintenance

    • Glycogenolysis: Rapid, short-term glucose release from stored glycogen.

    • Gluconeogenesis (GNG): Slower, long-term de-novo synthesis of glucose from non-carbohydrate precursors.

  3. Major Pathway Directions & Locations

    • Glycolysis: Glucose "\longrightarrow" Pyruvate (cytosolic, universal, energy-yielding).

    • Gluconeogenesis: Pyruvate "\longrightarrow" Glucose (mainly liver, minor kidney contribution, energy-consuming).

  4. Key Organ Roles

    • Liver: Central for both glycogen turnover and GNG; exports glucose to blood.

    • Kidney: Provides backup GNG (≈10% during prolonged fast).

    • Skeletal Muscle: Stores glycogen for local use only; lacks glucagon receptors and glucose-6-phosphatase, so cannot directly contribute glucose to blood.

  5. Non-Carbohydrate Precursors for GNG

    • Lactate: Via Cori cycle from anaerobic glycolysis.

    • Amino acids: Especially alanine and glutamine from muscle proteolysis (Alanine cycle).

    • Glycerol: From adipose triglyceride breakdown (fatty acid carbon cannot yield net glucose).

  6. Irreversible Glycolytic Steps and GNG Bypass Reactions

    • Memorize the three irreversible glycolytic steps and their corresponding GNG bypass enzymes:

      1. Pyruvate kinase: Bypassed by Pyruvate carboxylase (mitochondrial) and PEP-carboxy-kinase (cytosolic/mitochondrial).

      2. PFK-1: Bypassed by Fructose-1,6-bisphosphatase-1.

      3. Hexokinase/Glucokinase: Bypassed by Glucose-6-phosphatase (liver, kidney).

  7. Hormonal Control (Insulin vs. Glucagon)

    • Fed state: High insulin, low glucagon "\longrightarrow" favors glycogen synthesis & glycolysis, suppresses GNG.

    • Fasting state: Low insulin, high glucagon "\longrightarrow" activates glycogenolysis, GNG, lipolysis.

    • Crucial detail: Glucagon acts on liver & adipose, NOT skeletal muscle.

    • Adrenaline (epinephrine) plays a glucagon-like role in muscle during exercise.

  8. Glucagon Signal-Transduction Cascade (Classical G"_s" Pathway)

    • Understand the steps: Glucagon (peptide) "\longrightarrow" GPCR "\longrightarrow" G"_s" protein activation "\longrightarrow" Adenylyl cyclase "\longrightarrow" cAMP (amplification) "\longrightarrow" Protein kinase A (PKA) activation "\longrightarrow" phosphorylation program.

  9. Enzyme Regulation by Phosphorylation (PKA Targets)

    • Pyruvate kinase (liver): Phosphorylation by PKA inactivates it, shunting PEP towards GNG.

    • Bifunctional Enzyme PFK-2 / FBPase-2: Phosphorylation by PKA switches OFF kinase domain & ON phosphatase domain "\longrightarrow" "\downarrow F"2,6BP. This "\downarrow" PFK-1 (glycolysis) and "\uparrow" FBPase-1 (GNG).

    • Glycogen Metabolic Enzymes: PKA activates glycogen phosphorylase (breakdown) & inactivates glycogen synthase (synthesis) via phosphorylase kinase.

  10. Glycogen Metabolism Key Steps

    • Synthesis: UDP-Glucose as precursor, Glycogen synthase, branching enzyme.

    • Breakdown: Glycogen phosphorylase, debranching enzyme.

    • Distinguish glucose fate: Liver G1P "\longrightarrow" Glucose (exported); Muscle G1P "\longrightarrow" Glycolysis (local use).

  11. Integrated Cycles

    • Cori cycle: Lactate (muscle) "\leftrightarrow" Glucose (liver).

    • Alanine cycle: Transports nitrogen & carbon from muscle to liver for GNG.

  12. Key Equations (Contextual)

    • Overall hepatic gluconeogenesis (from 2 pyruvate):2\,\text{Pyruvate} + 4\,ATP + 2\,GTP + 2\,NADH + 6\,H2O \rightarrow \text{Glucose} + 4\,ADP + 2\,GDP + 6\,Pi + 2\,NAD^+ + 2\,H^+