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Glycogen Phosphorylase Regulation Overview

• Glycogen Phosphorylase (GP) is regulated by:

  • Allosteric interactions

  • Reversible phosphorylation

• ATP acts as the phosphate group source that activates GP.

• Protein Phosphatase 1 has distinct roles in phosphorylated glycogen phosphorylase recognized and dephosphorylation.

Isozymes of Glycogen Phosphorylase

• Differences in GP activity in liver versus muscle:

  • Different isozymes are expressed (90% identical amino acid sequence, 10% difference).

  • Liver glycogen maintains blood sugar levels.

  • Muscle glycogen provides glucose for energy production.

• Isozymes allow the enzyme to recognize different signals and respond accordingly.

Regulation of Liver Glycogen Phosphorylase

• Regulated by two layers:

  1. Phosphorylation

  2. Allosteric regulation

• GP exists as a dimer with identical subunits, each phosphorylated on a single serine residue.

• Phosphorylation leads to the more active "a" form, mediated by Phosphorylase Kinase.

  • Almost all phosphorylations are reversible; dephosphorylation is carried out by Phosphorylase Phosphatase converting to less active “b” form.

Phosphorylase Kinase and Ca²⁺ Sensitivity

• Phosphorylase Kinase responds to intracellular Ca²⁺ levels:

  • Increased Ca²⁺ results in increased phosphorylase kinase activity.

  • Triggered by signals related to muscle contraction (energy production state).

  • More phosphorylation leads to greater GP conversion to active form “a.”

Allosteric Regulation in Liver

• Liver GP is inhibited by glucose (an allosteric regulator).

• Binding of glucose causes conformational change blocking active site, reducing glycogen breakdown.

Regulation of Muscle Glycogen Phosphorylase

• The muscle isoform is regulated by:

  • Covalent phosphorylation

  • Allosteric regulation (different modifiers compared to liver).

• During contraction, AMP increases as ATP is consumed, reflecting a low-energy state, acting as a potent allosteric activator of muscle GP.

• ATP acts as an inhibitor, competing with AMP for enzyme binding.

• Glucose-6-phosphate also inhibits muscle GP, indicating sufficient glucose availability.

Activation of Glycogen Phosphorylase b in Muscle

• Only Glycogen Phosphorylase b is sensitive to allosteric modifiers; the phosphorylated form “a” remains active regardless of AMP, ATP, or glucose-6-P concentrations.

Tense and Relaxed Forms of Glycogen Phosphorylase

• Tense Form: Inactive enzyme state.

• Relaxed Form: Active enzyme state, influenced by AMP and glucose-6-P levels.

  • Phosphorylation retains enzyme in the relaxed state until phosphates are removed.

Adenylate Kinase Role

• Adenylate Kinase family transfers high energy phosphates (ATP, ADP to AMP):

  • Reaction example: $2 ext{ADP} <br>ightarrow ext{ATP} + ext{AMP}$.

• Assists in rapid energy production at the cost of generating AMP, similar to financial debt strategy.

Hormonal Regulation: Epinephrine and Glucagon

• Epinephrine (muscle) and Glucagon (liver) stimulate glycogen breakdown:

  • Epinephrine triggers rapid energy needs for muscle contraction.

  • Glucagon signals liver to release glucose due to low blood sugar.

• Both hormones activate GP via a regulatory cascade.

Mechanism of Hormonal Signaling

• Hormones activate GP through:

  1. Binding to receptors on target cells.

  2. Activation of Adenylate Cyclase, converting ATP to cyclic AMP (cAMP).

  3. cAMP activates Protein Kinase A, which then phosphorylates and activates Phosphorylase Kinase.

  4. Phosphorylase Kinase then phosphorylates GP transforming it to its active form, GP a.

Signal Amplification in Cascades

• Hormone binding leads to significant signal amplification:

  • One hormone activates many cAMP molecules, which activate multiple kinases.

  • Each kinase activates numerous Glycogen Phosphorylase through phosphorylation.

  • Increased enzyme action results in amplified glycogen breakdown responses.

Ca²⁺ and Glycogen Breakdown in Muscle

• Phosphorylase Kinase activated by Ca²⁺ binding to calmodulin, facilitating glycogen breakdown during muscle contractions.

• The coordination of calcium signaling and glycogen metabolism plays a crucial role in energy production during physical activity.

Reciprocal Regulation of Glycogen Metabolism

• Glycogen synthesis and breakdown are inversely regulated:

  • Phosphorylation activates glycolysis while inhibiting glycogen synthesis and vice versa.

• Protein Kinase A phosphorylates Glycogen Synthase thus inhibiting its activity while promoting glycogen breakdown.

Protein Phosphatase 1 Functionality

• Protein Phosphatase 1 (PP1) dephosphorylates key enzymes in glycogen metabolism:

  • Inactivates Phosphorylase Kinase and GP a, inhibiting glycogenolysis.

  • Activates Glycogen Synthase, promoting glycogen synthesis.

Insulin's Role in Glycogen Synthesis

• Insulin, secreted post-meal, promotes glycogen synthesis by:

  • Increasing glucose transporter availability (GLUT4) in muscle and fat cells, enhancing glucose uptake.

  • Activating pathways that decrease Glycogen Synthase Kinase activity, sustaining Glycogen Synthase activity.

• Promotes replenishment of glycogen stores to lower blood sugar.

Glucose Impact on Liver Glycogen Metabolism

• Post-meal, increased blood glucose leads to insulin release fostering glycogen storage.

• Glycogen Synthase activity increases only when Glycogen Phosphorylase is predominantly in its inactive state (b form).

Mechanism of Glucose Influence

• Free glucose binds to Glycogen Phosphorylase a, leading to de-phosphorylation by PP1, converting it to the inactive form (b).

• This allows PP1 to preferentially activate Glycogen Synthase by dephosphorylating it, aligning with coordinated metabolism regulation.

Glycogen Storage Diseases

• Various glycogen storage diseases involve defects in glycogen metabolism, predominantly affecting glycogenolysis.

• Example: von Gierke disease results from Glucose-6-Phosphatase deficiency, leading to elevated liver glycogen and hypoglycemia post-glucagon or epinephrine.

• Clinical findings include enlarged liver and hypoglycemic episodes due to inadequate release of glucose.