Comprehensive Study Notes on Glycogen Metabolism

Overview of Glycogen Storage and Function

• Glycogen Storage Sites and Capacity:

  • Liver: Glycogen can make up to $6\%$ of the liver's weight, approximately $100\,g$.
  • Muscle: Glycogen can make up to $2\%$ of skeletal muscle weight, approximately $400\,g$.
  • Minor Sites: Small amounts of glycogen are also stored in the kidneys, the intestine, and various other cells.

• The Biological Role of Liver Glycogen:

  • Glucose Reserve: Serves as a central reserve to maintain blood glucose levels between meals.
  • Release Mechanism: Free glucose is released directly into the bloodstream.
  • Duration: Liver stores typically last between $12$ to $24$ hours, though this depends heavily on the individual’s activity level.

• The Biological Role of Muscle Glycogen:

  • Local ATP Production: Serves as an immediate source of glucose for ATP production within the muscle cell itself.
  • Metabolic Path: Glucose is released from glycogen as Glucose 6-phosphate ($G6P$), which then enters glycolysis in that same muscle cell.
  • Lack of Glucose 6-phosphatase (G6Pase): Muscle tissue lacks the enzyme $G6Pase$, which is required to convert $G6P$ to free glucose. Consequently, muscle tissue releases little to no free glucose into the blood.
  • Blood Glucose Uptake: Exercise or metabolic demand causes muscles to absorb glucose from the blood to synthesize glycogen, effectively decreasing blood glucose levels.

Glycogen Structure and Composition

• General Composition:

  • Glycogen is a homopolymer of glucose molecules.
  • Synthesis occurs specifically when there is an excess of glucose available.

• Glycosidic Bonds:

  • Main Chains: Primarily composed of $\alpha-1,4$ glycosidic bonds between glucose residues.
  • Branch Points: Formed by $\alpha-1,6$ branch points.
  • Physical Shape: The branching creates a highly compact, tree-like structure, offering many nonreducing ends for rapid degradation and synthesis.

• End Types:

  • Reducing End: Attached to the protein primer, glycogenin.
  • Nonreducing Ends: Multiple "free" ends where glycogen synthase adds glucose or glycogen phosphorylase removes it.

Breakdown of Ingested vs. Endogenous Glycogen

• Exogenous (Dietary) Breakdown:

  • Enzymes: Salivary and pancreatic $\alpha-Amylases$ break some $\alpha-1,4$ glycosidic bonds.
  • Result: Final products are absorbed by intestinal cells and enter the bloodstream to provide energy or substrates to cells.

• Related Dietary Sugar Pathways:

  • Trehalose: Cleaved by trehalase into D-Glucose.
  • Lactose: Cleaved by lactase into D-Galactose and D-Glucose.
  • Sucrose: Cleaved by sucrase into D-Glucose and D-Fructose.

• Endogenous Glycogen Degradation (Glycogenolysis):

  • Glycogen Phosphorylase: Removes glucose residues one by one from the nonreducing ends of the glycogen chain. It uses inorganic phosphate ($P_i$) to produce Glucose 1-phosphate ($G1P$). The chain is shortened to $(glucose)_{n-1}$.
  • Debranching Enzyme: Handles branch points through two distinct activities:
    1. Transferase Activity: Shifts a block of three glucose residues from one outer branch to another, exposing a single glucose residue at the $\alpha-1,6$ linkage.
    2. $\alpha-1,6$-glucosidase Activity: Hydrolyzes the $\alpha-1,6$ glycosidic bond, releasing a single molecule of free glucose.

Phosphoglucomutase and Glucose 6-phosphatase

• Phosphoglucomutase Mechanism:

  • Reversibly converts $G1P$ to $G6P$.
  • Intermediate: The reaction proceeds via a Glucose 1,6-bisphosphate intermediate.
  • Reaction: $Glucose\,1-phosphate \rightleftharpoons Glucose\,6-phosphate$.

• Liver Specific Processing (G6Pase):

  • In the liver, $G6P$ is transported from the cytosol into the Endoplasmic Reticulum (ER) lumen via the $G6P$ transporter ($T1$).
  • Glucose 6-phosphatase: Located in the ER lumen, it dephosphorylates $G6P$ into free glucose and inorganic phosphate ($P_i$).
  • Transport out of ER: Glucose is moved back to the cytosol via transporter $T2$, and $P_i$ via transporter $T3$.
  • Release to Blood: Free glucose leaves the liver cell via the $GLUT2$ transporter to increase blood glucose concentration.

Glucose Transport and Phosphorylation Enzymes

• GLUT4 Proteins:

  • Specific transport proteins that enable glucose to enter muscle and adipose cells.
  • They are recruited to the cell membrane following the binding of insulin to its receptors.

• Hexokinase vs. Glucokinase:

  • Hexokinase (Muscle): Has a low $K_m$ for glucose (high affinity), meaning it operates at $V_{max}$ even at fasting glucose concentrations ($\sim 5\,mmol/L$).
  • Glucokinase (Liver): Acts as a glucose sensor. It has a higher $K_m$ (lower affinity) and higher $V_{max}$ than hexokinase. Its activity varies according to glucose concentration, allowing the liver to spare glucose for the brain and muscles during fasting while rapidly capturing it postprandially.

Biochemical Pathway of Glycogen Synthesis (Glycogenesis)

• Step 1: Activation of Glucose:

  • Glucose enters the cell and is phosphorylated to $G6P$.
  • $G6P$ is converted to $G1P$ by phosphoglucomutase.
  • $G1P$ reacts with $UTP$ (Uridine triphosphate) to form UDP-glucose (a sugar nucleotide) and pyrophosphate ($PP_i$). This is catalyzed by UDP-glucose pyrophosphorylase.
  • The hydrolysis of $PP_i$ into $2P_i$ by inorganic pyrophosphatase makes the reaction irreversible.

• Step 2: Priming by Glycogenin:

  • Glycogenin is a protein that acts as both a primer and an enzyme.
  • It has glucosyltransferase activity that attaches the first glucose from UDP-glucose to its own tyrosine residue ($Tyr\,194$).
  • It continues to add several more glucose units (repeats six times) via chain-extending activity until the chain is long enough for glycogen synthase to take over.

• Step 3: Chain Elongation (Glycogen Synthase):

  • Glycogen Synthase adds glucose units from UDP-glucose to the nonreducing ends of an existing glycogen chain ($n > 4$) via $\alpha-1,4$ bonds.
  • Byproduct: $UDP$ is released, which can be re-phosphorylated to $UTP$ using $ATP$.

• Step 4: Branching (Branching Enzyme):

  • Glucosyl 4:6 transferase (branching enzyme) creates branches.
  • When a chain reaches at least $11$ residues, the enzyme cuts a block of $6$ to $8$ residues and reattaches them via an $\alpha-1,6$ glycosidic bond.
  • Branches occur approximately every $8$ to $10$ glucose residues.

Regulation of Glycogen Metabolism

• Metabolite and Allosteric Regulation:

  • Muscle: Glycogen phosphorylase is activated allosterically by $AMP$ and $Ca^{2+}$ (via calmodulin-dependent phosphorylase kinase stimulated by neuronal activity).
  • Liver: Liver phosphorylase is activated by $Ca^{2+}$ (mediated by $\alpha-adrenergic$ receptors and epinephrine).
  • Inhibitors: Glycogen breakdown is inhibited by high levels of $G6P$ and $ATP$.

• Hormonal Regulation of Breakdown (Glycogenolysis):

  • Glucagon (Liver) and Epinephrine (Muscle/Liver): Bind to $\beta-adrenergic$ receptors, stimulating adenylyl cyclase to produce cAMP.
  • Phosphorylation Cascade:
    1. $cAMP$ activates Protein Kinase A (PKA).
    2. $PKA$ phosphorylates and activates Phosphorylase Kinase.
    3. Active Phosphorylase Kinase phosphorylates Glycogen Phosphorylase b (inactive) to Glycogen Phosphorylase a (active).
    4. Active glycogen phosphorylase cleaves glucose residues as $G1P$.

• Hormonal Regulation of Synthesis (Glycogenesis):

  • Insulin:
    1. Increases glucose import (via $GLUT4$ in muscle).
    2. Activates Protein Phosphatase 1 (PP1), which dephosphorylates and activates glycogen synthase.
    3. Activates Protein Kinase B (Akt), which phosphorylates and inactivates Glycogen Synthase Kinase 3 (GSK3).
  • GSK3 and Inactivation: $GSK3$ normally phosphorylates Glycogen Synthase a (active) to convert it to Glycogen Synthase b (inactive). By inhibiting $GSK3$, insulin keeps glycogen synthase in its active state.

Summary of Liver vs. Muscle Carbohydrate Control

• High Blood Glucose (Insulin Dominant):
  • Increases: PP1, PKB, Glycogen synthesis, Glycolysis.
  • Decreases: GSK-3, Phosphorylase kinase, Glycogen breakdown.
• Low Blood Glucose (Glucagon/Epinephrine Dominant):
  • Increases: cAMP, PKA, FBPase-2, Phosphorylase kinase, Glycogen breakdown, Gluconeogenesis (Liver).
  • Decreases: PFK-2, Glycolysis (Liver), Glycogen synthesis.

Glycogen Storage Diseases (GSD)

• Type 0: Deficiency in Glycogen synthase (Liver). Symptoms: Low blood glucose, high ketone bodies, early death.
• Type Ia (von Gierke): Deficiency in Glucose 6-phosphatase (Liver). Symptoms: Enlarged liver, kidney failure.
• Type Ib: Deficiency in Microsomal glucose 6-phosphate translocase (Liver). Symptoms: Same as Ia plus high susceptibility to bacterial infections.
• Type Ic: Deficiency in Microsomal $P_i$ transporter (Liver). Symptoms: Same as Ia.
• Type II (Pompe): Deficiency in Lysosomal glucosidase (Skeletal/Cardiac muscle). Symptoms: Infantile form causes death by age 2; juvenile form causes myopathy.
• Type IIIa (Cori or Forbes): Deficiency in Debranching enzyme (Liver, Skeletal/Cardiac muscle). Symptoms: Enlarged liver in infants, myopathy.
• Type IIIb: Deficiency in Liver debranching enzyme (Muscle normal). Symptoms: Enlarged liver in infants.
• Type IV (Andersen): Deficiency in Branching enzyme (Liver, Skeletal muscle). Symptoms: Enlarged liver/spleen, myoglobin in urine.
• Type V (McArdle): Deficiency in Muscle phosphorylase (Skeletal muscle). Symptoms: Exercise-induced cramps, myoglobin in urine.
• Type VI (Hers): Deficiency in Liver phosphorylase (Liver). Symptoms: Enlarged liver.
• Type VII (Tarui): Deficiency in Muscle PFK-1 (Muscle, Erythrocytes). Symptoms: Like Type V, plus hemolytic anemia.
• Type VIb, VIII, or IX: Deficiency in Phosphorylase kinase (Liver, Leukocytes, Muscle). Symptoms: Enlarged liver.
• Type XI (Fanconi-Bickel): Deficiency in Glucose transporter (GLUT2) (Liver). Symptoms: Failure to thrive, enlarged liver, rickets, kidney dysfunction.