Glycogen Metabolism

Glycogen Structure and Tissue Distribution:

Glycogen is a highly branched polymer of glucose residues. Its structure is optimised for rapid mobilisation by providing numerous ends for enzymatic action.

Molecular Architecture:

• Glycosidic Linkages: Most glucose units in glycogen are linked by α-1,4-glycosidic bonds. Approximately every tenth residue, an α-1,6-glycosidic bond creates a branch point.

• Solubility and Access: The branching not only increases the solubility of the polymer but also creates a large number of non-reducing ends, allowing multiple enzymes to work simultaneously to release glucose quickly.

• The Primer (Glycogenin): Glycogen synthesis cannot start de novo. It requires a protein primer called glycogenin, which consists of two 37kd subunits that bear an initial oligosaccharide of α-1,4-glucose units.

Tissue Distribution:

• Liver: Glycogen makes up about 10% of the liver's weight. Its role here is the maintenance of blood glucose levels for the entire body.

• Skeletal Muscle: While glycogen only constitutes about 2% of muscle mass, the total muscle mass of the body is so large that it stores the majority of the body's glycogen. Muscle glycogen is reserved strictly for the energy needs of the muscle itself.

• Brain: Stored in very small amounts, but critical for sustained cognitive function.

Glycogenolysis: The Degradation Pathway:

Glycogenolysis is the breakdown of glycogen into glucose 1-phosphate (G1P) and then into glucose 6-phosphate (G6P).

Key Enzymatic Steps:

1. Glycogen Phosphorylase: This is the rate-limiting enzyme. It cleaves α-1,4 bonds using orthophosphate (P_i) in a process called phosphorolysis. It acts on the non-reducing ends and continues until it reaches a point four residues away from a branch.

2. Debranching Process: A transferase moves a block of three glucose residues from one outer branch to another. Then, α-1,6-glucosidase hydrolyses the remaining single glucose residue at the branch point, releasing free glucose.

3. Phosphoglucomutase: This enzyme converts the released G1P into G6P by shifting the phosphate group from the C-1 position to the C-6 position via a serine residue in its catalytic site.

Tissue-Specific Fates of Glucose 6-Phosphate:

• Liver: The liver expresses glucose 6-phosphatase, an enzyme that removes the phosphate group to produce free glucose, which can then be exported into the bloodstream.

• Muscle: Muscle lacks this phosphatase. Instead, G6P enters glycolysis directly to produce ATP for muscle contraction.

Glycogenesis: The Synthesis Pathway:

Activation of Glucose:

• UDP-Glucose: Synthesis requires an activated Uridine diphosphate glucose (UDP-glucose), which is formed from G1P and UTP. The reaction is driven forward by the rapid hydrolysis of pyrophosphate (PP_i).

Chain Elongation and Branching:

• Glycogen Synthase: This is the key regulatory enzyme for synthesis. It transfers the glucosyl unit from UDP-glucose to the non-reducing end of a glycogen chain (forming α-1,4 bonds).

• Branching Enzyme: This enzyme creates α-1,6 linkages by breaking an α-1,4 link and moving a block of residues to a more interior site. This increases the number of terminal glucose residues available for rapid mobilisation.

Regulation of Glycogen Metabolism:

Metabolism is tightly regulated through allosteric interactions (meeting cellular needs) and covalent/hormonal modifications (meeting whole-body needs).

Allosteric Regulation (The Energy Charge):

• In Muscle:

  • Activators: AMP binds to phosphorylase, shifting it to the active 'R' state when energy levels are low.

  • Inhibitors: ATP and G6P inhibit phosphorylase, signalling that the cell has sufficient energy and glucose.

• In Liver:

  • The liver phosphorylase is primarily inhibited by glucose. When glucose is abundant, it binds to the enzyme and shifts it to the inactive 'T' state, preventing unnecessary glycogen breakdown

Hormonal Regulation (The Signalling Cascade):

• Glucagon and Epinephrine: These hormones trigger the breakdown of glycogen.

  • They bind to 7TM receptors, activating G proteins that increase cAMP levels.

    • cAMP activates Protein Kinase A (PKA).

      • PKA phosphorylates Phosphorylase Kinase, which in turn phosphorylates Glycogen Phosphorylase, activating it.

        • Simultaneously, PKA phosphorylates Glycogen Synthase, which inactivates it, preventing a futile cycle.

• Insulin: This hormone stimulates glycogen synthesis after a meal.

  • Insulin activates Protein Phosphatase 1 (PP1).

    • PP1 dephosphorylates glycogen synthase (activating it) and glycogen phosphorylase (inactivating it).

Glycogen Storage Diseases (GSDs):

GSDs result from genetic deficiencies in enzymes involved in glycogen metabolism, leading to abnormal amounts or structures of glycogen in tissues.