Biochemistry Chapter 18: Storage Mechanisms and Control in Carbohydrate Metabolism

Chapter 18: Storage Mechanisms and Control in Carbohydrate Metabolism

Overview of Glycogen

  • Glycogen is primarily stored in the liver and muscle.

  • Its structure is optimized for rapid energy release and prolonged energy storage.

  • The average chain length of glycogen branches is about 13 residues.

Glycogen Breakdown

  • Liver Function: Liver glycogen is converted to glucose-6-phosphate, which can be hydrolyzed to glucose for energy release.

  • Muscle Function: Muscle glycogen produces glucose-6-phosphate directly involved in glycolysis, bypassing the export to the bloodstream.

First Reaction of Glycogen Breakdown

  • Each glucose unit cleaved from glycogen results in glucose-1-phosphate.

  • This cleaving reaction is known as phosphorolysis, not hydrolysis.

  • Enzyme Involved: Glycogen phosphorylase, which cleaves the α(1 → 4) linkages without ATP consumption.

Second Reaction of Glycogen Breakdown

  • Glucose-1-phosphate is isomerized to glucose-6-phosphate via phosphoglucomutase.

  • Complete degradation requires debranching enzymes to handle α(1 → 6) linkages.

Glycogen Debranching

  • Debranching Enzyme: Transfers terminal glucose residues from branch points and hydrolyzes the α(1 → 6) bond at those points.

Glycogen Production

  1. Stage 1:

    • Glucose-1-phosphate reacts with UTP to form UDP-glucose and pyrophosphate (PPi).

    • Catalyzed by: UDP-glucose pyrophosphorylase.

  2. Stage 2:

    • UDPG is integrated into the growing glycogen chain via new α(1 → 4) glycosidic bonds, catalyzed by glycogen synthase.

    • Must extend an existing chain.

    • Branching enzyme creates α(1 → 6) linkages by transferring a segment from existing chains.

Energy Dynamics in Glycogen Production

  • Equation:
    \text{Glucose-1-phosphate + UTP} \rightarrow \text{UDPG + 2Pi} \quad (\Delta G^\circ' = -30.5 kJ mol^{-1}, -7.3 kcal mol^{-1})

Control of Glycogen Metabolism

  • Glycogen phosphorylase: Major regulator of glycogen metabolism, operates under allosteric control and covalent modification.

  • Glycogen synthase: Inactivate when phosphorylated; activated in a dephosphorylated state, with hormonal signals (glucagon, epinephrine) activating phosphorylation through cAMP-dependent protein kinase.

  • Enzymes involved in phosphorylation also undergo covalent modifications themselves.

Summary of Glycogen Metabolism

  • Glycogen serves as glucose storage; synthesis and degradation are energetically controlled and influenced by metabolic needs.

Gluconeogenesis

  • Process by which pyruvate is converted into glucose; not a direct reversal of glycolysis due to irreversible steps.

  • Different enzymes are involved in reversing glycolytic pathway steps, with notable steps including:

    • Pyruvate production from phosphoenolpyruvate.

    • Formation of fructose-1,6-bisphosphate from fructose-6-phosphate.

  • Net gluconeogenesis involves bypassing key glycolytic steps, involving different reactions and enzymes.

Conversion Steps

  1. Carboxylation of Pyruvate:

    • Catalyzed by pyruvate carboxylase, activated by acetyl-CoA, requiring biotin and ATP.

  2. Formation of Phosphoenolpyruvate: occurs in the mitochondrial and cytosolic compartments.

    • Involves several steps, ultimately producing glucose from pyruvate.