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Glycogen Metabolism, Oxidative Pentose Phosphate Pathway (OPPP), &…

- - - - Branched polymer: α-1,4-glycosidic bonds (linear), α-1,6-glycosidic bonds (branches every ~10 residues).
      - Major stores: Liver (maintains blood glucose) & skeletal muscle (energy for contraction).
      - Cytoplasmic granules: ~55,000 glucose units.
  - - - Key enzymes:
      - Glycogen phosphorylase: Cleaves α-1,4 bonds via phosphorolysis → glucose 1-phosphate (G1P).
      - Debranching enzyme: Removes α-1,6 branches (transferase + α-1,6-glucosidase).
      - Phosphoglucomutase: Converts G1P → glucose 6-phosphate (G6P).
      - Liver-specific step: Glucose 6-phosphatase converts G6P → glucose (released into blood).
  - - - Allosteric control:
      - Liver: Inhibited by glucose (shifts to T state).
      - Muscle: Activated by AMP (low energy), inhibited by ATP/G6P.
      - Covalent modification:
      - Phosphorylase a (active, phosphorylated) ↔ Phosphorylase b (inactive).
      - Hormonal triggers: Glucagon (liver), adrenaline (muscle) → cAMP cascade → PKA → phosphorylation.
  - - - UDP-glucose: Activated glucose donor (G1P + UTP → UDP-glucose + PPi).
      - Glycogen synthase: Adds glucose to non-reducing ends (requires primer glycogenin).
      - Branching enzyme: Creates α-1,6 linkages.
      - Regulation:
      - Activated by G6P (allosteric) & insulin (dephosphorylation via PP1).
      - Inhibited by phosphorylation (PKA, GSK3).
- - - - Oxidative phase: Generates NADPH.
      - Glucose 6-phosphate dehydrogenase: G6P → 6-phosphogluconolactone (NADPH).
      - 6-Phosphogluconate dehydrogenase: 6-phosphogluconate → ribulose 5-phosphate (Ru5P) + NADPH + CO₂.
      - Nonoxidative phase: Interconverts sugars (C3-C7).
      - Transketolase (2C transfer) & Transaldolase (3C transfer).
  - - - NADPH: Biosynthesis (fatty acids, cholesterol), antioxidant (reduces glutathione).
      - Ribose 5-phosphate (R5P): Nucleotide synthesis.
      - Link to glycolysis: F6P/GAP from OPPP enter glycolysis.
  - - - NADP+ levels: High NADP+ → activates G6P dehydrogenase.
      - Cellular needs:
      - NADPH > R5P: Convert R5P → F6P/GAP.
      - R5P > NADPH: Glycolysis intermediates → R5P.
- - - - Lipolysis: TAGs → fatty acids + glycerol (hormone-sensitive lipase).
      - Activation: Fatty acid + CoA → acyl-CoA (acyl-CoA synthetase; consumes 2 ATP).
      - Carnitine shuttle: Transports acyl-CoA into mitochondria (CPT I/II).
    - - Oxidation: Acyl-CoA → trans-Δ²-enoyl-CoA (FADH₂).
        
        Hydration: Enoyl-CoA → L-3-hydroxyacyl-CoA.
        
        Oxidation: 3-hydroxyacyl-CoA → 3-ketoacyl-CoA (NADH).
        
        Thiolysis: Cleaves to acetyl-CoA + shortened acyl-CoA.
    - - Palmitate (C16):
        
        7 cycles → 8 acetyl-CoA + 7 FADH₂ + 7 NADH.
        
        Total ATP ≈ 106 (after subtracting 2 ATP activation cost).
    - - Unsaturated FAs: Require isomerase (cis-Δ³ → trans-Δ²) & reductase (for polyunsaturated).
        Odd-chain FAs: Propionyl-CoA → succinyl-CoA (via methylmalonyl-CoA mutase, B₁₂-dependent).
    - - Synthesis (Liver): Acetyl-CoA → acetoacetate → β-hydroxybutyrate/acetone.
        
        Utilization (Peripheral tissues): Converted back to acetyl-CoA for TCA cycle.
        
        Regulatory role: High ketones suppress lipolysis.