Regulation of the TCA Cycle

Overview of Glucose Oxidation:

• Starting point: Glycolysis (cytosol)

  • Glucose → pyruvate

  • Produces:

    • 2 ATP (net)

    • 2 NADH

    • 2 pyruvate
      

• Fates of pyruvate:

  
  Anaerobic conditions / high glycolytic flux:

  • Pyruvate → lactate (via lactate dehydrogenase)

  • Purpose:

    • Regenerates NAD⁺ for glycolysis

    • Occurs when:

      • O₂ is limited (hypoxia, ischaemia)

      • Energy demand exceeds mitochondrial capacity (sprinting)

  • Consequence:

    • TCA cycle not used

    • ATP comes almost entirely from glycolysis
      

  Aerobic conditions:

  • Pyruvate enters mitochondria

  • Converted → acetyl-CoA → enters TCA cycle

  • Outcome:

    • Complete oxidation → CO₂

    • Majority of NADH & FADH₂ generated

    • These feed electrons into ETC (inner mitochondrial membrane)

    • ETC drives oxidative phosphorylation → bulk ATP production
      

• Organism differences

  • Yeast: can survive indefinitely on glycolysis + fermentation

  • Humans: can rely on glycolysis only temporarily

    • Require mitochondrial respiration for sustained metabolism

Mitochondrial Structure & Where Reactions Occur:

• Outer mitochondrial membrane (OMM)

  • Permeable to small molecules via porins

• Inner mitochondrial membrane (IMM)

  • Highly selective

  • Contains:

    • ETC complexes I–IV

    • ATP synthase

    • Transporters (e.g., pyruvate carrier)

• Intermembrane space

  • High proton concentration during oxidative phosphorylation

• Matrix

  • Location of:

    • Pyruvate oxidation (PDH/PDC)

    • TCA cycle

    • β-oxidation

    • Mitochondrial DNA, ribosomes, enzymes

Pyruvate Transport & Link Reaction (Pyruvate → Acetyl-CoA):

• Transport

  • Pyruvate enters mitochondrial matrix via:

    • Mitochondrial pyruvate carrier (MPC)

• Conversion by PDH complex (link reaction)

  • Reaction:

    • Pyruvate + CoA + NAD⁺ → Acetyl-CoA + CO₂ + NADH

  • Characteristics:

    • First CO₂ released from glucose

    • Irreversible

    • Rate-limiting for entry of carbs into TCA cycle

    • Commits carbon to full oxidation

The Pyruvate Dehydrogenase Complex (PDC):

• Size & structure

  • Giant multi-enzyme complex (4–10 MDa)

  • 3 catalytic components:

  E1: Pyruvate dehydrogenase (decarboxylase) (PDH)

  • Decarboxylates pyruvate, and removes CO₂ from pyruvate.

  • Requires TPP

  E2: Dihydrolipoyl transacetylase

  • Lipoamide “swinging arm” transfers acetyl group to CoA

  E3: Dihydrolipoyl dehydrogenase

  • Re-oxidises lipoamide

  • Uses FAD → FADH₂ → NADH

• Required coenzymes (5 total)

  • TPP (vitamin B1)

  • Lipoamide

  • CoA (vitamin B5 derivative)

  • FAD (vitamin B2)

  • NAD⁺ (vitamin B3)

• Products

  • Acetyl-CoA

  • NADH

  • CO₂

Regulation of PDC:

• Covalent regulation

  • PDC kinase (PDK)

    • Phosphorylates E1 → inactivates PDC

    • Activated by high-energy signals:

      • ↑ ATP

      • ↑ Acetyl-CoA

      • ↑ NADH

  • PDC phosphatase

    • Dephosphorylates E1 → activates PDC

    • Activated by:

      • Insulin

      • Ca²⁺ (major signal in muscle contraction)

      • Mg²⁺

• Summary

  • High-energy state → PDH OFF

  • Low-energy / exercise → PDH ON

TCA Cycle (Krebs Cycle):

• Entry step

  • Acetyl-CoA + oxaloacetate → citrate

  • Enzyme: citrate synthase

• Purpose

  • Extract high-energy electrons:

    • 3 NADH

    • 1 FADH₂

  • Produce:

    • 1 GTP (ATP equivalent)

    • 2 CO₂

  • Provide intermediates for biosynthesis

• Key point

  • TCA does not produce much ATP directly

  • NADH/FADH₂ → ETC → bulk ATP

Regulation of the TCA Cycle:

Citrate Synthase:

• Inhibited by:

  • ↑ ATP

  • ↑ NADH

  • ↑ Succinyl-CoA (competitive)

  • ↑ Citrate

• Strongly dependent on substrate availability (acetyl-CoA + OAA)
  

Isocitrate Dehydrogenase:

• Activated by:

  • ADP

• Inhibited by:

  • ATP

  • NADH

• Major rate-limiting enzyme
  

α-Ketoglutarate Dehydrogenase:

• Similar structure to PDC

• Inhibited by:

  • ↑ ATP

  • ↑ NADH

  • ↑ Succinyl-CoA

• No covalent regulation

Physiological & Pathological Relevance

Exercise:

• ↑ Ca²⁺ in muscle:

  • Activates PDC phosphatase → PDH ON

  • Boosts acetyl-CoA production for rapid ATP
    

Hypoxia:

• Low O₂ → mitochondrial ETC slows

• ↑ NADH / ↓ NAD⁺ → PDH inhibited

• Pyruvate diverted → lactate
  

Cancer (Warburg Effect):

• Tumours favour glycolysis even with O₂

• Benefits:

  • Fast ATP

  • Biosynthetic precursor production
    

Metabolic Disease:

• PDH can be pharmacologically activated

  • Example: dichloroacetate (DCA) inhibits PDK

  • Useful in diseases with impaired PDH or blood flow issues
    

Metabolic Imaging:

• ¹³C-labelled pyruvate:

  • Traces flux through PDH + TCA cycle via MRI

  • Used in studying diabetes → PDH often under-activated