Chapter 19: The Citric Acid Cycle

Cellular Respiration

  • O2 → CO2

  • More ATP produced from glucose than glycolysis

    • ∆G’º = -2,840 kJ/mol for fully oxidized Glucose

  • Three processes:

    • Citric Acid Cycle (TCA cycle)

      • Amphibolic - Role in cat/anabolism

      • Central metabolic pathway

      • Occurs in mitochondrial matrix

        • EXCEPTION: Succinate dehydrogenase (inner membrane)

    • Electron Transport

    • Oxidative phosphorylation

      • Occurs in inner membrane of mitochondria

    Metabolism:

    • Catabolism (oxidative breakdown)

      • Proteins, fats, carbs in three stages of cell. respiration

    • Anabolism (Reductive synthesis)

  • Three major stages:

1. Acetyl CoA Production

  • Oxidation of:

    • Fatty acids

    • Glucose

    • Amino acids

  • Yields Acetyl-CoA and some

    • ATP

    • NADH

    • FADH2

  • Can be supplied to TCA cycle from other carbon sources/carbohydrates

Pyruvate → Acetyl-CoA

  • Oxidative decarboxylation of pyruvate (from glycolysis)

  • Requires:

    • CoA-SH

    • NAD+

    • TPP

    • Lipoate

    • FAD

  • Produces:

    • CO2

    • Acetyl-CoA

    • NADH

  • Enzyme: Pyruvate Decarboxylase Complex (E1 + E2 + E3)

    • E1 = pyruvate dehydrogenase

    • E2 = Dihydrolipoyl transacetylase

    • E3 = Dihydrolipoyl dehydrogenase

  • ∆Gº’ = -33.4 kJ/mol

  • CHANNELING- move substrates from one E to the next

    • Minimizes side reactions

  • Regulation by ATP

2. Acetyl-CoA Oxidation via the citric acid cycle

  • Generates:

    • NADH

    • FADH2

    • GTP

Reaction 1: Acetyl-CoA + OAA → Citrate

  • Citrate formation

  • ∆Gº’ = -32.3 kJ/mol

  • Enzyme: Citrate synthase

  • First committed step

  • OAA creates binding site for A-CoA on enzyme

Reaction 2: Citrate → [cis-Aconitate] → Isocitrate

  • Enzyme: Aconitase

  • ∆Gº’ = 13.3 kJ/mol

  • Eliminate water from citrate → cis C=C bond

  • Citrate = bad oxidation

  • Isocitrate (2º alcohol) = Good oxidation

Reaction 3: Isocitrate → [oxalosuccinate] → a-Ketoglutarate

  • Oxidative decarboxylation

  • Enzyme: Isocitrate dehydrogenase

  • ∆Gº’ = -20.9 kJ/mol

Reaction 4: a-Ketoglutarate → Succinyl-CoA

  • Enzyme: a-Ketoglutarate dehydrogenase complex

    • Similar to pyruvate dehydrogenase complex:

      • Same coenzymes

        • Thiamine

        • Lipoid acid

        • CoA

        • FAD

        • NAD+

      • Same mechanisms

  • ∆Gº’ = -33.5 kJ/mol

Summary of Reactions 1-4

  • No net carbon gain/loss

    • Two new carbon atoms (Acetyl-CoA)

    • Loss of two CO2 carbons

      • Do not correspond to those from A-CoA

  • now 5-8 aim to regenerate oxaloacetate

Reaction 5: Succinyl-CoA → Succinate

  • Enzyme: Succinyl-CoA synthetase

  • ∆Gº’ = -2.9 kJ/mol

  • GDP + Pi go in

  • GTP + CoA-SH come out

Reaction 6: Succinate → Fumarate

  • Enzyme: Succinate dehydrogenase

  • ∆Gº’ = 0 kJ/mol

  • FAD required

    • FADH2 produced

Reaction 7: Fumarate → L-malate

  • Enzyme: Fumarase

  • ∆Gº’ = -3.8 kJ/mol

  • Carbanion transition site (no more C=C)

  • L-Malate = substrate of next step; D-Malate is not

Reaction 8: L-Malate → Oxaloacetate

  • Enzyme: Malate dehydrogenase

  • ∆Gº’ = 29.7 kJ/mol

Summary of Reactions 1-8:

  • In TCA cycle and Pyruvate dehydrogenase reaction:

    • 1 Pyruvate oxidized to 3 CO2

    • Accompanied by reduction of

      • NAD+ → NADH

      • FAD → FADH2

    • GDP → GTP

  • Captures 35-65% of glucose’ energy

  1. CONDENSATION C-C bond formation → citrate

  2. Isomerization by dehydration

    • then hydration

  3. Oxidative decarboxylation → 2 NADH

  4. Oxidative decarboxylation → 2 NADH

  5. Substrate-level phosphorylation → GTP

  6. Dehydrogenation → FADH2

  7. Hydration

  8. Dehydrogenation → NADH

Products of each step:

  1. Citrate

  2. a. cis-Aconitate

    b. Isocitrate

  3. a-Ketoglutarate

  4. Succinyl-CoA

  5. Succinate

  6. Fumarate

  7. Malate

  8. Oxaloacetate

OVERALL ∆Gº’ = -77.7 kJ/mol

Only unfavorable reaction: L-Malate → Oxaloacetate

Fate of its Components

  • Functions in:

    • oxidation of A-CoA to CO2

    • supplying intermediates in biosynthesis of carb., fatty acids, and amino aids

  • Dual role = possible depletion of intermediates

Anaplerotic Reactions- Replenish Citric Acid Cycle Components

  1. Pyruvate carboxylase (liver and kidney)

    MOST IMPORTANT

    • OAA generated

    • Allosterically ACTIVATED by A-CoA

  2. PEP carboxykinase (heart and skeletal muscle)

    • OAA generated

  3. PEP carboxylase (plants, yeast, bacteria)

    • OAA generated

  4. Malic enzyme (wide distribution)

    • L-Malate generated

Regulation of the TCA Cycle

  • Two major points:

    1. Production of A-CoA by: Pyruvate dehydrogenase complex

      - regulation at TWO levels:

      • Allosteric inhibition and activation

        • INHIBITORS:

          • ATP

          • A-CoA

          • NADH

          • Fatty Acids

        • ACTIVATORS:

          • AMP

          • CoA

          • NAD+

          • Ca2+

      • Phosphorylation

        • of E1 (pyruvate dehydrogenase)

        • At a specific Serine residue

    2. Entry of A-CoA into TCA by:

    • Citrate synthase

      • Inhibited by NADH, Succinyl-CoA, Citrate, ATP

      • Activated by ADP

    • Isocitrate Dehydrogenase

      • Inhibited by ATP

      • Activated by ADP

    • a-Ketoglutarate Dehydrogenase

      • Inhibited by NADH and Succinyl-CoA

3. Electron Transfer and Oxidative Phosphorylation

  • Electrons carried by NADH or FADH2

    • Funneled into chain of mitochondrial electron carrier respiratory chains

      • Reduce O2 to H2O

      • Drives production of ATP

Summary of Chapter 19:

  • TCA cycle:

    • Important catabolic process

      • Makes GTP

      • Makes reduced cofactors that yield ATP

    • important anabolic roles

  • Pyruvate dehydrogenase complex:

    • Converts pyruvate into Acetyl-CoA

  • Several cofactors involved in reactions that get energy from pyruvate

  • Organic chemistry rationalizes reactions of TCA