Citric Acid Cycle

Bypass Reactions in Glycolysis

  • Bypass #2:

    • Fructose-1,6-Bisphosphate to Fructose-6-Phosphate

    • Reactants: H2O, Pi

    • Enzyme: Fructose bisphosphate bisphosphatase

  • Bypass #3:

    • Glucose-6-Phosphate to Glucose

    • Reactants: H2O, Pi

    • Enzyme: Glucose-6-phosphatase

Enzyme Regulation in Glucose Metabolism

  • Phosphofructokinase (PFK-1) / Fructose-1,6-bisphosphatase:

    • Major regulatory point for glycolysis vs. gluconeogenesis

    • PFK-1: commits glucose to glycolysis

      • High ATP = Gluconeogenesis favored

      • High AMP = Glycolysis favored

Fructose 2,6-bisphosphate Role

  • Metabolism Mediation:

    • High glucagon = activation of protein kinase A → phosphorylates PFK2 → inactivates it

    • High insulin = dephosphorylation of PFK2 → activates PFK2

    • Active PFK2 → produces F-2,6-bisphosphate, activating PFK1

    • Result: Increased glycolysis

Aerobic Metabolism of Pyruvate

  • Pyruvate Oxidation:

    • Occurs in mitochondria (matrix and inner membrane)

    • Oxygen as the final electron acceptor

    • Produces Acetyl-CoA along with NADH and FADH2

Enzymatic Actions During Aerobic Metabolism

  • Substrates:

    • Pyruvate, oxygen, NAD, FAD, ADP

  • Products:

    • CO2, reduced NADH, FADH2, ATP

    • Enzymes organized in functional units (metabolons)

Pyruvate Dehydrogenase Complex

  • Oxidation of Pyruvate:

    • Pyruvate dehydrogenase is a multienzyme complex

    • Reaction: Decarboxylation, oxidation to C2, activation via thioester bond to CoA

    • Enzymatic Components:

      • E1: Pyruvate decarboxylase (complex 24 copies, coenzyme TPP)

      • E2: Dihydrolipoyl transacetylase (complex 24 copies, coenzymes lipoic acid, CoASH)

      • E3: Dihydrolipoyl dehydrogenase (complex 12 copies, coenzymes NAD+, FAD)

Steps of the Pyruvate Dehydrogenase Cycle

  1. Decarboxylation & TPP Formation: Requires TPP. Hydroxyethyl bound to TPP.

  2. Oxidation to Acetyl Group: Disulphide bond in lipoamide group reduced. Thioester bond forms.

  3. Formation of Acetyl-CoA: Transfer to CoA via thiol group.

  4. Reoxidation of Lipoamide: FAD reduced to FADH2, then reduces NAD+ to regenerate FAD.

Sum Reaction of Pyruvate Dehydrogenase

  • Reaction: Pyruvate + CoASH + NAD+ → Acetyl-CoA + CO2 + NADH + H+

Fate of Acetyl-CoA

  • Converted to NADH, FADH2, ATP, GTP, and CO2 during the TCA cycle.

TCA Cycle Steps Overview

  1. Formation of Citrate: Acetyl-CoA + Oxaloacetate → Citrate (exergonic).

  2. Isomerization: Citrate to Isocitrate by aconitase.

  3. Oxidation to α-Ketoglutarate: Loss of CO2, reduction of NAD+.

  4. Conversion to Succinyl-CoA: Identical reaction to pyruvate oxidation (NAD+ reduced).

  5. Conversion to Succinate: Regenerates Coenzyme A, produces ATP/GTP.

  6. Oxidation to Fumarate: Reduces one FAD to FADH2.

  7. Hydration to Malate: Involves a water molecule.

  8. Oxidation to Oxaloacetate: Converts NAD+ to NADH, driven by subsequent reactions.

TCA Cycle Output Summary

  • Reactants: Acetyl-CoA, Citrate

  • Intermediate Outputs: NADH, CO2, GTP (ATP)

  • Final Products: Oxaloacetate, Isocitrate, α-Ketoglutarate, Fumarate, Succinate.