Oxidative Phosphorylation

Citric Acid Cycle Steps

Page 1: Isomerization

  • Step 2: Citrate isomerized to Isocitrate by aconitase.

    • Involves a dehydration and subsequent rehydration.

    • Tertiary alcohols cannot oxidize without breaking carbon-carbon bonds, necessitating isomerization to generate a secondary alcohol (Isocitrate).

Oxidation of Isocitrate

  • Step 3: Oxidation of Isocitrate to α-Ketoglutarate by isocitrate dehydrogenase.

    • Results in loss of CO2 and reduces NAD+ (or NADP+) to NADH (or NADPH).

Oxidation of α-Ketoglutarate

  • Oxidation of α-Ketoglutarate to Succinyl-CoA by α-ketoglutarate dehydrogenase complex.

    • Results in loss of CO2 and reduces NAD+ to NADH.

    • Similar to the reaction converting pyruvate to acetyl-CoA, utilizing the same cofactors (TPP, lipoic acid, FADH) as the PDH complex.

Conversion to Succinate

  • Conversion of Succinyl-CoA to Succinate by succinyl-CoA synthetase.

    • Regenerates Coenzyme A and forms either ATP or GTP via substrate-level phosphorylation.

Oxidation to Fumarate

  • Oxidation of Succinate to Fumarate by succinate dehydrogenase.

    • Reduces one FAD to FADH2.

    • Succinate dehydrogenase exists tightly bound to the inner mitochondrial membrane.

Hydration to Malate

  • Hydration of Fumarate to Malate by fumarase.

    • Requires a water molecule for hydration.

Oxidation of Malate

  • Oxidation of Malate to Oxaloacetate by malate dehydrogenase.

    • Converts NAD+ into NADH.

    • Energetically unfavorable but driven forward by the next reaction, maintaining low oxaloacetate levels.

Summary of Citric Acid Cycle Outputs

  • Outputs:

    • Acetyl-CoA leads to: Citrate → Isocitrate → α-Ketoglutarate → Succinyl-CoA → Succinate → Fumarate → Malate → Oxaloacetate.

    • Produces NADH, FADH2, CO2, and GTP (ATP).

Amphibolic Nature of Citric Acid Cycle

Anabolic and Catabolic Functions

  • The citric acid cycle is amphibolic, serving both anabolic and catabolic purposes.

    • Compounds from the cycle can be siphoned off for use in other metabolic pathways.

Regulation of Cycle

Key Regulators

  • Important molecules: Pyruvate, ATP, Acetyl-CoA, NADH, ADP, Ca2+.

    • Regulation involves various key enzymes:

      • Citrate synthase, isocitrate dehydrogenase, and α-ketoglutarate dehydrogenase are all influenced by the concentrations of their product/reactants and energy status.

Oxidative Phosphorylation

Purpose of Reduced Carriers

  • Reduced NADH and FADH2 are generated in glycolysis and the citric acid cycle.

    • High-energy electrons in these carriers are passed along the mitochondrial electron transport chain, generating ATP.

    • The final electron acceptor is O2.

Mechanism

  • High-energy electrons in NADH and FADH2 are transferred through the mitochondrial inner membrane's electron transport chain.

    • Electrons lose energy (standard reduction potential, Eo’) as they are passed along.

    • NADH and FADH2 enter the chain from different pathways, converging at Coenzyme Q.

Final Electron Transfer

  • Complex IV transfers electrons from Cyt c to O2 via cytochromes.

    • Reaction: 4 cyt c (Fe+2) + O2 + 8H+ → 4 cyt c (Fe+3) + 4H+ + 2 H2O.

    • The transfer involves the pumping of protons across the mitochondrial membrane.

Inhibition and Gibbs Free Energy

Inhibition of Electron Transport

  • Various compounds can inhibit electron transport, which can serve as tools to determine carriers' order in the chain.

ΔGo’ Calculations

  • Energy considerations for ATP synthesis:

    • Two distinct activities:

      • Electron flow from NADH to O2 (ΔGo’ = -220 kJ/mol)

      • Phosphorylation of ADP to form ATP (ΔGo’ = +31 kJ/mol).

Mechanism of ATP Synthesis

  • Chemiosmotic coupling drives ATP synthesis.

    • Protons are pumped into the intermembrane space, generating a gradient.

    • ATP synthase harnesses this gradient to produce ATP via proton passage.

Final Notes

Yield from Glucose

  • Final balance for one glucose molecule:

    • Assigns 2.5 ATP/NADH and 1.5 ATP/FADH2, yielding approximately 31 ATP per glucose.

    • ΔGo’ for glucose oxidation: -2870 kJ/mol, with ATP hydrolysis being -30.5 kJ/mol, leading to an efficiency of conversion to ATP of about 32%.