OxPhos 1 2024

Oxidative Phosphorylation Overview

• Location: Occurs in the mitochondrion, specifically within the inner mitochondrial membrane.

• Function: Involves the oxidation of NADH and the reduction of O2, resulting in ATP synthesis through three major protein complexes and two mobile carriers (cytochromes and iron-sulphur proteins).

Major Metabolic Pathways

• Stage I: Breakdown of fats, polysaccharides, and proteins into their constituents:

  • Fatty acids and glycerol

  • Glucose and other sugars

  • Amino acids

• Stage II: Conversion to Acetyl CoA.

• Stage III: Entry into the Citric Acid Cycle (TCA), resulting in:

  • Production of 2 CO2

  • Generation of electrons for subsequent steps.

• End Product: ATP is produced through oxidative phosphorylation using O2.

Mechanism of Oxidative Phosphorylation

• Reaction: NADH + H+ + 1/2 O2 → NAD+ + H2O

• Coupling: The transfer of electrons from NADH to O2 is coupled to ADP phosphorylation, producing ATP.

Energy Yield from Glucose Oxidation

• Complete oxidation of glucose (C6H12O6 + 6O2 → 6CO2 + 6H2O) generates significant energy:

  • Approximately 2,870 kJ/mol through the entire process including glycolysis, TCA cycle, and electron transport.

• Energy Balance:

  • Reduced NADH stores most of the energy (DGo’ = -220 kJ/mol).

Electron Transport Chain (ETC) Composition

• Protein Complexes:

  • Complex I: NADH dehydrogenase

  • Complex III: Cytochrome b/c1 complex

  • Complex IV: Cytochrome oxidase

• Mobile Carriers: Includes Ubiquinone (UQ) and Cytochrome c (Cytc).

Redox Potential and Energy Coupling

• Coupling Mechanisms: The energy from electron transport is used to pump protons across the inner membrane, creating an electrochemical gradient that drives ATP synthesis.

• Redox potentials:

  • NADH oxidation involves electron transfer through various complexes with distinct redox potentials.

Uncouplers and Inhibition of ETC

• Uncouplers: Certain compounds can disrupt proton gradient maintenance, leading to impaired ATP production.

• Examples of Respiratory Poisons:

  • Rotenone, Antimycin, Cyanide inhibit specific components of the electron transport chain.

Chemiosmotic Hypothesis

• Postulates:

  1. H+ Pump: Energy from electron transport is used to pump protons across the membrane.

  2. Membrane Impermeability: Inner membrane is impermeable to protons; uncouplers increase permeability.

  3. ATP Synthase Mechanism: H+ flow back through ATP synthase drives ATP synthesis from ADP and Pi.

  4. Transmembrane Exchange: Ion transport mechanisms couple with proton entry, maintaining electroneutrality and favoring ATP production.

Key Experiments and Findings

• Ochoa’s Experiment: Shows that ADP stimulates mitochondrial respiration, indicating the obligatory coupling between respiration and ATP synthesis.

Summary of ATP Yield**

• Complete oxidation of 1 mol of glucose can yield up to 38 ATP molecules through NADH (30 ATP) and FADH2 production (4 ATP) + 2 ATP from glycolysis + 2 ATP from TCA cycle.