05: Oxidative Phosphorylation and Electron Transport Chain Notes

Oxidative Phosphorylation and Role of the Electron Transport Chain

Section Objectives

• Importance of Mitochondria:

• Key role in aerobic respiration (energy production using oxygen).

• High energy demands in liver and muscle tissues.

• Structural Features of Mitochondria:

• Outer Membrane: Permeable to small molecules and ions.

• Intermembrane Space: Space between the outer and inner membranes.

• Inner Membrane: Impermeable to most small molecules; houses electron transport chain.

• Matrix: Contains enzymes for metabolic processes and mitochondrial DNA.

• Enzymes of the Respiratory Complex:

• Include dehydrogenases and various electron carriers.

• Protein Complexes of the Electron Transport Chain:

• Four multi-subunit enzyme complexes responsible for electron transfer:

  • Complex I: NADH to ubiquinone.
  • Complex II: Succinate to ubiquinone.
  • Complex III: Ubiquinol to cytochrome c.
  • Complex IV: Cytochrome c to oxygen.

• Chemiosmotic Theory:

• Explains how proton movement generates ATP.

• Proton Motive Force:

• Resulting from a concentration and charge gradient of protons.

• Role of ATP Synthase:

• Catalyzes the conversion of ADP and inorganic phosphate to ATP.

• Production of ATP from NADH and FADH2:

• 2.5 ATP per NADH and 1.5 ATP per FADH2.

Mitochondria Structure

• Outer Membrane:

• Contains porin (voltage-gated anion channel) which allows small, anionic molecules to pass.

• Inner Membrane:

• Contains folds called cristae to increase surface area for reactions.

• Hydrophobic and requires specific transporters for molecule entry.

• Matrix:

• Contains enzymes for the TCA cycle and fatty acid oxidation.

• Houses mitochondrial DNA for synthesizing key proteins.

Respiratory Complex

• NADH and FADH2:
• Produced by the TCA cycle and glycolysis.
• Serve as electron donors in oxidative phosphorylation.

Mechanism of Electron Transfer

• Electron Transport Chain:

• Series of electron carriers including NAD+/NADH, FMN/FAD, ubiquinone.

• Transfer of electrons releases energy used to pump protons across the membrane.

• Coenzyme Q (Ubiquinone):

• Lipid-soluble, functions to transfer electrons and couple electron flow to proton movement.

• Heme Groups:

• Prosthetic groups that participate in electron transfer.

• Cytochromes:

• One-electron transporters characterized by iron-containing hemes.

• Three types: a, b, and c.

• Iron-Sulfur Proteins:

• Participate in electron transfer; consist of iron and sulfur associations.

The Electron Transport Chain (ETC)

• Complex I (NADH-Q reductase):

• Uses FMN, transfers electrons from NADH to ubiquinone, pumps 4 protons across the membrane.

• Complex II (Succinate-Q reductase):

• Links succinate oxidation with ubiquinone reduction; does not pump protons.

• Complex III (QH2-cytochrome c reductase):

• Couples electron transfer to cytochrome c and pumps protons across the membrane.

• Complex IV (Cytochrome c oxidase):

• Reduces molecular oxygen to water; 4 H+ are pumped for every two electrons.

Energy and ATP Production

• Flow of Electrons:

• Electrons move from a higher to lower energy state, releasing energy that is used to pump protons, establishing a proton gradient.

• Chemiosmotic Theory:

• Describes the relationship between proton gradients and ATP synthesis.

Proton Motive Force

• Mechanism:
• Movement of protons back into the matrix through ATP synthase drives ATP production.

Chemiosmotic Model

• ATP Production:
• 2.5 ATP produced per NADH and 1.5 ATP per FADH2 through the proton gradient established by the electron transport chain.

Summary

• Key Points:
• Complexes I, III, and IV act as proton pumps; Complex II does not.
• ATP synthase converts ADP and inorganic phosphate into ATP, relying on the passage of protons back into the matrix.
• Net H+ movements calculated for ATP yield: 10 H+/4 for NADH (approx. 2.5 ATP), 6/4 for FADH2 (approx. 1.5 ATP).