In-Depth Notes on the Electron Transport Chain and Oxidative Phosphorylation

Overview of Electron Transport Chain (ETC)

• Course Information:
  • Course: CHEM 38103 - Elements of Biochemistry
  • Instructor: Shivakumar Sonnaila
  • University: University of Arkansas
  • Semester: Spring 2025
  • Location: Estes Park, CO

Key Reactions in Cellular Respiration

• Glycolysis:

  • Input: Glucose + 2 Pᵢ + 2 ADP + 2 NAD⁺
  • Output: 2 Pyruvate + 2 ATP + 2 NADH + 2 H⁺ + 2 H₂O

• Conversion to Acetyl CoA:

  • Input: Pyruvate + CoA + NAD⁺
  • Output: Acetyl CoA + CO₂ + NADH + H⁺

• Citric Acid Cycle (Krebs Cycle):

  • Input: Acetyl CoA + 3 NAD⁺ + FAD + ADP + Pᵢ + 2 H₂O
  • Output: 2 CO₂ + 3 NADH + FADH₂ + ATP + 2 H⁺ + CoA

• Oxidative Phosphorylation:

  • Generates 26 of the 30 ATP from the complete oxidation of 1 glucose molecule to CO₂ and H₂O.

Oxidative Phosphorylation

• Functionality:
  • Major source of ATP in aerobic organisms.
  • Occurs in the inner mitochondrial membrane.
  • High-energy electrons from NADH and FADH₂ are transferred through protein complexes in the respiratory chain, reducing O₂ to H₂O.
  • Electron transfer drives protons (H⁺) out of the mitochondrial matrix, creating a proton gradient (proton-motive force).
  • ATP synthase uses this gradient to generate ATP as protons flow back into the matrix.
• Key Takeaway: Proton gradients serve as a form of energy currency in biological systems.

Mitochondrial Structure

• Components include:
  • Outer Membrane
  • Inner Membrane
  • Cristae (folds in the inner membrane, increase surface area)
  • Intermembrane Space
  • Matrix (site of multiple metabolic processes)

Components of the Electron Transport Chain (ETC)

1. Complex I: NADH: Ubiquinone Oxidoreductase

   • Function: Transfers electrons from NADH to Coenzyme Q (ubiquinone).
   • Donors: NADH
   • Acceptors: Ubiquinone
   • Cofactors: FMN, Fe-S clusters

2. Complex II: Succinate: Ubiquinone Oxidoreductase

   • Function: Transfers electrons from FADH₂ to Coenzyme Q.
   • Donors: FADH₂
   • Acceptors: Ubiquinone
   • Cofactors: FAD, Fe-S clusters

3. Complex III: Cytochrome c Oxidoreductase

   • Function: Transfers electrons from Ubiquinol to Cytochrome c.
   • Donors: Ubiquinol
   • Acceptors: Cytochrome c
   • Cofactors: Cytochromes (b, c₁), Fe-S clusters

4. Complex IV: Cytochrome c Oxidase

   • Function: Transfers electrons from Cytochrome c to O₂, forming H₂O.
   • Donors: Cytochrome c
   • Acceptors: O₂
   • Cofactors: Heme a, a₃; Cu²⁺ centers

5. ATP Synthase (Complex V)

   • Function: Synthesizes ATP using the established proton gradient.
   • Input: Proton gradient (H⁺), ADP + Pᵢ
   • Output: ATP

Proton Pumping and ATP Yield

• Protons Pumped Per Complex:

  • Complex I: 4 (from NADH)
  • Complex II: 0
  • Complex III: 4 (Q cycle)
  • Complex IV: 2
  • Total per NADH: 10 protons
  • Total per FADH₂: 6 protons

• ATP Synthesis Efficiency:

  • 1 NADH ≈ 2.5 ATP
  • 1 FADH₂ ≈ 1.5 ATP

Transport Mechanisms of NADH

• Glycerol 3-Phosphate Shuttle:
  • Allows cytoplasmic NADH to enter the mitochondrial electron transport chain, reducing dihydroxyacetone phosphate into glycerol 3-phosphate, enabling the transfer of electrons into the ETC.

Summary of ATP Yield from Glucose Oxidation

• Complete Oxidation Process:
  • Glycolysis: 2 ATP + 2 NADH (yielding 3 ATP)
  • Pyruvate to Acetyl CoA: 2 NADH (yielding 5 ATP)
  • Citric Acid Cycle: 2 ATP + 6 NADH (yielding 15 ATP) + 2 FADH₂ (yielding 3 ATP)
• Net Yield per Glucose:
  • Total ATP: Approximately 30
  • Note: Modern values reflect a net yield of 30 ATP, correcting earlier estimates of 36 ATP due to improvements in understanding of proton pumping and ATP synthesis.

Important Notes

• Proton gradients are integral to ATP generation.
• Complexes I and II contribute to proton pumping efficiency for ATP production.
• The choice of shuttle mechanisms (glycerol 3-phosphate vs. malate-aspartate) affects overall ATP yield from glucose oxidation.