CH 20: The Electron-Transport Chain

Biochemistry: Electron-Transport Chain Notes

Chapter Overview and Key Sections

  • Oxidative Phosphorylation

    • Captures energy from high-energy electrons to synthesize ATP.

    • Electrons flow from NADH and FADH2 to O2 through redox reactions in the electron-transport chain (ETC).

    • Generates a proton gradient that drives ATP synthesis.

20.1: Oxidative Phosphorylation in Mitochondria

  • Mitochondrial Structure

    • Outer membrane: permeable due to mitochondrial porin channels.

    • Inner membrane: site of electron transport and ATP synthesis, folded into structures called cristae.

  • Surface Area

    • Human average mitochondrial surface area is approximately 1.7 m².

20.2: Electron Transfer in Oxidative Phosphorylation

  • Reduction Potential (E0′)

    • Indicates the tendency of a molecule to donate or accept electrons.

    • Strong reducing agents have negative E0′, strong oxidizing agents have positive E0′.

    • Implications:

    • Lower E0′: lower affinity for electrons.

    • Higher E0′: higher affinity for electrons.

  • Standard Reduction Potential Table

    • Includes various redox couples with respective E0′ values.

Free Energy and Electron-Transport Chain

  • Energy is released during electron transfer to oxygen and utilized to create a proton gradient that is pivotal for ATP synthesis.

  • Relation between reduction potential and Gibbs free energy—used to establish the driving force for ATP production.

The Electron-Transport Chain Components

Main Components

  • Protein Complexes in ETC

    • Comprised of four large protein complexes:

    • Complex I: NADH-Q oxidoreductase

    • Complex II: Succinate-Q reductase

    • Complex III: Q-Cytochrome c oxidoreductase

    • Complex IV: Cytochrome c oxidase

Electron Carriers

  • Includes:

    • Flavin mononucleotide (FMN)

    • Iron-sulfur (Fe-S) proteins

    • Cytochromes with hemes

    • Coenzyme Q (Ubiquinone)

Section 20.3: Proton Pumps & Citric Acid Cycle Link

  • Electron flow from NADH to O2 through proton pumps embedded in the inner mitochondrial membrane.

  • Proton pumps:

    • Complex I: Pumps out 4 protons

    • Complex III: Pumps out additional 4 protons

    • Complex IV: Pumps chemical protons and also reduces O2 to H2O

Complex Specific Functions

Complex I: NADH-Q Oxidoreductase

  • Transfers electrons from NADH to Ubiquinone (Q), forming QH2 while pumping 4 protons out of the mitochondria.

Complex II: Succinate-Q Reductase

  • Integrates into the citric acid cycle, reducing Q to QH2 using FADH2. It is not a proton pump.

Complex III: Q-Cytochrome c Oxidoreductase

  • Utilizes electrons from QH2 to reduce cytochrome c and pumps protons.

  • Q Cycle: Mechanism for efficient electron transfer between QH2 and cytochrome c.

Complex IV: Cytochrome c Oxidase

  • Catalyzes the reduction of O2 to H2O with protons from the matrix and pumps out additional protons for the gradient.

ROS and Cellular Damage

  • Reactive Oxygen Species (ROS) produced from partial reduction of O2 can lead to cellular damage.

  • Enzymes like superoxide dismutase help mitigate ROS effects.

Summary of Key Concepts

  • Proton Motive Force: Established by the ETC, crucial for ATP synthesis.

  • Pathological Implications: Understanding of ROS's role in disease.

  • Components: Detailed knowledge of complexes and associated enzymatic reactions is critical.

  • Quizzes and exercises enhance understanding of ETC functions and consequences of inhibitor actions (like amytal).