BCH 210 (Electron Transport chain)

The Electron Transport Chain Overview

  • Date: February 19, 2003

  • Presenter: Bryant Miles

Citric Acid Cycle and Electron Transport

  • The citric acid cycle oxidizes acetate into:

    • 2 molecules of CO2

    • Captures electrons as:

      • 3 NADH molecules

      • 1 FADH2 molecule

  • Reduced molecules (NADH, FADH2) contain high transfer potential electrons.

  • Electrons are transferred to O2 to form H2O.

  • Major energy source for ATP production through oxidative phosphorylation.

Free Energy Changes

  • Standard free energy change for the reaction:

    • NADH + H+ + 1/2 O2 → NAD+ + H2O

    • Reduction potentials:

      • NAD+/NADH: E° = -0.315 V

      • O2/H2O: E'° = +0.816 V

  • Overall change in reduction potential:

    • ΔE'° = +0.816 - (-0.315) = +1.136 V

  • ΔG° = -nFΔE'° = -219 kJ/mol

  • ATP synthesis calculation:

    • ATP = 219 / 30.5 = 7.2 ATP

Mitochondrial Structure

Mitochondria

  • Sites of electron transport and oxidative phosphorylation.

  • Eukaryotic cells (e.g., mammalian cells) have mitochondrial structures.

  • Mammalian cells contain 800 to 2,500 mitochondria each.

  • Exception: Red blood cells lack mitochondria.

Membrane Structure

  • Outer membrane:

    • Contains porins (transmembrane proteins) that allow <10,000 Da molecules to diffuse freely.

    • Chemically equivalent to the cytosol.

  • Inner membrane:

    • Contains 80% proteins contributing to electron transport and ATP synthesis.

    • Impermeable to ions and molecules; requires transport proteins for crossing.

    • Folded into cristae, increasing surface area.

  • Matrix:

    • Encloses enzymes essential for metabolism (e.g., pyruvate dehydrogenase, citric acid cycle enzymes).

    • Contains mitochondrial genome, ribosomes, and tRNAs.

Components of the Electron Transport Chain

  1. NADH:

    • Generated in the matrix by specific dehydrogenase reactions.

    • Transfers 2 electrons as a hydride (NAD+ + H+ + 2e- → NADH).

  2. Flavoproteins:

    • Contain FAD or FMN.

    • Accept/donate electrons either one at a time or two at a time.

    • Typical standard reduction potential: ~0 V.

  3. Coenzyme Q (Ubiquinone):

    • Soluble electron carrier in the lipid bilayer.

    • Able to accept/donate one or two electrons.

  4. Cytochromes:

    • Contain heme groups for electron transfer.

    • Types include cytochromes b, c, and a/a3.

    • Distinct absorption spectra.

  5. Iron-Sulfur Proteins:

    • Participate in single electron transfers with Fe2+ and Fe3+ states.

    • Characteristics include various clusters like FeS, Fe2S2, Fe3S4, and Fe4S4.

  6. Copper Proteins:

    • Involved in one-electron transfers with Cu+ and Cu2+ states.

Overview of the Electron Transport Chain

  • Electrons move through the chain from donors to oxygen as the final acceptor.

  • Organized into 4 complexes (I-IV):

    1. Complex I:

      • NADH dehydrogenase (NADH-Coenzyme Q reductase). Links glycolysis, citric acid cycle, and fatty acid oxidation to electron transport.

    2. Complex II:

      • Succinate dehydrogenase, linking the citric acid cycle to electron transport.

    3. Complex III:

      • Coenzyme Q reductase.

    4. Complex IV:

      • Cytochrome c oxidase.

Functional Dynamics of Complexes

  • Complex I:

    • Transfers electrons from NADH to CoQ, generating a proton gradient.

    • Contains FMN and iron-sulfur clusters for electron transfer.

  • Complex II:

    • Catalyzes the conversion of succinate to fumarate while transferring electrons to CoQ.

    • Composed of four subunits, with covalently bound FAD.

    • Does not drive proton transport, resulting in lower ATP yield (1.5 ATP per FADH2 vs 2.5 ATP per NADH).

Advanced Cytochrome and Q-cycle in Complex III

  • The Q-cycle involves a series of electron transfers facilitated by CoQH2.

  • First step releases protons into the intermembrane space and generates semiquinone.

  • Further interactions ensure efficient electron transfer to cytochrome C, contributing to ATP synthesis.

Complex IV Functionality

  • Also known as cytochrome c oxidase; completes the reduction of oxygen.

  • Accepts electrons from cytochrome C to form water.

  • Involves a series of electron transfers and associated proton movements, supporting the final step in ATP synthesis.