Electron Transport Chain Study Notes

Overview and Cellular Localization of Oxidative Phosphorylation

• Oxidative phosphorylation and the electron transport chain (ETC) are responsible for generating the bulk of cellular $ATP$

• In eukaryotic organisms, the components of the electron transport chain and oxidative phosphorylation machinery are embedded within the inner mitochondrial membrane and the cristae

• In bacterial cells, electron transport chain complexes and ATP synthase are situated directly on the plasma membrane

Structural Components and Cofactors of the Electron Transport Chain

• Complex I: Known as NADH-ubiquinone oxidoreductase, which carries out both oxidation and reduction processes

• Complex II: Known as Succinate-ubiquinone reductase

• Complex III: Known as Ubiquinone-cytochrome c oxidoreductase

• Complex IV: Known as Cytochrome c oxidase

• Mobile Carriers and Structural Units:

  • Ubiquinone incorporates an isoprene unit

  • Cytochrome c contains cytochrome structural components

• Iron Cofactor:

  • Iron ($Fe$) serves as an essential cofactor within the complexes and cytochromes

  • Iron is classified strictly as an inorganic chemical component (cofactor), distinct from an organic coenzyme

Electron Transport Pathways and Terminal Electron Acceptor

• Pathway for NADH Electrons:

  • $NADH$ donates electrons directly to Complex I, oxidizing $NADH$ to $NAD^+$ and releasing protons ($H^+$)

  • Electrons travel from Complex I to ubiquinone, then to Complex III, then to cytochrome c, and finally to Complex IV

• Pathway for FADH2 Electrons:

  • $FADH_2$ donates electrons to Complex II, leaving $2H^+$ protons

  • Electrons travel from Complex II to ubiquinone, then to Complex III, then to cytochrome c, and finally to Complex IV

• Terminal Electron Acceptance:

  • Complex IV transfers the final electrons to oxygen ($O_2$), which acts as the last electron acceptor

Proton Pumping and Electrochemical Gradient Generation

• Proton Pumps:

  • Complex I, Complex III, and Complex IV function actively as proton pumps

  • Complex II does not function as a proton pump

• Gradient Formation:

  • As electrons move through Complexes I, III, and IV, protons ($H^+$) are pumped out of the mitochondrial matrix and into the intermembrane space

  • This active transport creates a concentration gradient of protons ($H^+$ proton gradient) across the inner mitochondrial membrane

Chemiosmosis and Complex V (ATP Synthase)

• Peter Mitchell's Chemiosmotic Hypothesis:

  • Chemiosmosis explains that the energy from a proton gradient drives ATP synthesis

  • Bacteria placed in an acidic solution demonstrate this process by synthesizing $ATP$ due to the surrounding excess of protons ($H^+$) relative to their intracellular environment

• Complex V Function and Mechanism:

  • Complex V is ATP Synthase

  • Because protons ($H^+$) are charged ions, they cannot directly diffuse through the hydrophobic phospholipid bilayer of the inner membrane

  • Protons diffuse down their concentration gradient back into the matrix exclusively by passing through the channel provided by Complex V

  • This diffusion of protons down their gradient is exergonic

  • The energy freed by the passive return of protons through Complex V powers the phosphorylation reaction:

$ADP + P_i \rightarrow ATP$

Inhibitors and Toxins of the Electron Transport Chain

• Cyanide Toxicity:

  • Cyanide acts as a severe biological toxin by targeting Complex IV

  • Cyanide specifically binds to the iron ($Fe$) cofactor present inside Complex IV

  • The binding of cyanide to iron causes irreversible denaturation of Complex IV

  • Irreversible denaturation of Complex IV halts the entire electron transport chain, preventing proton gradient maintenance and terminating ATP generation via Complex V