Metabolic: lecture 24

Mitochondrial Electron Transport

• Mitochondrial electron transport (MET) involves four inner mitochondrial membrane complexes and two mobile electron carriers (Coenzyme Q and cytochrome c)
• All components are embedded in the membrane.
• The process involves a large positive change in free energy. The free energy relative to oxygen is measured in kJ/mol

Complexes and Electron Carriers

• NADH dehydrogenase (Complex I): Transfers electrons from NADH to Ubiquinone (Coenzyme Q). Accompanies a transfer of protons from the matrix to intermembrane space. About 4 protons are transported for each NADH reacted.

• Succinate dehydrogenase (Complex II): Converts succinate to fumarate. Captures and donates electrons to the electron transport chain, generating ATP. Does not transport protons across the inner mitochondrial membrane.

• Ubiquinone:cytochrome c oxidoreductase (Complex III): Uses two electrons from QH2 to reduce two molecules of cytochrome c. Translocates four additional protons to the intermembrane space. Clearance of electrons from reduced quinones creates a Q-cycle.

• Cytochrome c oxidase (Complex IV): Transfers four electrons from cytochrome c to oxygen. Two ions (CuA & CuB) are involved in accepting electrons and bonding to oxygen, forming water. Four protons are picked up from the matrix. Two additional protons are transported across the membrane.

The Q Cycle

• The Q cycle is a process where electrons from QH2 are transported to cytochrome c and additional protons are pumped into the intermembrane space. Two QH2 molecules are oxidized releasing protons into the intermembrane space. A single molecule of Q becomes re-reduced, transferring 4 protons (for two electrons)
• Four protons are transported across the membrane per two electrons that reach cytochrome c.
• Two of the four protons come from QH2.
• Two molecules of QH₂ become oxidized, releasing protons into the intermembrane space. One molecule of Q becomes re-reduced, resulting in a net transfer of four protons per reduced coenzyme Q

Cytochrome C

• Cytochrome c is a heme-containing protein.
• The heme iron can exist in ferric (Fe3+, oxidized) or ferrous (Fe2+, reduced) forms.
• It carries a single electron from Complex III to Complex IV.
• Ubiquinone moves through the membrane while Cytochrome c moves through the intermembrane space.

Cytochrome Oxidase (Complex IV)

• Cytochrome oxidase is a membrane protein with 13 subunits.
• It contains two heme groups and two copper ions (CuA & CuB).
• It transfers four electrons from cytochrome c to oxygen, forming two water molecules.
• Four protons are picked up from the matrix and two additional protons are transported across the membrane.

Proton Motive Force (PMF)

• Proteins in the ETC create an electrochemical proton gradient across the membrane.
• Three methods accomplish this:
  • Actively transporting protons across the membrane (Complexes I, III, and IV)
  • Chemically removing protons from the matrix during the reduction of CoQ and oxygen.
  • Releasing protons into the intermembrane space during oxidation of QH2

Chemiosmotic Model for ATP Synthesis

• Electron transport sets up a proton-motive force.
• The energy of the proton-motive force drives ATP synthesis.