Ch 20 + 21

• Oxidative Phosphorylation in eukaryotic cells, the vast majority of ATP synthesis occurs during oxidative phosphorylation
• Oxidative phosphorylation is linked to electron transport
• The electron transport chain transfers high-energy electrons donated by the reduced electron carriers NADH and FADH2 to oxygen
• A pH gradient (proton gradient) within the mitochondrion as a result of electron transport
• Collectively, the citric acid cycle and oxidative phosphorylation are called cellular respiration, or simply respiration
• Oxidative Phosphorylation in Eukaryotes

  • Aerobic processes occur in the mitochondria
  • The electron-transport chain and ATP synthesis occur in the mitochondria
• Electron-transport chain

  Series of intermediate carriers that transfer electrons from NADH and FADH2 to O2 exergonic process
• The citric acid cycle occurs in the mitochondrial matrix
• Mitochondrion

  • The inner membrane is the site of electron transport and ATP synthesis
  • The citric acid cycle and fatty acid oxidation occur in the matrix
• Reduction potential E0', or redox potential

  A measure of a molecule's tendency to donate or accept electrons
• Reducing agent (reductant)

  Readily donates electrons and has a negative E0'
• Oxidizing agent (oxidant)

  Readily accepts electrons and has a positive E0'
• Electrons move from a more negative reduction potential to a more positive one
• Reduction of H+ to hydrogen gas is considered to have a voltage (E) of zero
• Oxidant (oxidizing agent)

  The acceptor of electrons in an oxidation–reduction reaction
• Reductant (reducing agent)

  The donor of electrons in an oxidation–reduction reaction
• The more positive the E0 value - the greater the reduction potential (will be reduced)
• Standard reduction potentials

  • Provide a basis for comparison among redox reactions
  • Help predict sequence of reactions in the electron transport chain
• In all reactions, electrons are passed from the reduced form of one reactant (a lower reduction potential) to the oxidized form of the next reactant(a high reduction potential )in the chain
• Energy is released when high-energy electrons are transferred to oxygen
• Faraday = 96.48 kJ/Vmol
• Electron-transport chain

  • Composed of four large protein complexes
  • The electrons donated by NADH and FADH2 are passed to electron carriers in the protein complexes
• Electron carriershave these prosthetic groups:

  • Flavin mononucleotide (FMN)
  • Iron associated with sulfur in proteins (iron–sulfur proteins)
  • Iron incorporated into hemes that are embedded in proteins called cytochromes
  • A mobile electron carrier called coenzyme Q (Q)
• Complex I and II are both flavoprotein
• Iron–sulfur clusters

  • A single iron ion bound by four cysteine residues
  • 2Fe-2S cluster with iron ions bridged by sulfide ions
  • 4Fe-4S cluster
• Complex I, II, and III all contain Iron-sulfur clusters
• Heme

  A component of Cytochrome c Oxidase
• All cytochromes contain a heme group
• Coenzyme Q

  • Derived from isoprene
  • Also known as ubiquinone
  • Binds protons (QH2) as well as electrons and can exist in several oxidation states
  • Oxidized and reduced Q are present in the inner mitochondrial membrane in what is called the Q pool
• The reduction of ubiquinone (Q) to ubiquinol (QH2) proceeds through a semiquinone intermediate (QH•)
• Electron-transport chain

  • Electrons flow from NADH to O2 through three large protein complexes (I, III, IV) embedded in the inner mitochondria membrane
  • These complexes pump protons out of the mitochondria, generating a proton gradient
• Protein complexes in the electron-transport chain

  • NADH-Q oxidoreductase (Complex I)
  • Q-cytochrome c oxidoreductase (Complex III)
  • Cytochrome c oxidase (Complex IV)
  • Succinate Q-reductase (Complex II)
• Succinate-Q reductase is not a proton pump
• The electron affinity of the components increases as electrons move down the chain
• Components of the Mitochondrial Electron-Transport Chain

  • NADH-Q oxidoreductase (Complex I)
  • Succinate-Q reductase (Complex II)
  • Q-cytochrome c oxidoreductase (Complex III)
  • Cytochrome c oxidase (Complex IV)
  • Cytochrome c
• NADH-Q Oxidoreductase (Complex I)

  • The electrons from NADH are passed along to Q to form QH2
  • QH2 leaves the enzyme for the Q pool in the hydrophobic interior of the inner mitochondrial membrane
  • Four protons are simultaneously pumped out of the mitochondria
• Succinate-Q reductase (Complex II)

  • Succinate dehydrogenase of the citric acid cycle is a part of this complex
  • The FADH2 generated in the citric acid cycle reduces iron–sulfur protein, which then reduces Q to QH2
• Q Cycle

  • Mechanism for coupling electron transfer from QH2 to cytochrome c
  • In one cycle, four protons are pumped out of the mitochondria and two more are removed from the matrix
• Cytochromes

  • All contain a heme group
  • Differences in the side chain depending on the heme
  • In each heme group, the iron is successively reduced to Fe (II) and reoxidized to Fe (III)
• Different types of cytochromes are distinguished by lowercase letters (a, b, c), and further distinction is made possible with subscripts (c1)