1.7

• Electron transport chain


  1. NADH and FADH2 from citric acid cycle pass to electron transport chain

  2. Electrons and hydrogen ions from NADH and FADH2 are passed to intermediate carriers

  3. Electrons and hydrogen ions ultimately react with molecular oxygen to produce water
• Electron transport chain


  Also called the respiratory chain
• Citric acid cycle


  1. Oxaloacetate

  2. Citrate

  3. Isocitrate

  4. α-Ketoglutarate

  5. Succinyl CoA

  6. Succinate

  7. Fumarate

  8. Malate
• NADH and FADH2 produced in the citric acid cycle pass to the electron transport chain
• The electron transport chain is a series of biochemical reactions in which electrons and hydrogen ions from NADH and FADH2 are passed to intermediate carriers and then ultimately react with molecular oxygen to produce water
• NADH and FADH2 are oxidized in the electron transport chain process
• Electron transport chain reactions

  1. FADH2 oxidation to FAD

  2. NADH oxidation to NAD

  3. Oxygen reduction to water
• Electron transport chain


  • Enzymes and electron carriers located along the inner mitochondrial membrane
  • Four distinct protein complexes containing molecules needed for the electron transport chain process
• Protein complexes in the electron transport chain


  • Complex I: NADH–coenzyme Q reductase
  • Complex II: Succinate–coenzyme Q reductase
  • Complex III: Coenzyme Q–cytochrome c reductase
  • Complex IV: Cytochrome c oxidase
• Coenzyme Q and cytochrome c


  Mobile electron carriers that shuttle electrons between the various complexes
• Complex I: NADH–Coenzyme Q Reductase


  1. NADH oxidation to NAD

  2. FMN reduction to FMNH2

  3. Electron transfer through iron-sulfur proteins

  4. Electron transfer to coenzyme Q
• FMN


  Electron carrier, synthesized from riboflavin
• Iron-sulfur proteins


  Electron carriers, undergo Fe3+ to Fe2+ reduction
• NADH supplies


  Electrons and one hydrogen ion to FMN
• The other hydrogen ion comes from the matrix solution
• Complex II: Succinate–Coenzyme Q Reductase

  1. FAD reduction to FADH2

  2. Electron transfer through iron-sulfur proteins

  3. Electron transfer to coenzyme Q
• FAD


  Electron carrier, synthesized from riboflavin
• FADH2 carries electrons


  From complex II to complex III
• Complex III: Coenzyme Q–Cytochrome c Reductase

  1. Electron transfer from CoQH2 to iron-sulfur proteins

  2. Electron transfer to cytochromes b, c1, and c
• Cytochrome c is the only water-soluble cytochrome, it delivers electrons to complex IV
• CoQH2 is the initial substrate


  For complex III
• The hydrogen ions produced from the oxidation of CoQH2 go into cellular solution
• Electron movement through complex III

  1. Electron carrier CoQH2 initiates

  2. Electrons passed to cyt c in several steps
• Iron/sulfur protein (FeSP)


  Nonheme iron protein, often containing sulfur bound to iron in cysteine
• A feature shared by all steps in the ETC is that as each electron carrier passes electrons along the chain, it becomes reoxidized and thus able to accept more electrons
• Cytochromes


  • Iron of the heme is involved in redox reactions, changing between 2+ and 3+ oxidation states
  • Heme is bound to protein to prevent combining with oxygen as in hemoglobin
• Electron transfer through complex IV (cytochrome c oxidase)

  1. Electrons pass through copper and iron centers

  2. Reduction of one O2 molecule requires 4 electrons through complex IV, one at a time
• Summary of electron flow through the four complexes of the electron transport chain

  • Complex I
  • Complex II
  • Complex III
  • Complex IV
• Both hydrogen and electrons from NADH and FADH2 participate in the reactions involving enzyme complexes I and II
• Following the formation of CoQH2, hydrogen ions no longer directly participate in enzyme complex reactions, instead they become part of the cellular solution
• Oxidative phosphorylation


  Biochemical process by which ATP is synthesized from ADP as a result of the transfer of electrons and hydrogen ions from NADH or FADH2 to O2 through the electron carriers in the electron transport chain
• Coupled reactions


  Pairs of biochemical reactions that occur concurrently, where energy released by one reaction is used in the other reaction
• Oxidative phosphorylation and the oxidation reactions of the electron transport chain are coupled systems
• Three of the four protein complexes involved in the ETC (I, III, and IV) also serve as "proton pumps", transferring protons from the matrix side of the inner mitochondrial membrane to the intermembrane space
• For every two electrons passed through the ETC, four protons cross the inner mitochondrial membrane through complex I, four through complex III, and two more through complex IV
• The proton flow causes a buildup of H+ ions (protons) in the intermembrane space, creating a proton gradient that drives ATP synthesis