Chemotrophic Energy Metabolism: Aerobic Respiration II

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Chemotrophic Energy Metabolism Aerobic
Respiration II

• Reading Becker, ch. 14, pp. 415-438

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ATP Yield By the End of the TCA Cycle
ATP NADH FADH2 Glycolysis 2
4 0 TCA Cycle 2
6 2 4 10
2 Where does all the ATP come from during
aerobic respiration? From complete oxidation of
the large number of reduced coenzymes formed
during glycolysis and the TCA cycle.
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Electron Transport Chain and ATP Synthesis

• Re-oxidation of reduced coenzymes from TCA cycle
• Electrons are transferred in a multi-step process
  to the terminal electron acceptor in the chain,
  O2
• Re-oxidation is exergonic and drives synthesis of
  ATP

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Coenzyme Oxidation

• NADH H 1/2O2 ? NADH H2O
• DG -52.4 kcal/mol
• FADH2 1/2O2 ? FAD H2O
• DG -45.9 kcal/mol

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5 Types of Carriers in the ETC

• Flavoproteins
• Membrane-bound proteins
• Contain an attached FAD or FMN (flavin
  mononucleotide)
• FAD or FMN accept 2 electrons and 1 proton
  from reduced coenzymes
• Coenzymes are re-oxidized, flavoproteins are
  reduced

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5 Types of Carriers in the ETC

• Iron-Sulfur Proteins (Fe-S)
• Membrane-bound proteins
• Contain an Fe-S center
• Fe3 (ferric iron) accepts 1 electron and is
  reduced to Fe2 (ferrous iron)
• Cytochromes
• Membrane-bound proteins
• Covalently linked to a heme group, which is
  complexed to Fe
• Fe3 accepts 1 electron and is reduced to Fe2

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5 Types of Carriers in the ETC

• Copper-containing Cytochromes
• Membrane-bound proteins
• Covalently linked to a heme group, which is
  complexed to a Fe-Cu center
• Cu2 (cupric) accepts 1 electron and is reduced
  to Cu (cuprous)
• Coenzyme Q (CoQ or Ubiquinone)
• Not a protein!
• Hydrophobic molecule embedded in the inner
  mitochondrial membrane
• Accepts 2 electrons and 2 protons

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Redox Pairs

• Redox pair
• 2 molecules that are interconvertible by the
  loss or gain of electrons
• For example
• Oxidized/Reduced
• NAD/NADH
• FAD/FADH2
• Fe3/Fe2
• O2/H2O

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Reduction Potential

• A measure of the relative reducing power of a
  redox pair (measured in V)
• E
• Positive E values associated with redox pairs in
  which the oxidized form is a good electron
  acceptor (reduction occurs readily)
• Negative E values associated with redox pairs in
  which the oxidized form is a poor electron
  acceptor (reduction does not occur readily)

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Reduction Potential
The reduced form of any redox pair will
spontaneously reduce the oxidized form of any
pair with a more positive E
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Spontaneity of Reduction Reactions

• Change in Reduction Potential (DE)
• DE Eacceptor Edonor
• Positive DE associated with spontaneous
  reactions

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Change in Reduction Potential (DE)

• Is the transfer of electrons from NADH to CoQ
  spontaneous?
• NAD ? NADH E -0.32 V
• CoQ ? CoQH2 E 0.04 V
• DE 0.04 V (-0.32 V) 0.36 V
• DE reduction of CoQ by NADH is spontaneous

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Relationship Between DE and DG

• DG -nFDE
• Where n the number of electrons transferred
• F 23,062 cal/mol-V
• Example Transfer of electrons from NADH to CoQ
• n 2
• DE 0.36 V
• DG -2(23,062 cal/mol-V)(0.36 V)
• -16,604 cal/mol
• -16.6 kcal/mol
• When DE is positive, DG is negative
• Spontaneous reactions DE, -DG

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What Is the Electron Transport Chain?

• A sequence of redox reactions
• Electrons are transferred from reduced form of
  one redox pair to the oxidized form of a second
  redox pair that has a more positive reduction
  potential
• These redox reactions are spontaneous, liberating
  energy
• Liberated energy can be harnessed to drive the
  synthesis of ATP

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Carrier Proteins of ETC Are Organized Into
Respiratory Complexes

• 4 major respiratory complexes (I, II, III, IV)
• Multi-protein complexes consisting of several
  carrier proteins
• Within each complex, electrons are
• transferred from one carrier to another
• carrier with a more positive reduction potential

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The Chemiosmotic Model

• Transfer of electrons from one carrier to the
  next in the ETC is coupled to pumping of protons
  out of the mitochondrion up their electrochemical
  gradient
• Protons re-enter the mitochondrion down their
  concentration gradient in an exergonic reaction
  which drives the synthesis of ATP

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The Chemiosmotic Model
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Evidence For Proton Pumping

• Isolated mitochondria
• Incubated in presence
• of either NADH or
• succinate, initially in
• the absence of O2
• Added O2 and measured
• pH of the solution
• Result With O2, pH decreased
• Conclusion Protons are being pumped out of the
  mitochondrion during aerobic respiration

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