Organization of the Electron transfer Chain

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Organization of the Electron transfer Chain

• The electron carriers are organized in membranes
  as multienzyme complexes
• There are four distinct membrane-embedded
  supramolecular complexes
• Each catalyzes electron transfer through a
  portion of the chain
• Q and Cyt c serve as mobile carriers that shuttle
  electrons between the complexes

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• Complex I and II do NOT operate in series
• They both accomplish the same thing Transfer e-
  from NADH or succinate to Q

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Specific inhibitors of electron transport act at
specific points
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• As electrons are transferred along the
  respiratory chain there is a decline in
  Free-Energy
• There are 3 major free energy jumps at complexes
  I, III and IV
• These are the sites at which ATP is synthesized

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How is the free energy released from the
oxidation of NADH harnessed to yield ATP?

• The energy released is used to eject protons from
  the mitochondrial matrix to the inter-membrane
  space
• At
• (i) Complex I
• (ii) Complex III
• (iii) Complex IV

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• Therefore, the energy of electron transfer is
  used to power a proton pump
• This produce a proton gradient
• The mitochondrial matrix becomes appreciably more
  alkaline than the cytoplasm.

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• A proton gradient is an intermediary form of
  energy
• It takes energy to produce it and its dissipation
  would result in the release of free energy

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• Intermediary form of energy for 2 reasons
• Chemical concentration gradient
• It costs energy to move against a concentration
  gradient
• Free energy is released when a chemical move
  down its concentration gradient

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• (ii) Electrical gradient
• When protons are ejected without any accompanying
  anions an electrical imbalance is created
• the inside contains more negative charges than
  the outside
• this is a membrane potential (difference in
  electrical potential across the membrane)

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• It costs energy to pump a positive charge out of
  the mitochondria when there is a ribbon of
  negative charges inside the membrane
• Therefore, there is a release of energy when a
  positive charge enters such a mitochondria

Proton Motive Force
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The free energy change for the creation of an
electrochemical gradient

• ?G RT ln(C2/C1) ZF ??
• C2 H in the intermembrane space
• C1 H in the matrix
• Z Absolute value of the electrical charge (ie
  1 for H)
• ?? difference in transmembrane electrical
  potential (volts)
• Calculated ?G for pumping a proton out at 250C
• ?G 20 kJ/mol

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• For each NADH oxidized
• 10 H are pumped into the intermembrane space
• Therefore roughly 200 kJ of the 220 kJ released
  by the oxidation of a mole of NADH is conserved
  in the proton gradient
• (? G for oxidation of NADH 220 kJ/mole)
• For each FADH2 oxidized
• - 6 H are transferred

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Phosphorylation (ATP Synthesis)
ATP Synthase
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Chemiosmotic Theory
Peter Mitchell (1961)
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Chemiosmotic Theory

• Free energy of electron transport is conserved by
  pumping H from the matrix to the IM space
• This creates an electrochemical H gradient
  across the inner mito. Membrane
• These H then flow back passively into the matrix
  through a pore associated with ATP Synthase
• This dissipates the electrochemical potential
  contained in the proton motive force.
• This energy is harnessed to synthesize ATP