Oxidative Phosphorylation

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Oxidative Phosphorylation
2
What is the most important feature of
mitochondria with respect to generation of energy?

• 1. There are many mitochondria per cell
• 2. Mitochondria contain enzymes of the citric
  acid cycle
• 3. Mitochondrial membranes are impermeable to
  ions and small molecules
• 4. Mitochondria have their own DNA genome

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Coupled Transport Across Mitochondrial Membranes

• Pyruvate, ADP, Pi pumped in, ATP moved out
• Carrier proteins can couple transport to
  energetically favorable flow of H into the
  matrix
• ADP co-transported (antiport) with ATP driven by
  charge difference across membrane
• Thus proton gradient drives formation of ATP and
  transport of metabolites

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Coupled Transport in Mitochondria
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Proton Gradient Produces Most of Cells ATP

• Glycolysis 2 ATP/ molecule glucose in absence of
  O2
• Oxidative phosphorylation provides 2.5 ATP/NADH,
  1.5 ATP/FADH2
• Complete oxidation of glucose yields 30
  ATP/molecule glucose
• Also generate large ATP from oxidation of fats
• Maintains high ATP/ADP ratio in cells

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ATP Production

• ATP main energy currency of the cell
• Glycolysis preserves 10 free energy of glucose
• Majority of ATP synthesized in membrane-bound
  process in mitochondria
• Oxidative phosphorylation
• Stage 1 Electrons passed along
  electron-transport chain, release energy to pump
  H
• Stage 2 H flow down electrochemical gradient to
  synthesize ATP
• Chemiosmotic coupling

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Structure of Mitochondria

• Present in large numbers in cell
• Can be localized at sites of high ATP consumption
• Inner and outer membranes
• Outer membrane permeable to proteins lt5000 D
• Inner membrane impermeable to ions and most small
  molecules in absence of transport proteins
• Matrix and intermembrane space
• Matrix contains molecules that can be transported
  across inner membrane

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Mitochondria
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Inner Membrane

• Contains transmembrane ATP synthase
• Proteins of electron transport chains
• Distinct lipid composition
• Contains a variety of transport proteins
• Pyruvate, fatty acids
• Highly convoluted to provide greater surface area
• Cristae

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What tissues would be affected in diseases where
mitochondria are abnormal?

• 1. Brain
• 2. Liver
• 3. Heart
• 4. Eyes
• 5. Skeletal muscle

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High Energy Electrons from Citric Acid Cycle

• Acetyl CoA ? CO2 H2O
• NADH and FADH2 generated
• High energy electrons transferred to inner
  mitochondrial membrane
• Electrons enter electron transport chain
• Regeneration of NAD and FAD

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Electron-Transport Chains

• Membrane-based energy-conservation
• Protons are moved by the transfer of electrons
• Water is a reservoir for donation and acceptance
  of protons
• When an electron is acquired it brings along a
  negative charge
• Charge neutralized by rapid addition of a proton
  from water
• Net effect is transfer of entire H atom, He-
• When a molecule is oxidized, H atom dissociated
  into electron and proton
• Electron passed to electron acceptor, proton to
  water

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Chemiosmotic Process

• Chemiosmotic process converts high energy
  electrons to ATP
• High energy electrons passed along electron
  transport chain to molecular O2 to form H2O
• Energy released pumps protons across membrane
• Discharge of proton gradient provides energy to
  ATP synthase to make ATP

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Mitochondria Catalyze Conversion of Energy
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Electron Transfer

• Each electron transfer is an oxidation-reduction
  reaction
• Electrons pass readily from molecules with low
  affinity for electrons to molecules with a high
  affinity
• Electron transfer occurs in proteins that can
  pump protons from water across membrane to
  intermembrane space
• Creates electrochemical gradient

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Proton Gradient

• Gradient of proton concentration across inner
  membrane
• pH gradient
• Matrix pH 8, intermembrane space pH 7
• Generates a membrane potential
• Inside of membrane negatively charged
• Outside of membrane positively charged

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The Electrochemical Gradient Across the Inner
Mitochondrial Membrane Consists of Membrane
Potential
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And A Smaller Force Due to the Proton
Concentration
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ATP Synthase

• Membrane-bound, multisubunit enzyme
• Creates a hydrophilic pathway across inner
  mitochondrial membrane
• Allows protons to flow down electrochemical
  gradient
• As ions flow through channel, drive reaction ADP
  Pi ? ATP
• Energy-converting molecular motor
• Can produce 100 molecules ATP per second
• Reversible

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The Electrochemical Gradient Drives ATP Synthesis
by Oxidative Phosphorylation
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Electron Transport Chains

• Protons are moved by the transfer of electrons
• Water is a reservoir for donation and acceptance
  of protons
• When an electron is acquired it brings along a
  negative charge
• Charge neutralized by rapid addition of a proton
  from water
• Net effect is transfer of entire H atom, He-
• When a molecule is oxidized, H atom dissociates
  into electron and proton
• Electron passed to electron acceptor, proton to
  water

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Protons in Water Are Highly Mobile