OXIDATIVE PHOSPHORYLATION

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OXIDATIVE PHOSPHORYLATION
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ATP is the single currency of life

• Adenosine triphosphate
• ATP is the most important molecule for capturing
  and transferring free energy
• Hydrolysis of ATP to ADP Pi yields 7.3
  kcal/mol energy that can be used to power e.g.
  protein synthesis, muscle contraction or
  transport of molecules

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Oxidative phosphorylation generates ATP

• In aerobic oxidation, sugars and fatty acids are
  metabolized to C02 and H20 .
• The released energy is converted to chemical
  energy of phoshoanhydride bonds in ATP.

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Oxidative phosphorylation is the last stage of
catabolism
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NADH and FADH2
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NADH and FADH2

• Glycolysis, TCA cycle and fatty acid oxidation
  generate NADH and FADH2
• NADH and FADH2 are energy rich molecules because
  each contains a pair of electrons that have a
  high transfer potential
• In oxidative phosphorylation the electon
  transferring potentila of NADH and FADH2 is
  converted to phosphate-transfer potential of ATP

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Mitochondrion
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Oxidative phosphorylation

• ATP is formed as electrons are transferred
  from NADH or FADH2 to 02 by a series of electron
  carriers.

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Proton motive force and chemiosmotic coupling

• The immediate energy sources that power ATP
  synthesis are proton gradient and electric
  potential (voltage gradient) across the membrane.
• Proton gradient and electric potential are
  collectively called proton-motive force.

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Proton motive force and chemiosmotic coupling

• The proton motive force is generated by stepwise
  movement of electrons by electron carriers that
  leads to pumping of protons out of the
  mitochondrial matrix.
• Oxidation of NADH and phosphorylation of ADP are
  coupled by a generation of proton gradient.

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Proton motive force and chemiosmotic coupling
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Energy is released gradually in the electron
transfer chain

• Most free energy released when glucose is
  oxidised to carbon dioxide is retained in the
  reduced coenzymes NADH and FADH2
• Respiration electrons are released from from
  NADH and FADH2 to oxygen
• NADH H 1/2 02 NAD H20

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Energy is released gradually in the electron
transfer chain

• NADH H 1/2 02 NAD H20 -52.6 kcal/mol
• ADP Pi ATP 7.6 kcal/mol
• ATP production is maximised by releasing the free
  energy in small increments in the electron
  transfer chain (a.k.a respiratory chain).
• Electron transfer chain contains four
  multiprotein complexes. Three of these are
  electron driven proton pumps that create the
  proton gradient

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The electron transfer chain
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The electron transfer chain
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The electron transfer chain
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Electron transfer is driven by redox potential
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Redox potential

• Oxidation-reduction potential
• Oxidant electron reductant
• Substance that can exist as a reduced and
  oxidices form is referred to a redox couple
• Redox potential of such couple is measured
  against H -gt H2 couple.
• Redox potential of H -gt H2 couple is defined as
  0 V (volts).

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Redox potential

• A negative redox potential means that a substance
  has lower affinity for electrons than hydrogen.
  Positive redox potential means higher affinity.
• Strong oxidising agents have positive redox
  potential
• In the respiratory chain the electrons are
  transferred to higher redox potential values,
  that is, to higher affinity electron carriers.

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Electron transfer is driven by redox potential
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Prosthetic groups act as electron carriers
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Complexes in the chain are transmembrane proteins
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Coenzyme Q and cytochrome c shuttle electrons
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Cytochromes are heme containing proteins

• Cytochromes are covalently linked to heme, an
  iron-containing prosthetic group similar to that
  in hemoglobion or myoglobin.
• Electron transport occurs by by oxidation and
  reduction of the Fe atom in the centre of the
  heme
• Different cytochromes have slightly different
  heme groups that generate different
  environment for Fe-ion and thus different
  tendency to accept an electron

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ATP Synthase
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ATP Synthase

• ATP synthase or F0F1 complex has two components
  that are both itself multiprotein complexes
• F0 is transmembrane complex that forms a
  regulated H channel
• F1 is protrudes in the matrix and contains the
  sites for ATP formation

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ATP Synthase

• Proton translocation through F0 powers rotation
  of one subunits of F1
• Three confromations, one binds ADP and Pi so
  tightly that they spontaneously form ATP.

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Several toxins can block oxidative phosphorylation
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Transporters traffic ATP and ADP
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Malate/aspartate shuttle and glycerol phosphate
shuttle are needed for oxidation of cytosolic
NADH
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Respiratory control

• Mitochondria can only oxidise FADH and NADH only
  as long as there is ADP and Pi available.
  Electron flow ceases if ATP is not produced.
• ADP increases when ATP is consumed e.g. in muscle
  work. Oxidative phosphorylation is regulated by
  ATP consumption.

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Brown-fat mitochondria contain an uncoupler of
oxidative phosphorylation

• Brown fat specialised to produce heat
• Newborns brown-fat thermogenesis
• Thermogenin protein, a proton transporter that is
  not connected to ATP synthesis.
• Energy released by NADH oxidation converted to
  heat.

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Oxidative Phosphorylation - Summary