Chapter 14 - Electron Transport and Oxidative Phosphorylation

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Chapter 14 - Electron Transport and Oxidative
Phosphorylation

• The cheetah, whose capacity for aerobic
  metabolism makes it one of the fastest animals

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Oxidative Phosphorylation in Mitochondria

• Oxidative phosphorylation is the process by which
  NADH and FADH2 are oxidized and ATP is formed
• NADH and FADH2 are reduced coenzymes from the
  oxidation of glucose by glycolysis and the citric
  acid cycle

The Respiratory Electron-transport Chain (ETC) is
a series of enzyme complexes embedded in the
inner mitochondrial membrane, which oxidize NADH
and QH2. Oxidation energy is used to transport
protons across the inner mitochondrial membrane,
creating a proton gradient ATP synthase is an
enzyme that uses the proton gradient energy to
produce ATP
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Mitochondria are energy centers of a cell
Cytosol
Mitochondria
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Fig 14.2
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Fig 14.6Structure of the mitochondrion

• Final stages of aerobic oxidation of biomolecules
  in eukaryotes occur in the mitochondrion
• Site of citric acid cycle and fatty acid
  oxidation which generate reduced coenzymes
• Contains electron transport chain to oxidize
  reduced coenzymes

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Overview of oxidative phosphorylation
Fig 14.1
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Electron Flow in Oxidative Phosphorylation

• Five oligomeric assemblies of proteins associated
  with oxidative phosphorylation are found in the
  inner mitochondrial membrane
• Complexes I-IV contain multiple cofactors, and
  are involved in electron transport
• Electrons flow through complexes I-IV
• Complexes I, III and IV pump protons across the
  inner mitochondrial membrane as electrons are
  transferred
• Mobile coenzymes ubiquinone (Q) and cytochrome c
  serve as links between electron transport
  complexes
• Complex IV reduces O2 to water
• Complex V is ATP synthase, which uses the
  generated proton gradient across the membrane to
  make ATP

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Cofactors in Electron Transport

• NADH donates electrons two at a time to complex I
  of the electron transport chain
• Flavin coenzymes are then reduced
• (Complex I) FMN FMNH2
• (Complex II) FAD FADH2
• FMNH2 and FADH2 donate one electron at a time to
  ubiquinone (U or coenzyme Q)
• All subsequent steps in electron transport
  proceed by one electron transfers

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Mobile electron carriers
1. Ubiquinone (Q)Q is a lipid soluble molecule
that diffuses within the lipid bilayer, accepting
electrons from Complex I and Complex II and
passing them to Complex III. 2. Cytochrome
cAssociated with the outer face of the inner
mitochondrial membrane. Transports electrons
from Complex III to Complex IV.
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Iron in metalloenzymes

• Iron undergoes reversible oxidation and
  reduction
• Fe3 e- (reduced substrate)
• Fe2 (oxidized substrate)
• Enzyme heme groups and cytochromes contain iron
• Nonheme iron exists in iron-sulfur clusters (iron
  is bound by sulfide ions and S- groups from
  cysteines)
• Iron-sulfur clusters can accept only one e- in a
  reaction

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Iron-sulfur clusters

• Iron atoms are complexed with an equal number of
  sulfide ions (S2-) and with thiolate groups of
  Cys side chains

• Heme consists of a tetrapyrrole Porphyrin ring
  system complexed with iron

Heme Fe(II)-protoporphyrin IX
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Complex I. NADH-ubiquinone oxidoreductase

• Transfers two electrons from NADH as a hydride
  ion (H-) to flavin mononucleotide (FMN), to
  iron-sulfur complexes, to ubiquinone (Q), making
  QH2

• About 4 protons (H) are translocated across the
  inner mitochondrial membrane per 2 electrons
  transferred

Fig 14.9
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Complex II. Succinate-ubiquinone oxidoreductase

• Also known as succinate dehydrogenase complex
• Transfers electrons from succinate to flavin
  adenine dinucleotide (FAD) as a hydride ion
  (H-), to an iron-sulfur complex, to ubiquinone
  (Q), making QH2
• Complex II does not pump protons

Fig 14.11
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Complex III. Ubiquinol-cytochrome c oxidoreductase

• Transfers electrons from QH2 to cytochrome c,
  mediated by iron-sulfur and other cytochromes
• Electron transfer from QH2 is accompanied by
  the translocation of 4 H across the inner
  mitochondrial membrane

Fig 14.14
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Complex IV. Cytochrome c oxidase

• Uses four-electrons from the soluble electron
  carrier cytochrome c to reduce oxygen (O2) to
  water (H2O)
• Uses Iron atoms (hemes of cytochrome a) and
  copper atoms
• Pumps two protons (H) across the inner
  mitochondrial membrane per pair of electrons, or
  four H for each O2 reduced

Fig 14.19
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Complex V ATP Synthase

• F0F1 ATP Synthase uses the proton gradient energy
  for the synthesis of ATP
• Composed of a knob-and-stalk structure
• F1 (knob) contains the catalytic subunits
• F0 (stalk) has a proton channel which spans the
  membrane.

• Estimated passage of 3 protons (H) per ATP
  synthesized

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Knob-and-stalk structure of ATP synthase
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Mechanism of ATP Synthase

• There are 3 active sites, one in each b subunit
• Passage of protons through the Fo channel causes
  the c-e-g unit to rotate inside the a3b3 hexamer,
  opening and closing the b-subunits, which make ATP

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Fig 14.20 Transport of ATP, ADP and Pi across
the inner mitochondrial membrane

• Adenine nucleotide translocase unidirectional
  exchange of ATP for ADP (antiport)
• Symport of Pi and H is electroneutral

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The PO Ratio
molecules of ADP phosphorylated PO ratio
----------------------------------------- atoms
of oxygen reduced

• Translocation of 3H required by ATP synthase for
  each ATP produced
• 1 H needed for transport of Pi, ADP and ATP
• Net 4 H transported for each ATP synthesized

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Calculation of the PO ratio
Complex I III IV
H translocated/2e- 4 4 2 Since
4 H are required for each ATP synthesized
For NADH 10 H translocated / O (2e-) P/O
(10 H/ 4 H) 2.5 ATP/O For succinate (FADH2)
substrate 6 H/ O (2e-) P/O (6 H/ 4 H)
1.5 ATP/O
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Regulation of Oxidative Phosphorylation

• Overall rate of oxidative phosphorylation depends
  upon substrate availability and cellular energy
  demand
• Important substrates NADH, O2, ADP
• In eukaryotes intramitochondrial ratio ATP/ADP is
  a secondary control mechanism
• High ratio inhibits oxidative phosphorylation as
  ATP binds to a subunit of Complex IV