Lecture 40: reading 782 793

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Lecture 40 reading 782 - 793 Mitochondria
continued.
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(No Transcript)
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Electrons are donated from NADH to the electron
transport chain. The final acceptor is Oxygen.
This is where most of the oxygen you breath gets
used.
Each complex pumps protons out as electrons are
transferred through the complex.
The three transmembrane complexes are free to
diffuse laterally in the plane of the membrane.
Electrons are transferred between the complexes
by the mobile electron carriers.
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• A reaction involving the transfer of electrons is
  called a redox reaction.
• A redox pair refers to the reduced and oxidized
  forms of a molecule.
• NADH and NAD are a redox pair.
• The reaction is NADH ltgt NAD H 2e-
• The redox potential of a redox pair provides a
  measure of the affinity a molecule has for
  electrons.
• The standard redox potential for a redox pair is
  designated Eo, and these values are available in
  books.
• The more negative the value of Eo, the greater
  is the tendency of the reduced molecule to
  release an electron.
• The more positive the redox potential, the
  stronger the attraction for electrons.

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A redox reaction always involves one molecule
being oxidized and another molecule being
reduced. There must be an electron acceptor -
otherwise the electron would be attracted back to
the molecule being oxidized. Hence, two redox
pairs must be involved in an electron transport
reaction. NADH ltgt NAD H 2e- Eo -320
mV H2O ltgt 1/2O2 2H 2e- Eo 820
mV Each redox pair has a redox potential.
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To describe a chemical reaction involving
electron transfer, you need to combine the two
redox pairs that describe the molecule being
oxidized and the molecule being reduced. The
direction in which to write the reactions depends
on which molecule prefers to release electrons.
Since Eo(NADH) lt Eo(H2O) , then NADH will
release electrons and oxygen will receive the
electrons (recall low Eo corresponds to low
affinity for electrons). Hence NADH ltgt NAD
H 2e- 1/2O2 2H 2e- ltgt H2O
NADH 1/2O2 H ltgt NAD H2O ?Eo (Eo
of electron acceptor) - (Eo of electron
donor) ?Eo (820 mV) - (-320 mV) 1140 mV
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The ?Go of redox reaction is calculate by the
following equation ?Go -nF?Eo n
electrons F Faraday constant ?Eo (Eo of
electron acceptor) - (Eo of electron donor) ?Eo
(820 mV) - (-320 mV) 1140 mV ?Go
-(2)(0.023 kcal mV-1 mole-1)(1140 mV) ?Go
-52.44 kcal/mole What you should know 1) The
greater the redox potential of a half reaction,
the higher the attraction for electrons. 2)
Given the redox potential for two redox pairs, be
able to predict the favorable direction of the
full reaction. 3) ?Go is the standard free
energy change for a reaction and Eo is the redox
potential of a redox pair. 4) Favorable reactions
have a negative ?G and a positive ?E. You dont
need to know How to do the calculations at the
top of the at the top of the slide or on the
previous slide.
Note that even though the reaction is reversed,
the sign is not reversed in the equation.
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Back to the respiratory chain. NADH is oxidized
and the 2 e- reduce electron carriers in the NADH
dehydrogenase complex.
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• The direction of electron transfer is dictated
  by
• 1. The redox potential of the electron carriers.
• 2. Intimate contact between the electron
  carriers.
• For example, even though transfer of electrons
  between the b-c1 complex and cytochrome oxidase
  is favorable, the two complexes do not come close
  enough for direct transfer of the electrons.

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Many of the electron carriers in the electron
transport chain are iron atoms that undergo
cycles of reduction and oxidation Fe3 1e- ltgt
Fe2 (from left to right is a reduction) NADH
dehydrogenase complex several iron-sulfur centers
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cytochrome b-c1 complex an iron-sulfur center
and several hemes.
Heme is composed of an iron bound by a porphyrin
ring (blue)
Cytochromes are proteins containing a heme.
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cytochrome oxidase complex copper and hemes.
The complex contains two cytochromes as
implicated by the two hemes.
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Mobile electron carriers shuttle electrons
between the transmembrane complexes.
Ubiquinone, the mobile carrier between the NADH
dehydrogenase and the cytochrome b-c1, is a small
lipophilic molecule.
Note that this is not associated with a protein.
Cytochrome c, the mobile carrier between the
cytochrome b-c1 complex and the cytochrome
oxidase complex, is a small heme containing
protein.
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Iron undergoes cycles of reduction and oxidation
in all three membrane complexes. For there to be
a preferred direction of electron transport, the
irons in different complexes must be adjusted to
different redox potentials. The surroundings of
each iron set the redox potentials. Based on
the order in which they appear in the electron
transport chain, we can deduce that the irons in
an iron-sulfur complex usually has a lower redox
potential than the iron in a heme complex.
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A closing remark Oxidative phosphorylation is
remarkably efficient. The free energy change for
transferring 2 electrons from NADH to O2 is
sufficient to generate 7 ATP under standard
conditions. In reality, the cell generates 3
ATP. This is an efficiency of 40.