REDUCTIONOXIDATION

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BIOC 460 DR. TISCHLER LECTURE 24
REDUCTION-OXIDATION RESPIRATORY CHAIN
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OBJECTIVES
1. Describe the relationship between the
reduction potential difference (?Eo) and a)
reduction-oxidation (redox) reactions b) the
free energy difference (?Go) of such a
reaction. 2. Essential features of oxidative
phosphorylation and role of redox reactions in
the electron transport (respiratory) chain.
3. For the respiratory chain major complexes (I
to IV) a) name the components that immediately
donate electrons to and accept electrons from
each b) list the functional groups in each
that are important in the electron
transfer c) explain how energy from the
respiratory chain complexes is conserved and
which produce sufficient energy to make
ATP. 4. Explain how pH and charge gradients are
formed across the mitochondrial inner membrane.
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REDOX FACTS

• Reduction-oxidation (redox) couple
• pair of molecules of which one is reduced and
  the other is oxidized (e.g., lactate-pyruvate,
• NADH-NAD, and FADH2-FAD
• each pair constitutes a half reaction
• the reductant of one pair donates electrons
  and the oxidant of the other pair accepts the
  electrons
• Red1 Ox2 ? Ox1 Red2

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REDOX FACTS (continued)
Reduction potential (EO) how well one substance
reduces another (donates electrons).

• ?EO
• standard reduction potential difference
  between two half reactions
• similar to the ?GO (standard free energy
  difference)
• ?EO is positive for favorable reactions (?GO
  is negative for favorable reactions)

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Oxidative process
Phosphorylation process
O2
e-
ADPPi
outer membrane
inner membrane
H2O
ATP
H
H
intermembrane space
matrix
Figure 1. Essential features of oxidative
phosphorylation. redox reactions of respiratory
chain use electrons to reduce oxygen to
water energy generated moves protons from matrix
to intermembrane space inward movement of protons
recovers this energy to promote formation of ATP
in the matrix.
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Table 1. Reduction potential of the
lactate/pyruvate and NADH/NAD redox pairs.
Oxidant Reductant Eo Pyruvate Lactate -0.19
V NAD NADH -0.32 V
Thus ?Eo (-0.19 V) - (-0.32 V) 0.13
V Arithmetically subtracting the reactions
gives Pyruvate - NAD Lactate - NADH
which after rearrangement yields Pyruvate
NADH ? Lactate NAD
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Table 2. Reduction potential of the H2O/½O2 and
NADH/NAD redox pairs.
Oxidant Reductant Eo ½O2 H2O 0.82 V
NAD NADH -0.32 V
the potential difference is calculated as ?Eo
(0.82 V) - (-0.32 V) 1.14 V.
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?Eo' 0.07V
?Eo' 0.19V
?Eo' 0.53V
?Eo' 0.42V
Figure 2. Overview of the respiratory chain
showing the progression of reduction potentials
from strong to weak reductants culminating in
oxygen as the ultimate electron acceptor. The
?Eo? values are the potential differences across
the four complexes
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Table 3. Summary of redox complexes of the
electron transport chain
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Figure 3. Complex I of the respiratory chain
that links NADH and coenzyme Q. DH is NADH
dehydrogenase.
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Figure 4. Complex II of the respiratory chain.
SDH is succinic dehydrogenase an enzyme of the
citric acid cycle.
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Figure 5. Complex III of the respiratory chain
linking CoQ and cytochrome C.
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Figure 6. Complexes III and IV linked by
cytochrome C with complex IV reducing oxygen to
water.
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High H
High H

H
H
H
H
H
H
H
H
Low H
Matrix
H
H

Cytoplasm
Cristae
Figure 7. Generation of a pH gradient (H) and
charge difference (negative in the matrix) across
the inner membrane constitute the protonmotive
force that can be used to drive ATP synthesis and
transport processes.
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Medical Scenario I Against his oncologist's
advice, P.J. purchases laetrile in Mexico to
treat his lung cancer. Without proper medical
guidance, he exceeds the recommended dosage
considerably, figuring that it will hasten its
purported destruction of proliferating cells.
Instead the overdose leads to his death. You are
a student working with the oncologist. You study
the effects of laetrile and learn that it is a
cyanoglycoside found in high concentrations in
the pits of certain fruits. Hydrolysis of this
and similar compounds releases a
cyanide-containing benzaldehyde derivative which
itself spontaneously degrades, releasing cyanide.
With your knowledge of the respiratory chain
explain why laetriles improper use led to the
patients death?
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Medical Scenario II Respiratory chain defects
are associated with lactic academia. Lactic
academia occurs because the high concentration of
NADH favors the formation of lactate from
pyruvate. Blood lactate may be elevated 30-fold
or more. Because elevated NADH inhibits pyruvate
dehydrogenase, blood pyruvate is also increased,
though to a lesser extent than for lactate.
Elevated pyruvate increases alanine production.
Defects in each complex of the respiratory chain
have been identified.