The introduction of Electron Transport Chain

1. Electron Transport
and
Oxidative Phosphorylation

2. Lesson Learning Outcomes
Upon completion of this lecture, students should be
able to:
• understand the electron transport chain
• understand the oxidative phosphorylation
(production of ATP)

3. Electron Transport Chain
• Carried out by four closely related multi subunit
membrane-bound complexes and two electron carriers,
coenzyme Q and cytochrome C
in a series of oxidation-reduction reactions, electrons
from FADH2 and NADH are transferred from one
complex to the next until they reach O2
O2 is reduced to H2O
– as a result of electron transport, protons are pumped
across the inner membrane to the intermembrane
space
O2 + 4 H+ + 4 e- 2 H2 O

4. Electron Transport Chain
Proton gradient across inner mitochondrial membrane

5. Electron Transport Chain
the proton gradient establishes a voltage gradient
the proton and voltage gradients together provide
the mechanism to couple electron transport with
phosphorylation of ADP
ADP + Pi ATP + H2O

7. Complex I
• NADH-CoQ oxidoreductase
electrons are passed from NADH to co-enzyme Q
through a series of reaction
N
H3 C
H3 C
N H
O
O
N N
CH2
H3 C
3
H C
N H
O
O
2
H
N
N N
CH H
2 H+ + 2 e -
( CHO H ) 3
CH2 OPO3
2 -
Flavin mononucleotide
(FMN)
( CHO H ) 3
CH2 OPO3
2 -
Dihydroflavin mononucleotide
(FMNH 2)

8. Complex I
electrons are then passed to the iron-sulfur
clusters
finally, electrons are passed to coenzyme Q (also
called ubiquinone)
O
C H 3 O C H 3
C H 3
O
C o e n z y m e Q
(o x i dized f o r m ) O H
C H 3 O C H 3
C H 3
C H 3 O ( C H 2 C H = C C H 2 ) n H
2
O H
C o e n z y m e Q H
( r e d u c e d f o r m )
re d u c t i o n
+ 2 e -
o x i da t io n
C H 3 O ( C H 2 C H = C C H 2 ) n H + 2 H +

9. Complex I
the overall equation for the reaction of complex I is
this transfer of electrons is strongly exergonic and is
sufficient to drive the phosphorylation of ADP
Fe-S( ox) + CoQH2
NADH + H+ + E-FMN
E-FMNH2 + 2 Fe-S( ox)
Fe-S( red) + CoQ + 2 H+
NAD+ + E-FMNH2
E-FMN + 2 Fe-S( red) + 2 H+
NADH + H+ + CoQ NAD+
+ CoQH2 G°' = -81 kJ•mol -1
-1
ADP + Pi ATP + H2 O G°' =+30.5 kJ•mol

11. Complex II
Succinate-coenzyme Q oxidoreductase
◾the overall reaction is exergonic, but not enough to
drive ATP production
◾no H+ is pumped out of the matrix during this step
C O O-
CH2
CH2
C O O-
Succinate
+ E- FA D
C
C
H
H
C O O-
- O O C
+ E- FA D H 2
E - FAD H 2 + C o Q
-1
Succinate + C o Q
Fumarate
E - FAD + C o Q H 2
Fumarate + C o Q H 2 G°' = -13.5 kJ•mol

12. Redox Forms of CoQ
CH3 O CH3
O
CH3 O CH3
OH
CH3 O
CH3 O
R
CH3
O•
O-
e-
e-
CH3 O R
O
Coenzyme Q
CH3 O R
OH
Coenzyme QH 2
Coenzyme Q -
2 H+ + e-
2 H+ + e-

14. Complex III
CoQH2-cytochrome c oxidoreductase
◾this decrease in free energy is sufficient to drive the
phosphorylation of ADP to ATP
◾(DG0’ = + 30.5 kJ•mol-1)
CoQH2 + 2Cyt c[Fe( III)]
CoQ + 2Cyt c[Fe( II)] + 2H+ G°' = -34.2 kJ•mol -1

16. Complex IV
• Cytochrome oxidase:
complex IV contains cytochrome a, cytochrome a3,
and Cu(II), which are also involved in the electron
transport
complex IV is the link with molecular oxygen
NADH + H+ + 1 O2 NAD+ + H2 O
2

18. Electron Transport Chain

19. Coupling Oxidation and Phosphorylation
the energy-releasing oxidations give rise to proton
pumping and a pH gradient across the inner
mitochondrial membrane
in addition, differences in the concentration of
ions across the membrane generates a voltage
gradient
a coupling process converts the electrochemical
potential to the chemical energy of ATP
the coupling factor is ATP synthase, a complex
protein oligomer, separate from the electron
transport complexes

21. ATP Synthase

22. Coupling Oxidation and Phosphorylation
P/O ratio: gives the number of moles of Pi consumed
in phosphorylation to the number of moles of oxygen
atoms consumed in oxidation
◾P/O = 2.5 moles of ATP when NADH is oxidized
◾P/O = 1.5 moles of ATP when FADH2 is oxidized
⦿Biochemist had used integral values of 3 and 2 for the P/O ratios for
oxidation of NADH and FADH2
AT P + H 2 O
2 H +
+ 2 e - H 2 O
Phosphorylation (P)
A D P + Pi
Oxidation (O)
1 / 2 O 2 +

23. Mechanism of Ox/Phos
• The mechanism by which the proton gradient leads
to the production of ATP depends on ion channels
through the inner mitochondrial membrane.
protons flow back into the matrix through
channels
the flow of protons is accompanied by formation
of ATP
the details of how phosphorylation takes place as
a result of the linkage to the proton gradient are
not explicitly specified by this mechanism

24. The overall effect of electron transport reaction series is to move protons
out of the matrix into the intermembrane space, creating a difference in pH
across the membrane.

25. Formation of ATP accompanies the flow of protons
back into the mitochondrial matrix.

28. Shuttle Mechanisms
⚫Shuttle mechanisms: transport metabolites between
mitochondria and cytosol
⚫Glycerol phosphate shuttle:
Glycolysis in the cytosol produces NADH
NADH does not cross the mitochondrial
membrane, but glycerol phosphate and
dihydroxyacetone phosphate do
Through the glycerol phosphate shuttle, 1.5 ATP
are produced in the mitochondria for each
cytosolic NADH
Has been found in insect flight muscle, mammalian
muscle and brain

29. The Glycerol-Phosphate Shuttle

30. The Malate-Aspartate Shuttle
• The Malate-Aspartate Shuttle:
Has been found in mammalian kidney, liver, and
heart
Malate crosses the mitochondrial membrane,
while oxaloacetate cannot
The transfer of electrons from NADH in the cytosol
produces NADH in the mitochondria
In the malate-aspartate shuttle, 2.5 mitochondrial
ATP are produced for each cytosolic NADH
 More efficient shuttle mechanism than glycerol-phosphate shuttle

31. The Malate-Aspartate Shuttle

32. Summary
• Shuttle mechanisms transfer electrons, but not
NADH, from the cytosol across the mitochondrial
membrane
• In the malate-aspartate shuttle, 2.5 molecules of ATP
are produced for each molecule of cytosolic NADH,
rather than 1.5 ATP in the glycerol-phosphate
shuttle, a point that affects the overall yield of ATP in
these tissues

33. The ATP Yield from Complete Oxidation
of Glucose
• In the complete oxidation of glucose, a total of
30 or 32 molecules of ATP are produced for
each molecule of glucose, depending on the
shuttle mechanism

35. End of lecture