1. Berg • Tymoczko • Stryer
Biochemistry
Sixth Edition
Chapter 18:
Oxidative Phosphorylation
Copyright © 2007 by W. H. Freeman and Company

5. Oxidative Phosphorylation and
Mitochondria Transport Systems
Mitochondria = power house of the cell
glyco. 
TCA  NADH, FADH2 (energy rich mols)
f.a.oxi.  each has a pair of e- (having a transfer pot.)
2 e-
02 Energy released! (used for ATP)
Oxidative Phoshorylation: the process in which ATP is
formed as electrons are transferred from NADH or FADH2
to O2 by a series of electron carriers

6. Some Features…
1. Oxidative phosphorylation is carried out by respiratory
assemblies that are located in the inner membrane.
– TCA is in the matrix
2. The oxidation of NADH  2.5 ATP
3. FADH2  1.5 ATP
– Oxidation and phosphorylation are COUPLED
4. Respiratory assemblies contain numerous electron
carriers
– Such as cytochromes
5. When electrons are transferred H+ are pumped out
6. ATP is formed when H+ flow back to the mitochondria

7. Some Features Continued…
 Thus oxidation and phosphorylation are coupled by
a proton gradient across the inner mitochondria
membrane
– So, we produce ATP through this
– Glycolysis and TCA cycle can continue only if NADH and
FADH2 are somehow reoxidized to NAD+ and FAD

9. Release of Free Energy During Electron
Transport
1. Electrons transferred
electron donor (reductant)  electron acceptor (oxidant)
They can be transferred
– H-
– H+
– Pure electrons
2. When a compound loses its electrons becomes
oxidant
cyt b (Fe ++) + cyt c1 (Fe +++)  cyt b (Fe +++) + cyt c1 (Fe ++)
red. X oxi. Y oxi. X’ red. Y’
Red. X and Oxi. X’ Redox
Red. Y’ and Oxi. Y Pairs

10. Release of Free Energy Continued…
3. PAIRS differ in their tendency to lose electrons
– It is a characteristic of a pair
– Can be quantitatively specified by a constant… E0 (volts)
– E0: standard reduction potential
– The more negative E0, the higher the tendency of the reductant
to lose electrons
– The more positive E0, the higher the tendency of the oxidant to
accept electrons
– Electron transfer: more –E0 ---------- more +E0
4. Free energy decreases as electrons are transferred
Go = -nF E0
where “n” is the number of electrons transferred, and F is Faraday’s constant (23, 062)
E0 = E0 (electron accepting pair) – E0 (electron donating pair)

19. What Are the Electron Carriers in mt?
 Most of the electron carriers in mitochondria are
integral proteins
 There are four types of electron transfers
1. Direct transfer of electrons
Fe+3  Fe+2
2. As a hydrogen atom
H+ + electron
3. As a hydride ion
:H- (has 2 electrons)
4. Direct combination of an organic reductant with O2

21. Flow of electrons and protons
thru the respiratory chain

22. How Is This Order Found?
1. NADH, UQ, cytb, cytc1, c, a, and a3 is the order
– Their standard reduction potentials have been determined
experimentally!
– The order  increased E0 because electrons tend to flow from more
negative E0 to more positive E0
2. Isolated mitochondria are incubated with a source of electrons but
without O2
– a, a3 is oxidized first
– c, c1, b are second, third, and fourth respectively
– When the entire chain of carriers is reduced experimentally by
providing an electron source but no O2 (electron acceptor) then O2
suddenly introduced into the system
– The rate at which each electron carrier becomes oxidized shows the
order in which the carriers function
– The carrier nearest O2 is oxidized first, then second, third, etc.

27. Action of Dehydrogenases
 Most of the electrons come from
 Electron acceptors NAD or FMN, FAD
Reduced subs + NAD+  ox. sub + NADH + H+
Reduced subs + NADP+  ox. Sub + NADPH + H+
 In addition to FAD and NAD, there are three other
types of electron carrying groups
– Ubiquinone
– Iron containing proteins (cytochromes, Fe-S proteins)
 Ubiquinone = CoQ or = UQ
– When it accepts 1 electron  UQH (semiquinone)
– When it accepts 2 electrons  UQH2 (ubiquinal)

30. Oxidation
states of
quinones

32. Oxidation states of flavins.
• The reduction of flavin mononucleotide (FMN) to FMNH2
proceeds through a semiquinone intermediate.

37. Complex I
 NADH dehydrogenase
(NADH Q reductase)
 Huge protein
– 25 pp
 FMN, Fe-S
 I electron  UQ

39. Complex II
 Succinate Q Recuctase (Succinate dhydrogenase)
– Is the only membrane bound enzyme in the TCA cycle
– Contains  FAD, Fe-S
 II  electrons  UQ
 Cytochrome: an electron transferring protein that
contains a heme prosthetic group!

42. Complex III

43. Complex III
 Cyt reductase (UQ-cyt c oxido reductase or cyt
bc1 complex)
– Contains cyt b, c1, Fe-S proteins and at least six other
protein subunits
 UQ is 2e- carrier, cyts are 1e- carriers
– This switch is done in a series of reactions (called Q
cycle)
 Electron transfer in III seems to be complicated
but it’s not
 Net reaction:
– UQH2  UQ and cyt c is reduced
– H+ is pumped out also

44. Complex IV
 Cyto oxidase
– Contains a, a3, and CuA, CuB
 The detail of this electron transfer in
Complex IV is not known
 It also functions as a proton pump

52. ATP Production in Mitochondria

53. Chemiosmotic Theory

57. Mitchel’s Theory
 The electrochemical potential
difference resulting from the
asymmetric distribution of the H+ is
used to drive the mech. responsible
for the formation of ATP

61. Chemiosmotic Theory Continued…
G = RT ln(C2/C1) + ZF
[ + ] [ + ]
When H+ is pumped against electrochemical gradient
G=+
When protons flow back inside, this G becomes
available to do the work!!

62. Oxidation and ATP synthesis
are coupled

69. ATP Synthase

82. Uncoupled Mitochondria in Brown Fat
Produces Heat
 This is done by DNP or other uncouplers
 They carry protons across the inner mitochondria membrane
 In the presence of DNP, electron transport is normal but ATP is
not formed
 Proton-motive force is gone or disrupted
 Uncoupling is also seen in brown adipose tissue
 It is useful to maintain BT in hibernating animals, newborns, and
mammals adapted to the cold
 It has lots of mitochondria
 IMM  thermogenin (uncoupling protein)
 Thermogenin generates heat by short-circuiting the mitochondrial
proton battery

83. Shuttle Systems
 Required for cytosolic NADH oxidation
 NADH dehydrogenase IMM can accept electrons only
from NADH in the matrix
 We also make cytosolic NADH by glycolysis
 They also have to be reoxidized to NAD+
 IMM is not permeable to cytosolic NADH
– We therefore need shuttle systems
 Electrons are transferred from NADH to Complex III
(not I), providing only enough energy to make 2 ATP
(G-3-P shuttle)
 It is active in muscle (insect flight) and brain
 Net reaction:
– NADH + H+ + E- FAD  NAD+ + E-FADH2
(cytosolic) (mitochondrial) (cyto) (mito)
 So, 2ATP is formed UQ

87. Malate-aspartate Shuttle
 Heart
 Liver
 Cytoplasmic NADH is brought to
mitochondria by this shuttle
 This shuttle works only if NADH/NAD+
increase in the cytosol (then
mitochondria)
 No energy consumed
 No ATP lost

91. Regulation of ATP Producing
Pathways
 Coordinately regulated
– Glycolysis
– TCA
– FA oxidation
– a.a. oxidation
– Oxidative phosphorylation
 Interlocking regulatory mech.
 ATP, ADP controls all of them
 Acetyl CoA and and citrate

94. Regulation of Oxidative Phosphorylation
 Intracellular [ADP]
If no ADP  no ATP
– The dependence of the rate of O2 consumption on the
[ADP] (Pi acceptor) is called “acceptor control”
acceptor control ratio = ADP-induced O2 consumption
O2 consumption without ADP
 Mass action ratio:
ATP is high normally
[ADP][Pi]
So, system is fully phosphorylated.
ATP used, ratio decreases, rate of oxidative phosphorylation
increases.

100. Tumor Cells
 Regulation is gone in catabolic
processes
 Glycolysis is faster than TCA
 They use more Glc, but cannot
oxidize pyruvate
 Pyruvate  lactate
(PH decreases in tm.)

102. Mutations in Mitochondrial Genes
 Mutations in mitochondrial genes cause human
disease.
 DNA has 37 genes (16, 569 bp), 13 of them encode
respiratory chain proteins.
 LHON- Leber’s Hereditary Opti-neuropathy
– CNS problems
– Loss of vision
– Inherited from women.
– A single base change ND4
Arg  His (Complex 1)
– Result: defective electron transfer from NADH to UQ.
Succinate  UQ okay, but NADH  UQ not.

110. 3 Stages of
Catabolism

111. Summary
 Electron flow results in pumping out H+ and the
generation of membrane potential!
 ATP is made when protons flow back to the matrix!
 F0F1 complex
 Proton motive force, PH gradient, membrane potential
The flow of two electrons through each of three
proton-pumping complexes generates a gradient
sufficient to synthesize one mole of ATP!

112. The proton gradient is an
interconvertible form of free energy
 Proton gradients are a central
interconvertible currency of free
energy in biological systems.
• Active transport of Ca
• Rotation of bacterial flagella
• Transfer of e from NADP+ to NADPH
• Generate heat in hybernation