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Electron transport chain
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•
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All oxidative steps in the degradation of carbohydrates, fats, and amino
acids converge in the final stage of cellular respiration in which the energy
of oxidation drivers the synthesis of ATP – oxidative phosphorylation
This accounts for most of the ATP generated
Occurs in the mitochondria
...

Freely permeable to small
molecules and ions

Inner membrane
...
Contains:Respiratory
electron carriers (complexes
I-IV), ADP-ATP translocases,
ATP synthase and other
membrane transporters

Matrix
...
) or two to form
ubiquinol (QH2)

•

Like flavoprotein carriers it can act at a junction between a two-electron donor and a
one–electron acceptor
• Because its small and hydrophobic, it is freely diffusible within the lipid bilayer of
the inner mitochondrial membrane and can shuttle reducing equivalents
between other less mobile carriers in the membrane
• And because it carries both electrons and protons, it plays a central role in
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coupling electron flow to proton movements

 The cytochromes
• Are proteins with iron-containing heme prosthetic group

 Iron-sulfur proteins
• In these proteins the iron is present not as heme but in association with inorganic
sulfur atoms or with the sulfur atoms of cysteine residues or both
• These iron–sulfur centers (Fe-S) range from simple structures with a single Fe atom
to more complex Fe-S centers with two or four Fe atoms
• All iron–sulfur proteins participate in one-electron transfers in which one iron atom of
the iron–sulfur cluster is oxidized or reduced
Most of the electron carriers are integral proteins with prosthetic groups capable of
accepting or donating either one or two electrons
• Three types of electron transfers occur in oxidative phosphorylation
• Direct transfer as in the reduction of Fe3+ to Fe2+
• Transfer as a hydrogen atom (H+ + ē)
• Transfer as a hydride ion (:H–)

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Electron carriers function in order of increasing reduction potential, because
electrons tend to flow from carriers of lower E′o to carriers of higher E′o
Standard reduction potentials
Redox reaction (half-reaction)

E′o (V)

2H+ + 2ē → H2

–0
...
320

NADP+ + H+ + 2ē → NADPH

–0
...
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Ubiquinone + 2H+ + 2ē → Ubiquinol
Cyt b (Fe3+) + ē → Cyt b (Fe2+)

0
...
077

Cyt c1 (Fe3+) + ē → Cyt c1 (Fe2+)

0
...
254

Cyt a (Fe3+) + ē → Cyt a (Fe2+)

0
...
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½

O2 + 2H+ + 2ē → H2O

0
...
3
Complex
I

– 0
...
077
0
...
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Complex
IV
0
...

(In brackets are the prosthetic groups of the respective enzymes)
Pyruvate

(Lipoate)
FAD
α-Ketoglutarate

NADH

Glutamate, malate, isocitrate,3hydroxybutyrate, 3-hydroxyacylCoA, proline

NADH dehydrogenase (NADH:ubiquinone oxidoreductase) (FMN, Fe-S)
(Complex I)
Glycerol- 3-phosphate
Fatty acyl-CoA

FAD Q

Succinate-Q reductase (Succinate
dehydrogenase )(Complex II)(FAD, Fe-S)

Cytochrome reductase (Ubiquinone:cytochrome c(Heme b, Heme c1, Fe-S)
oxidoreductase ) (Complex III)
Cyt c

Cytochrome oxidase (Complex IV)

O2

(Heme a, Heme a3, CuA, CuB)

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 Complex I (NADH dehydrogenase) : Transfers electrons from NADH to
ubiquinone
• The reduced NADH of the chain is oxidized by a metalloflavoprotein
enzyme NADH dehydrogenase
• It’s a large, L-shaped enzyme composed of 42 polypeptide chains that
includes an FMN-containing flavoprotein and at least 6 iron-sulfur clusters

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NADH:ubiquinone oxidoreductase (Complex I)
...

• This electron transfer also drives the expulsion from the matrix of four
protons per pair of electrons
...

This electrochemical potential drives ATP synthesis
• Amytal ( a barbiturate), rotenone (a plant product commonly used as an
insecticide) and piericidin (an antibiotic) inhibit electron flow from Fe-S
centers to ubiquinone
• Ubiquinol (QH2, the fully reduced form diffuses in the inner mitochondrial
membrane from complex I to complex III)

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 Complex II (Succinate-Q reductase)
...
Result –less ATP formed from oxidation
of FADH than NADH

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 Complex III (Cytochrome reductase): transfers electrons from ubiquinol
to cytochrome c

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•

Cytochrome reductase contains cytochromes b and c1 and iron–sulfur
protein

•

Cyt b has two heme groups bL(b-566) and bH(b-560)

•

Ubiquitol transfers one of its two electrons to the Fe-S cluster in the
enzyme
...

A second molecule of QH2 then reacts with the complex as above
...
After
accepting an electron cytochrome c moves to complex IV to donate the
electron
The flow of a pair of electrons through the complex leads to the effective net
transfer of 4(?2)H+ to the cytosolic side
...
From here electrons pass
through heme a to the Fe-Cu center
(cyt a3 and CuB)
...
Delivery of two
more electrons from cyt c converts the
peroxyl derivative to two molecules of
water consuming four
“substrate”protons from the matrix
...

Cyanide, azide and carbon
monoxide inhibit cytochrome
oxidase
Cyanide and azide react with the ferric form
of heme a3, whereas carbon monoxide
inhibit the ferrous form
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ATP synthesis
• Protons are translocated to the exterior of the inner mitochondrial membrane by
oxidation in the respiratory chain
– Since the membrane is impermeable to ions, protons accumulate outside the
membrane, creating an electrochemical potential difference across the
membrane
– electrochemical potential difference consists of
• Chemical potential difference (pH)
• Electrical potential difference (Charge)
• According to the chemiosmotic theory the electrochemical potential difference
generates a proton-motive force as protons flow back passively into the matrix
through a proton pore associated with ATP synthase
...
Three ATP
molecules are generated per revolution

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F1

ATP
ADP + Pi

γ

H+

Inside

C C C C

Outside

Mitochondrial inner
membrane

F0

H+

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•

•

Inhibitors of passage of electrons to O2 can block ATP synthesis
...
Electron flow and ATP
synthesis are coupled; neither reaction occurs without the other
– This is explained by the chemiosmotic theory
...
The proton gradient builds up until the free energy of
pumping protons against this gradient exceeds the energy released by the
transfer of electrons from NADH to O2
...
Enters the mitochondrial matrix
in the protonated form
...
Uncouple electron transfers from oxidative phosphorylation by
dissipating the electrical contribution to the electrochemical gradient
– Electron transport from NADH to O2 proceeds normally but ATP is not formed
...
5 ATP molecules and
each FADH2 generates about 6/4 or 1
...
0 and 2
...
Because the antiporter moves 4
negative charges out for 3 negative
3H+ charges that move in, its activity is
favoured by the transmembrane
electrochemical gradient, which gives the
matrix a net negative charge; the protonmotive force drives the ADP-ATP
exchange
...
This too is favoured by the
transmembrane proton gradient
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Electrons from cytosolic NADH enter the mitochondria by shuttles
•
•

The inner mitochondrial membrane is impermeable to NADH and NAD+
NADH is formed by glycolysis in the cytosol and NAD+ must be regenerated for
glycolysis to continue
...

 The glycerophosphate shuttle
– One carrier is glycerol 3-phosphate in the glycerophosphate shuttle which
readily traverses the outer mitochondrial membrane
...
Catalyzed by cytosolic glycerol 3-phosphate
dehydrogenase
...
In the process an electron
pair is transferred to FAD to form FADH2
...

– Since the glycerophosphate shuttle is linked to the flavoprotein rather than NAD+
only 1
...
5 molecules of ATP are formed
...

• The NADH can then pass on the electrons to the respiratory chain where
about 2
...

-In contrast to the glycerophosphate shuttle, this shuttle is reversible
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Creatine phosphate shuttle facilitates transport of high-energy phosphate
from mitochondria
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•
•
•
•
•
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ATP and ADP do not diffuse freely from the inner mitochondrial membrane
A specific protein adenine nucleotide translocase or adenine nucleotide transporter
enables these highly charged molecules to traverse the permeability barrier
Flows of ATP and ADP are coupled
...

Atractyloside inhibits this transporter
The shuttle acts as a dynamic system for transfer of high energy phosphate from the
mitochondria in active tissues like muscles
...

CKa, creatine kinase is concerned with large requirements for ATP eg muscle
contraction; CKc, creatine kinase for maintaining equilibrium between creatine and
creatine phosphate and ATP/ADP; CKg, creatine kinase for coupling glycolysis to
creatine phosphate synthesis

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Energy- requiring
processes eg
muscle contraction

ATP

ADP

CKa
ATP

Creatine

ADP

Creatine phosphate

CKc

ATP

CKg

ADP

Glycolysis

Outer mitochondrial
membrane
CKm

ATP

ADP

Inter-membrane
space

ANT

Oxidative
phosphorylation

Inner mitochondrial
membrane
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Regulation of oxidative phosphorylation
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Oxidative phosphorylation produces most of the ATP made in aerobic cells
Oxidative phosphorylation is regulated by cellular energy needs
The rate of respiration in mitochondria is generally limited by the availability of Pi
acceptor, ADP - This is called the acceptor control of respiration
Energy status of the cell can be measured by intracellular concentration of ADP or
the mass–action ratio of the ADP-ATP system i
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Briefly explain this difference
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Briefly describe the chemiosmotic hypothesis for oxidative phosphorylation

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