Transcript O 2

Mitochondrial Electron
Transport
• The cheetah, whose
capacity for
aerobic metabolism
makes it one of the
fastest animals
Fatty Acids
Acetyl Co A
Pyruvate
Glucose
Citric acid
cycle supplies
NADH and
FADH2 to the
electron
transport
chain
Amino Acids
Reduced coenzymes NADH and FADH2 are
formed in matrix from:
(1) Oxidative decarboxilation of pyruvate to
acetyl CoA
(2) Aerobic oxidation of acetyl CoA by the
citric acid cycle
(3) Oxidation of fatty acids and amino acids
The NADH and FADH2 are energy-rich
molecules because each contains a pair of
electrons having a high transfer potential.
The reduced and oxidized forms of NAD
The reduced and oxidized forms of FAD
Electrons of NADH or FADH2 are used to
reduce molecular oxygen to water.
A large amount of free energy is liberated.
The electrons from NADH and FADH2 are not
transported directly to O2 but are transferred
through series of electron carriers that undergo
reversible reduction and oxidation.
The flow of electrons through carriers leads to
the pumping of protons out of the mitochondrial
matrix.
The resulting
distribution of
protons
generates a pH
gradient and a
transmembrane
electrical
potential that
creates a
protonmotive
force.
ATP is synthesized when protons flow back to the
mitochondrial matrix through an enzyme complex
ATP synthase.
The oxidation of fuels and the phosphorylation of
ADP are coupled by a proton gradient across the
inner mitochondrial membrane.
Oxidative
phosphorylation is
the process in which
ATP is formed as a
result of the
transfer of electrons
from NADH or
FADH2 to O2 by a
series of electron
carriers.
OXIDATIVE PHOSPHORYLATION IN
EUKARYOTES TAKES PLACE IN MITOCHONDRIA
Two membranes:
outer membrane
inner membrane (folded into
cristae)
Two compartments:
(1) the intermembrane space
(2) the matrix
The outer membrane
is permeable to small
molecules and ions
because it contains
pore-forming protein
(porin).
The inner membrane
is impermeable to ions
and polar molecules.
Contains transporters
(translocases).
Location of mitochondrial complexes
• Inner mitochondrial membrane:
Electron transport chain
ATP synthase
• Mitochondrial matrix:
Pyruvate dehydrogenase complex
Citric acid cycle
Fatty acid oxidation
THE ELECTRON TRANSPORT CHAIN
Series of enzyme complexes (electron carriers)
embedded in the inner mitochondrial membrane,
which oxidize NADH2 and FADH2 and transport
electrons to oxygen is called respiratory
electron-transport chain (ETC).
The sequence of electron carriers in ETC
NADH
FMN
Fe-S
succinate FAD Fe-S
Co-Q
Fe-S
cyt b
cyt c1
cyt c
cyt a
cyt a3
O2
High-Energy Electrons: Redox Potentials
and Free-Energy Changes
In oxidative phosphorylation, the electron
transfer potential of NADH or FADH2 is
converted into the phosphoryl transfer
potential of ATP.
Phosphoryl transfer potential is G°' (energy
released during the hydrolysis of activated phosphate compound). G°' for ATP = -7.3 kcal mol-1
Electron transfer potential is expressed as E'o,
the (also called redox potential, reduction
potential, or oxidation-reduction potential).
E'o (reduction potential) is a measure of how easily a
compound can be reduced (how easily it can accept
electron).
All compounds are compared to reduction potential of
hydrogen wich is 0.0 V.
The larger the value of E'o of a carrier in ETC the better
it functions as an electron acceptor (oxidizing factor).
Electrons flow through the ETC components spontaneously
in the direction of increasing reduction potentials.
E'o of NADH = -0.32 volts (strong reducing agent)
E'o of O2 = +0.82 volts (strong oxidizing agent)
NADH
FMN
Fe-S
succinate FAD Fe-S
Co-Q
Fe-S
cyt b
cyt c1
cyt c
cyt a
cyt a3
O2
Important characteristic of ETC is the amount of
energy released upon electron transfer from one
carrier to another.
This energy can be calculated using the formula:
Go’=-nFE’o
n – number of electrons transferred from one carrier
to another;
F – the Faraday constant (23.06 kcal/volt mol);
E’o – the difference in reduction potential between
two carriers.
When two electrons pass from NADH to O2 :
Go’=-2*96,5*(+0,82-(-0,32)) = -52.6 kcal/mol
THE RESPIRATORY CHAIN
CONSISTS OF FOUR
COMPLEXES
Components of electrontransport chain are arranged
in the inner membrane of
mitochondria in packages
called respiratory
assemblies (complexes).
FMN
II
III
IV
III
I
NADH
I
Fe-S
II
succinate FAD Fe-S
Co-Q
Fe-S
cyt b
IV
cyt c1
cyt c
cyt a
cyt a3
O2
The energy is released not in a single step of electron
transfer but in incremental amount at each complex.
26.8
Energy released at three specific steps in the chain is
collected in form of transmembrane proton gradient and
used to drive the synthesis of ATP.
Complexes I-IV
• Mobile coenzymes: ubiquinone
(Q) and cytochrome c serve as
links between ETC complexes
• Complex IV reduces O2 to water
Complex I (NADH-ubiquinone oxidoreductase)
Transfers electrons from NADH to Co Q (ubiquinone)
Consist of:
- enzyme NADH dehydrogenase (FMN - prosthetic group)
- iron-sulfur clusters.
NADH reduces FMN to FMNH2.
Electrons from FMNH2 pass to a Fe-S clusters.
Fe-S proteins convey electrons to ubiquinone.
QH2 is formed.
The flow of two electrons from NADH to coenzym Q leads
to the pumping of four hydrogen ions out of the matrix.
NADH
FMN
matrix
Fe-S
Iron-sulfur clusters
contains two or four iron
ions and two or four
inorganic sulfides.
Clusters are coordinated
by four cysteine residues.
Iron ions in Fe-S
complexes cycle between
Fe2+ or Fe3+ states.
NADH-Q oxidoreductase - an
enormous enzyme consisting of
34 polypeptide chains.
L-shaped (horizontal arm lying
in the membrane and a vertical
arm that projects into the
matrix).
Complex II (succinate-ubiquinon oxidoreductase)
Transfers electrons from succinate to Co Q.
Form 1 consist of:
- enzyme succinate dehydrogenase (FAD – prosthetic group)
- iron-sulfur clusters.
Succinate reduces FAD to FADH2.
Then electrons pass to Fe-S proteins which reduce Q to QH2
Form 2 and 3 contains enzymes acyl-CoA dehydrogenase
(oxidation of fatty acids) and glycerol phosphate dehydrogenase
(oxidation of glycerol) which direct the transfer of electrons
from acyl CoA to Fe-S proteins.
Complex II does not contribute to proton gradient.
All electrons must pass through the ubiquinone (Q)ubiquinole (QH2) pair.
Ubiquinone Q:
- lipid soluble molecule,
- smallest and most hydrophobic of all the carriers
- diffuses within the lipid bilayer
- accepts electrons from I and II complexes and
passes them to complex III.
Complex III (ubiquinol-cytochrome c oxidoreductase)
Transfers electrons from ubiquinol to cytochrome c.
Consist of: cytochrome b, Fe-S clusters and cytochrome c1.
Cytochromes – electron transferring proteins containing a
heme prosthetic group (Fe2+  Fe3+).
Oxidation of one QH2 is accompanied by the translocation
of 4 H+ across the inner mitochondrial membrane. Two H+
are from the matrix, two from QH2
Q-cytochrome c
oxidoreductase is a dimer.
Each monomer contains
11 subunits.
Q-cytochrome c
oxidoreductase contains
three hemes: two b-type
hemes within cytochrome b,
and one c-type heme within
cytochrome c1.
Enzyme also contains an
iron-sulfur protein with an
2Fe-2S center.
Q cycle
two molecules of QH2 are oxidized to form two
molecules of Q,
one molecule of Q is reduced to QH2,
two molecules of cytochrome c are reduced,
four protons are released on the cytoplasmic side,
two protons are removed from the mitochondrial
matrix
Complex IV (cytochrome c oxidase)
Transfers electrons from cytochrome c to O2.
Composed of: cytochromes a and a3.
Catalyzes a four-electron reduction of molecular oxygen (O2) to
water (H2O):
O2 + 4e- + 4H+  2H2O
Translocates 2H+ into the intermembrane space
Cytochrome c
oxidase
consists of 13
subunits and
contains two
hemes (two
iron atom) and
three copper
ions, arranged
as two copper
centers.
The Catalytic Cycle of Cytochrome c Oxidise
The four protons used for the
production of two molecules of
water come from the matrix.
The consumption of these four
protons contributes to the
proton gradient.
Cytochrome c oxidase pumps
four additional protons from
the matrix to the cytoplasmic
side of the membrane in the
course of each reaction cycle
(mechanism under study).
Totally eight protons are
removed from the matrix in
one reaction cycle (4 electrons)
Cellular Defense Against Reactive
Oxygen Species
If oxygen accepts four electrons - two molecules of H2O are
produced
single electron - superoxide anion (O2.-)
two electrons – peroxide (O22-).
O2.-, O22- and, particularly, their reaction products are harmful to
cell components - reactive oxygen species or ROS.
DEFENSE
superoxide dismutase (manganese-containing version in
mitochondria and a copper-zinc-dependent in cytosol)
O2.- + O2.- + 2H+ = H2O2 + O2
catalase
H2O2 + H2O2 = O2 + 2 H2O
antioxidant vitamins: vitamins E and C reduced glutathione