Download Electron Transport and oxidative phosphorylation (ATP Synthesis)

Electron Transport and
oxidative phosphorylation
(ATP Synthesis)
Dr. Howaida Nounou
Biochemistry department
Sciences college
The Metabolic Pathway of Cellular
Respiration
All of the reactions involved in cellular respiration can
be grouped into three main stages
Glycolysis – occurs in cytoplasm
The Krebs cycle – occurs in matrix of
mitochondria
Electron transport – occurs across the
mitochondrial membrane
Mitochondrion
Cytosol
High-energy
electrons
carried
mainly by
NADH
High-energy
electrons
carried
by NADH
Glycolysis
Glucose
2
Pyruvic
acid
Krebs
Cycle
Electron
Transport
A Road Map for Cellular Respiration
Redox Reactions
Chemical reactions that transfer electrons from one
substance to another are called oxidation-reduction
reactions
REDOX short for oxidation-reduction reactions
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
REDOX FACTS
A:H
Reductant
B
B:H
Oxidant + e- Reductant
(acceptor)
(donor)
²
A
Oxidant + e-
both oxidation and reduction must occur
simultaneously
Electrons can move through a chain of donors
and acceptors
In the electron transport chain, electrons flow
down a gradient
Electrons move from a carrier with low reduction
potential (high tendency to donate electrons)
toward carriers with higher reduction potential
(high tendency to accept electrons)
Oxidative
process
Phosphorylation
process
O2 e inner
membrane
H 2O
H+
ADP+
Pi
H+
outer
membrane
ATP
intermembrane
space
matrix
Figure: 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.
Succinate
II
NADH
I
Coenzyme Q
III
Cytochrome C
IV
electron flow
The components of the RC are arranged in order of
increasing redox potential
½ O2
RC exists as four large, multisubunit protein complexes
The respiratory electron transport chain
complex I is a NADHubiquinone reductase
complex II is succinate
dehydrogenase (part of the
TCA cycle)
complex III is the
ubiquinone -cytochrome c
reductase
complex IV is cytochrome
oxidase
Figure: Complex I of the
respiratory chain that links NADH
and coenzyme Q.
NADH Dehydrogenase accepts 2e-- from NADH and transfers
them to ubiquinone (coenzyme Q), an electron carrier
Uses two bound cofactors to accomplish this: FMN (Flavin
mononucleotide) and 6 iron-sulfur (Fe-S) protein
Pumps 4 H+ out to inter-membrane space
Complex II: Succinate-CoQ reductase
Prosthetic groups: FAD; Fe-S
Succinate
FAD
SDH
Fumarate
FADH2
CoQ
SDH is succinic dehydrogenase an enzyme of the citric acid
cycle (associated with membrane)
2 e- transferred from succinate to CoQ
1 mole FADH2 produced
Electrons from
complex I or II
CoQ
cyt b/cyt c1
Complex III: cytochrome reductase
Prosthetic groups: heme b; heme c1; Fe-S
cyt c
Figure: Complex III of the respiratory chain linking
CoQ and cytochrome C.
Is composed of cytochome b, cytochrom C1 and iron sulphur
proteins
Accepts e- from coenzyme Q and transfers e- to cytochrome c
coupled with the transfer of protons from the matrix to the
intermembrane space
Figure: Complex IV -cytochrome
oxidase- reducing oxygen to water
Contains cytochromes a/a3 and 2 Cu ions involved in e- transfers
Cytochrome oxidase passes electrons from cytochrome c through
a series of heme groups and Cu ions to O2, reducing it to H2O (end
product) and pumping one proton into the intermembrane space
for each e-
Coenzymes and cytochromes in the
complexes act as e- donors & acceptors
Flavin MonoNucleotide (FMN), in Complex I, functions like FAD
(Flavin adenine dinucleotide , which is an electron acceptor that
helps electron transfer during Krebs Cycle and Electron Transport
Chain in cellular respiration).
iron-sulfur (Fe-S proteins): Fe-S centers transfer e- in Complexes I,
II and III
Coenzyme Q (ubiquinone), lipid soluble, floats in the membrane
and doesn’t require protein
Cytochromes (b, c1, c, a, a3; contain heme): transfer e- in Complexes
III and IV, Cytc is the only soluble cytochrome
NAD+, FMN, CQ are carriers of e- and hydrogen while cytochromes
are carriers of electrons only.
Summary of redox complexes of the electron transport chain
Complex
designation
I–
NADHNADH-Q
reductase
Functional
groups
FMN (flavin
monomononucleotide);
FeFe-S
Function
oxidizes NADH to
NAD+;
transfers electrons to
coenzyme Q
Poisons
Rotenone
II – SuccinateSuccinate- FAD; FeFe-S
Q reductase
oxidizes succinate to
fumarate with reduction
of FAD to FADH2;
electron transfer to CoQ
III Cytochrome
reductase
heme b;
heme c1;
FeFe-S
transfers electrons
between coenzyme Q
and cytochrome C
(C becomes reduced)
Antimycin A
IV Cytochrome C
oxidase
heme aa- a3 ;
Cu
oxidizes cytochrome C;
reduces ½O2 to H2O
Carbon monoxide
Cyanide
Overall Reaction….
NADH + H+ + ½ O2 NAD+ + H2O
Taking into account the protons pumped out:
complex I
4 H+
complex III 4 H+
complex IV 2 H+
The equation for electron transfer is:
NADH + 11 H+matr + ½ O2 NAD+ + 10 H+intermem + H2O
Oxidative
Phosphorylation
Definition
Process in which ATP is formed as a result of transfer of
electrons from NADH or FADH2 by a series of electron
carriers
The electron transport chain generates no ATP directly.
Rather, its function is to break the large free energy
drop from food to oxygen into a series of smaller steps
that release energy.
ATP yield
Only 4 of 38 ATP ultimately produced by respiration of
glucose are derived from substrate-level
phosphorylation (2 from glycolysis and 2 from TCA)
The vast majority of the ATP (90%) comes from the
energy in the electrons carried by NADH and FADH2
ATP-synthase (complex V),
ATPV) present in the inner
mitochondrial membrane, actually makes ATP from ADP
and Pi.
ATP used the energy of an
existing proton gradient to
power ATP synthesis.
This proton gradient develops
between the intermembrane
space and the matrix.
This concentration of H+ is the
proton--motive force.
proton
force
22
The ATP synthase molecules are the
only place that will allow H+ to
diffuse back to the matrix (exergonic
flow of H+).
This flow of H+ is used by the
enzyme to generate ATP a process
called chemiosmosis
chemiosmosis..
Chemiosmosis: (osmos = push)
is the oxidative phosphorylation that
results in ATP production in the inner
membrane of mitochondria.
Properties of ATP Synthase
Multisubunit transmembrane protein
Molecular mass = ~450 kDa
Functional units
F0: water-insoluble transmembrane protein
(up to 8 different subunits)
F1: water-soluble peripheral membrane protein (5
subunits) ,contains the catalytic site for ATP synthesis
Flow of 3 protons through ATP synthase
leads to phosphorylation of 1 ADP
During respiration, most energy flows from glucose -> NADH ->
electron transport chain -> proton
proton--motive force -> ATP.
One sixsix-carbon glucose molecule is oxidized to 6 CO2 molecules.
Some ATP is produced by substrate
substrate--level phosphorylation during
glycolysis and the Krebs cycle, but most ATP comes from oxidative
chain).
phosphorylation (through electron transport chain).
Each NADH from the Krebs cycle and the conversion of pyruvate
contributes enough energy to generate a maximum of 3 ATP.
ATP.
Each FADH2 from the Krebs cycle can be used to generate about
2ATP
ATP..
Energy produced in electron transport chain gives a maximum
yield of 34 ATP by oxidative phosphorylation via ATPATP-synthase.
These
compounds prevent the passage of e- by binding a
component of the ETC blocking the oxidation/reduction reaction
Inhibitors of ElectronTransport Chain
and oxidative phosphorylation
Be familiar with the actions of inhibitors in the red boxes
Uncouplers:
Compounds that increase the permeability of the inner
mitochondrial membrane to protons.
Protons renters the matrix at sites other than ATP
synthase through holes made by these compounds .
These compounds have no effect on electron transport
chain , but they uncouple oxidative phosphorylation.
The energy produced by the transport of electron is
released as heat rather than being used to synthesis ATP.
Examples:
2,4 dinitrophenol, dinitrocresol, pentachlorophenol,
thyroxine, calcium, mchlorocarbonylcyanide
phenyl hydrazone (cccp).
Adding Up the ATP
Cytosol
Mitochondrion
Glycolysis
Glucose
2
Pyruvic
acid
2
AcetylCoA
Krebs
Cycle
Electron
Transport
Maximum
per
glucose:
by direct
synthesis
by
direct
synthesis
by
ATP
synthase
Protein
complex
Electron
carrier
Inner
mitochondrial
membrane
Electron
flow
Electron transport chain
ATP synthase
31
Food
Polysaccharides
Sugars
Glycerol
Fats
Fatty acids
Proteins
Amino acids
Amino groups
Glycolysis
AcetylCoA
Krebs
Cycle
Electron
Transport
32