Lecture 13
Electron Transport
Glucose + 02--- CO2 + H20
Reduced
High energy
Oxidized
Low energy
Mitochondria
The pumps
Overview electron transport
ADP
ATP
Working of respiratory chain is different from enzymatic reactions where energy is passed down from
one energy rich bond to another. Here chemical reaction is coupled to physical force.
1
separation of protons and electrons of hydrogen
2
Passage of electrons down cascade-four membrane protein complexes (proton pumps expel protons)
3
Electrons travel down chain and are picked up by oxygen and combine with proton forming H2O
4
Proton accumulate outside mito and are pumped back through inner membrane-mol motor rotates by
flow of protons driving synthesis of ATP from ADP+Pi
Summary
1
2
3
4
Complex1 accepts H+ from NADH2 converting it back to NAD. Electrons are transferred to
carrier-ubiquinone. 2 Hydrogen protons + 2 additional protons are expelled/electron (Free energy
of electron transfer is used)
Complex2 accepts hydrogen from succinate and transfers electrons to ubiquinone (CoenzyQ)
Complex3 re-oxidizes CoenzyQ and expels four protons/electron. Electrons are transferred to
carrier CytochromeC (Free energy of electron transfer is used to expel proton).
Complex4 re-oxidizes CytC and electrons are transferred to oxygen. Free energy of electron
transfer is used to expel 4 protons
Electrons do not occur in free form. Proteins provide grps that bind and release electrons. These are
in close physical proximity.
Successive transfer of electrons must be exergonic- free energy must be negative
Cofactors that bind/release electrons
1
Flavins
2
Iron-sulfur cluster (Fe held by 4 sulfur)
3
Porphyrin (heme- Fe2+ to Fe3+)
4
Copper ions
Two electron carriers are mobile
A
Ubiquinone (CoenzymeQ)- carries electrons as hydrogen and moves within inner membrane.
Transports electrons from complex1 and 2 to complex3
B
CytochromeC (contains Heme) located between inner and outer membrane. Transports electrons
from complex3 tocomplex4
Std reduction potential
Free energy of electron movement from one cofactor to
next must be negative.
Cofactors down the chain have higher affinity for electronsthey have higher redox potential.
Redox Carrier
DE0’
NAD+
FMN (complex1)
Fe-S (complex1)
FAD (complex2)
Ubiquinone (carrier)
Heme (complex3)
CytochromeC (carrier)
Heme (complex4)
Oxygen
-0.32V
-0.30V
-0.03V
-0.04V
0.04V
0.235V
0.25V
0.38V
0.82V
Complex 1-4
The gradient
Path of electrons
Electrons are stripped of their protons, the protons are released at cytosolic side. This accounts
for some of the proton translocation.
The mechanism by which the additional protons are expelled from the mitochondria is not known.
Possible that binding and release of electrons causes conformational changes in protein complexes
(analogous to allosteric change) and these may help expel protons.
Path of electrons 1-2
Path of electrons 3
Path of electrons 4
Protonmotive force
Cytosol
Mitochondrial matrix
ATP is synthesized AFTER substrate is vanished in form of CO2 and H2O.
The energy is stored in PROTON MOTIVE force.
Proton conc in cytosol is 10 times higher than in mitochondrial matrix. Proton moving down the gradient
generates free energy (6kJ/mol)
Each proton ejected leaves one negative charge inside mitochondria- generates electrostatic
membrane potential- 150mV negative inside=15kJ/mol
Total protonmotive force is 21kJ/mol
H+ pump
ATP synthase
Mito matrix
Cytoplasm
1
Protons flow between a and F0
subunit (with 10 c chains).
2
This causes F0 subunit to rotate
(aspartate in c chain in F0 interacts
with proton and undergo
conformational change resulting in
rotation). The proton is translocated
and released into mitochondrial
matrix
3
g subunit rubs against ab hexamer
4
ab hexamer that rub undergo
conformational change
5
b subunit contains active site that
binds ADP+Pi
6
b subunit utilizes energy transmitted
from conformational change to
synthesize ATP
7
b subunit gets everted and ATP is
released.
ADP to ATP
ATP transport