5.17.06 Electron Transport and Oxidative Phosphorylation
What does the citric acid (Kreb’s cycle) accomplish?
What does the citric acid (Kreb’s) cycle accomplish?
• Carbons in pyruvate are fully oxidized to CO2
• Some GTPs (later converted to ATPs) are generated by
substrate level phosphorylation
• 8 NADH and 2 FADH2 are stockpiled
NADH and FADH2 are high energy molecules
and they can be used as reducing agents by the
cell.
But much of the stored energy is not directly
“accessible” to the cell in this form
What happens in the mitochondria to convert
the potential energy in NADH into the form of
ATP?
1
NADH and FADH2 are high energy molecules:
As electrons drop from the
top to the bottom of the scale,
energy is released
2
• Substances vary in
their tendancy to
become oxidized or
reduced.
• This tendancy is
expressed as the
reduction
potential. As
electrons drop
from the top to the
bottom of the scale,
energy is released.
• Glucose has a
reduction potential
of (-0.43 V) – so
the exergonic
oxidation of
glucose is coupled
to the endergonic
reduction of NAD+
How to power ATP synthesis
• build a dam
• pile up protons on one side
• poke a hole -- use the rush
of protons through the hole
to turn a turbine which then
makes ATP
3
The inner mitochondrial membrane is 70% protein and
30% phospholipid by weight
An electron micrograph of
the inside surface of the
inner mitochondrial
membrane in a plant cell.
Densely packed particles
are visible -- due to
protruding portions of
ATP synthase and the
respiratory enzyme
complexes
4
A Closer look at the inner mitochondrial membrane

 ATP synthase
ATP
Transporter
The inner mt membrane is 70% protein and 30% phospholipid
by weight
• many of the proteins belong to the electron transport chain
• also includes ATP synthase: converts the stored potential
energy in the electrochemical proton gradient into chemical
energy
matrix
space between inner membranes
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THE ELECTRON TRANSPORT CHAINS CONSISTS
OF MEMBRANE ASSOCIATED ELECTRON
CARRIERS
These systems have two basic functions:
1. to accept electrons from an electron donor and to
transfer them to an electron acceptor
2. to conserve some of the energy released during
electron tranasfer for the synthesis of ATP
The electron transport chain reoxidizes the
coenzymes NADH and FADH and channels the free
energy into the synthesis of ATP
• NADH and FADH gained electrons when
oxidizing other compounds
• they transfer these electrons to the electrontransport chain: electron transport chain
reoxidizes the coenzymes NADH and FADH
and channels the free energy into the synthesis
of ATP
6
NADH is a high energy
molecule:
Brock 5.19
Reduction potential
of the components
of the electron
transport chain of
the mitochondrion
of eukaryotic cells
and the plasma
membrane of some
bacterial cells.
By breaking up the
complete oxidaton
into a series of
discrete steps,
energy “recapture”
is possible
• Substances vary in their tendancy to become oxidized or reduced.
• This tendancy is expressed as the reduction potential. As
electrons drop from the top to the bottom of the scale, energy is
released.
• NOTICE the molecule at the bottom!
7
How to power ATP synthesis
• build a dam
• pile up protons on one side
• poke a hole -- use the rush of protons through
the hole to turn a turbine which then makes ATP
Oxidative Phosphorylation:
the production of ATP using energy derived from the
redox reactions of an electron transport chain
Chemiosmosis:
the production of ATP from ADP using the energy of
hydrogen ion gradients
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How protons can be
pumped across membranes:
As an electron passes along
an electron-transport chain
embedded in a lipidbilayer, it can bind and
release a proton at each
step. In this diagram
electron carrier B picks up
a proton (H+) from one side
of the membrane when it
accepts an electron from
carrier A. It releases the
proton to the other side of
the membrane when it
donates its electron to
carrier C
Matrix (inside the inner membrane of the mt) is
above the membrane (gray bar). The
intermembrane space is below the membrane.
animation of electron transport
http://www.sp.uconn.edu/~terry/images/anim/ETS_slow.html
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ALBERTS animation 14.2
• The energy released is used to transport H+ ions across the
inner mitochondrial membrane to the space between the two
membranes
• In this way, a gradient of H+ ions is maintained across the
inner membrane
• This gradient serves as a source of energy (like a battery) that
is tapped to drive a variety of energy-requiring reactions
• The most prominent of these reactions is the generation of
ATP
ADP + Pi -----> ATP
•
10
How to power ATP synthesis
• build a dam
• pile up protons on one side
• poke a hole -- use the rush of protons
through the hole to turn a turbine which
then makes ATP
11
Potential energy in gradient converted to
mechanical energy which via conformtional
changes in the cytoplasmic portion of the ATPase is
converted to chemical energy in ATP
• ATP synthase is
imbedded in the
inner mitochondrial
membrane
• below & left in this
figure is the matrix
of the mitochondria
(compartment
contained within the
inner mitochondrial
membrane)
• water soluble
catalytic domain is in
matrix
• spinning ion
transport channel
(embedded in lipid
bilayer)
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ATP synthase: enzyme that uses energy from
the proton gradient to produce ATP from ADP
+ Pi
• inner mitochondrial membrane of all
eukaryotic cells
• the thylakoid membrane of chloroplasts of
plant cells
• the plasma membrane of prokaryotic cells
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Now for some serious quaternary structure!
ATP synthase — energy converter. The enzyme consists of two rotary motors, F0
and F1 which are coupled via their drive shafts. The transmembrane F0 motor has
one a, two b and nine to twelve c subunits. The soluble F1 motor has three α and
three β subunits, and one each of the other subunits.
During ATP synthesis, F0 channels protons across the membrane to drive rotation.
Nature 410: 878 4/19/01
•
The rotating subunits are the c polypeptide in Fo and the γ
polypeptide in F1
• The rotation of Fo (caused by movement of protons) drives the
rotation of γ
• This rotation drives the conformational transitions of the
catalytic subunits which, in turn, alters the nucleotide binding
site affinities.
• As a consequence, conformational energy flows from the
catalytic subunit into the bound ADP and Pi to promote their
dehydration into ATP.
ALBERTS animation 14.3
HTTP://WWW.TCD.IE/BIOCHEMISTRY/IUBMB-NICHOLSON/SWF/GLYCOLYSIS.SWF
Step 32 in this animation substrate level ATP synthesis in glyclolysis
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MORE optional Stuff on ATP synthase for those who are
amused by this protein:
Look at First two links
ATP synthase
do cross section alpha, beta gamma
http://www.cnr.berkeley.edu/~hongwang/Project/ATP_synthase/
ATP synthase
http://rsb.info.nih.gov/NeuroChem/biomach/ATPsyn.html
This cartoon is adapted from fig. 2 of Cross. The 3 shades of red represent the 3 different
conformational states of the catalytic subunits. The central asymmetric black object represents
the gamma subunit which is caused to rotate by themitochondrial proton efflux. This rotation
drives the conformational transitions of the catalytic subunits which, in turn,alters the nucleotide
binding site affinities. As a consequence, conformational energy flows from the catalytic subunit
into
the bound ADP and Pi to promote their dehydration into ATP.
http://teddy.berkeley.edu:1024/ATP_synthase/
ATP synthase: the rotory engine in the cell:
http://www.res.titech.ac.jp/~seibutu/main.html?right/~seibutu/projects/f1_e.html
in vitro rotation of an actin filament attached to ATP synthase
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What fraction of the potential energy can a respiring
cell extract from a glucose molecule?
A few billion years of evolution have ensured that
the aerobic system is
40 - 54%
efficient
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