Electron transport chain and oxidative

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Electron transport chain and oxidative phosphoryl
ation Where DOES most of the ATP come
from? Remember that we have 10 molecules of
NADH and 2 molecules of FADH2 These contain
high-energy electrons Electron transport chain
the controlled release of this energy Oxidative
phosphorylation ATP synthesis Linked by
electrochemical proton gradient
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What are the electron carriers? All (except
coenzyme Q) have prosthetic groups that can be
oxidized or reduced All (except cytochrome c) are
hydrophobic (how do you know?) Can be
distinguished by absorption spectra Flavoproteins
- use FAD or FMN as a prosthetic group. Can
transfer both electrons and protons Iron-sulfur
proteins- Fe and S complexed to cysteine groups.
Fe is electron donor and Acceptor does not pick
up protons
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Cytochromes- also contain iron, but as heme. At
least 5 different kinds b,c,c1,a, a3 All are
integral membrane proteins except cytochrome c.
Cytochrome c is not part of a complex and can
diffuse rapidly. Cytochromes a and a3 also
contain copper they form iron-copper
centers. Donates and accepts electrons Also binds
oxygen until 4 protons and electrons are also
bound, to form water That is one reason that you
need iron and copper in your diet!
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Coenzyme Q- a quinone, not a protein also known
as ubiquinone Found in interior of inner
membrane Can transfer electrons and protons Can
itself pick up and transfer protons to
intermembrane space Carriers are arranged
according to their reduction potentials
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I
II
III
IV
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Comparison of the complexes Complex electrons
electrons protons from
to transferred? I NADH coenzyme Q
yes II succinate coenzyme Q
no (via FAD) III coenzyme Q cytochrome c
yes IV cytochrome c oxygen
yes (see previous diagram for complex components)
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Complex IV (cytochrome oxidase) is a
terminal oxidase passes electrons directly to
oxygen Cyanide and azide bind directly to
Fe-Cu center of this and therefore block all
electron transport These complexes are mobile
within the membrane lots of unsaturated
phospholipids and very little cholesterol (very
fluid membranes)
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Electron transport does not yield ATP but the
energy released is couple to ATP synthesis Chemi
osmosis model (Mitchell, 1961) energy released
from electron transfer is accompanied by pumping
of protons across membrane. Gradient
provides driving force for ATP synthesis Took
almost 20 years to confirm this the connection
is provided by the F0F1-ATP complex (ATP
synthase)
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• The evidence
• Protons are pumped out of the mitochondrial
• Matrix. Mitochondria were suspended in
• medium
• In the presence of oxygen, pH of medium fell
• (same with chloroplasts). Conclusion
• Protons are being pumped out. (How is still
• not entirely clear)
• 2. ETC carriers must be oriented so that
• pumping occurs in one direction. Labeling
• experiments confirmed this.

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3. Synthetic vesicles with complexes I, III or
IV produced proton gradients 4. Oxidative
phosphorylation will not occur unless an intact
membrane is formed 5. Anything that abolished
the proton gradient (an uncoupling agent)
abolishes ATP formation 6. Proton gradient
drives ATP synthesis proton motive force tends
to drive protons back across membrane (about 3-4
protons per ATP molecule
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7. Artificial proton gradients can also drive
ATP synthesis ATP synthase has distinct
components F1- binding site/catalytic site for
ATP formation F0- forms and stabilizes proton
channel stalk (within F0) transmits energy
from F0 to F1
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F0
F1
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Chemiosmosis uses energy stored in a
proton gradient to drive cellular
work Mitochondria- ATP synthesis Chloroplasts-
likewise (except that high-energy electrons are
excited by light) Prokaryotes- ATP synthesis (on
plasma membrane) pumping of various
molecules flagellar movement
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Approximately 3 ATP per NADH 2 per
FADH2 Prokaryotes generally gain more ATP from
a molecule of glucose than do eukaryotes Why? NA
DH produced during glycolysis (in cyto- plasm)
must be shuttled into mitochondrion Process
ultimately requires transfer of electrons to
FAD, and thus 2 fewer ATP molecules
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Actual yield is probably less than that,
because proton pump is used to drive transport of
other Materials across inner membrane. Neverthele
ss, the process is highly efficient (40-50 of
energy released through oxidation of glucose is
actually conserved, through ATP formation
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Control of cellular respiration Feedback
control ATP synthesis increases if demand is
high Demand is low intermediates can be
diverted into other pathways Regulation of
certain enzymes is critical
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