Transcript Chapter 20 - Evangel University

Mary K. Campbell
Shawn O. Farrell
http://academic.cengage.com/chemistry/campbell
Chapter 20
Electron Transport and
Oxidative Phosphorylation
Paul D. Adams • University of Arkansas
The Role of Electron Transport in Metabolism
• Electron transport is carried out by four closely
related multisubunit membrane-bound complexes
and two electron carriers, coenzyme ___ and
cytochrome ___
• In a series of oxidation-reduction reactions, electrons
from FADH2 and NADH are transferred from one
complex to the next until they reach _____
• O2 is reduced to H2O
• As a result of electron transport, ____________
are pumped across the inner membrane to the
intermembrane space, creating a pH gradient
ATP Production in the Mitochondrion
• The production of ATP in the mitochondria is the
result of oxidative phosphorylation
• The proton gradient establishes a voltage gradient
• The proton and voltage gradients together provide
the mechanism to couple electron transport with
_______________________________________
Establishment of the Proton Gradient
Summary
• Electron transport from
one carrier to another
creates a proton gradient
across the inner
mitochondrial membrane
• The proton gradient is
coupled to the
production of ATP in
aerobic metabolism
Reduction Potentials
• A useful way to look at electron transport is to
consider the change in free energy associated with
the movement of electrons from one carrier to
another
• If we have two electron carriers, for example NADH
and coenzyme Q, are electrons more likely to be
transferred from NADH to coenzyme Q, or vice versa?
• What we need to know is the _____________
_____________ for each carrier
• A carrier of high reduction potential will tend to be
_____________ if it is paired with a carrier of
_____________ reduction potential
Reduction Potentials
Reduction Potentials
Summary
• Standard reduction potentials provide a
basis for comparison among redox
reactions
• The sequence of reactions in the electron
transport chain can be predicted by using
reduction potentials.
Electron Transport Complexes
• Complex I: NADH-CoQ
oxidoreductase
• Electrons are passed from
NADH to FMN
NADH + H+ E-FMN
NAD+ + E-FMNH2
Electron Transport Complexes
• Electrons are then passed to the iron-sulfur clusters
• The last step of Complex I involves electrons being
passed to coenzyme Q (also called ubiquinone)
Energetics of Electron Transport
• The transfer of electrons is strongly ____________
and sufficient to drive the phosphorylation of ADP
Electron Transport Complexes
Complex II: Succinate-coenzyme Q oxidoreductase
Succinate + E-FAD
Fumarate + E-FADH2
• The overall reaction is exergonic (-13.5 kJ/mol),
but not enough to drive ATP production
• No H+ is pumped out of the matrix during this step
Electron Transport Complexes
Complex III: CoQH2-cytochrome c oxidoreductase
CoQH2 + 2Cyt c[Fe(III)]
CoQ + 2Cyt c[Fe(II)] + 2H+
• This reactions of this complex results in a decrease in
free energy that is sufficient to drive the phosphorylation
of ADP to ATP
• The flow of electrons from reduced CoQ, a quinone that
can exist in 3 forms, is known as the _____________
Oxidized and Reduced Forms of CoQ
Electron Transport Complexes
Complex IV: Cytochrome c oxidase
• Catalyzes the final step in electron transport
2Cyt c[Fe(II)] + 2H+ + ½O2
2 Cyt c[Fe(III)] + H2O
• Complex IV contains cytochrome a, cytochrome a3,
and Cu(II), which are also involved in the electron
transport
• Complex IV is the link to __________ __________
Electron Flow
The Energetics of Electron Transport Reactions
The Heme Group of Cytochromes
• All cytochromes contain a _____________ group
• Side chain differences depending on the heme
Connection between Electron Transport & Phosphorylation
• The energy-releasing oxidations give rise to proton
pumping and a pH gradient across the _____________
_____________ membrane
• Differences in the concentration of ions across the
membrane generates a _____________ _____________
• A coupling process converts the electrochemical potential
to the chemical energy of ATP
• The coupling factor is ATP _____________, a complex
protein oligomer, separate from the electron transport
complexes
• Uncouplers inhibit the phosphorylation of ADP without
affecting electron transport; examples are
________________, _____________, & _____________
Uncouplers
P/O Ratio
• P/O ratio: the number of moles of Pi consumed in
_____________ to the number of moles of oxygen
atoms consumed in _____________
• Phosphorylation: ADP + Pi ATP + H2O
• Oxidation: 1/2O2 + 2H+ + 2e- H2O
• P/O = ______ when NADH is oxidized
• P/O = ______ when FADH2 is oxidized
Summary
• The coupling of electron transport to oxidative
phosphorylation requires a multisubunit
membrane-bound enzyme, ATP synthase.
• This enzyme has a channel for protons to flow
from the intermembrane space into the
mitochondrial matrix.
• The proton flow is coupled to ATP production in a
process that appears to involve a conformational
change of the enzyme.
Chemiosmotic Coupling
• Chemiosmotic coupling
• based on a ________ concentration gradient between
the intermembrane space and the matrix
• a proton gradient exists because the various proteins
that serve as electron carriers are not symmetrically
oriented with respect to the two sides of the inner
mitochondrial membrane
• these proteins take up protons from the matrix when
they are reduced and release them to the
intermembrane space when they are reoxidized
• the reactions of NADH, CoQ, and O2 all require
_______________________
Chemiosmotic Coupling
Chemiosmotic Coupling
Evidence for chemiosmotic coupling suggested (Mitchell 1961):
• A system with definite inside and outside _____________
(closed vesicles) is essential.
• Submitochondrial vesicles can be prepared, which carry
out oxidative phosphorylation and have an asymmetric
orientation of respiratory complexes.
• A model system for oxidative phosphorylation can be
constructed with proton pumping in the absence of electron
transport; the model system consists of reconstituted
membrane vesicles, mitochondrial ATP synthase, and a
proton pump.
• The existence of the pH gradient has been demonstrated
and _____________ _____________
Chemiosmotic Coupling
• The mechanism by which the proton gradient leads
to the production of ATP depends on ion channels
through the inner mitochondrial membrane
• Protons flow back into the matrix through channels in
the F0 unit of ATP _____________
• The flow of protons is accompanied by formation of
ATP in the F1 unit of ATP _____________
• The details of how phosphorylation takes place as a
result of the linkage to the proton gradient are not
explicitly specified by this mechanism
Chemiosmotic Coupling
Conformational Coupling
• The proton gradient leads to changes in conformation in
a number of proteins, including ATP _______________
• There are 3 sites for substrate on ATP _____________,
and 3 possible conformations:
- Open (O); a low affinity for substrate
- Loose-binding (L); not catalytically active,
binds ADP & Pi
- Tight-binding (T); catalytically active, binds ATP
• These sites interconvert as a result of proton flux
through ATP synthase
• Proton flux converts L to T, which produces ATP
• Proton flux converts T to O, releasing ATP
Release of ATP from
ATP Synthase
Summary
• In chemiosmotic coupling, the proton gradient
is the crux of the matter. The flow of protons
through pores in the synthase drives ATP
production.
• In conformational coupling, a change in the
shape of the synthase releases bound ATP
that has already been formed.
Respirator Inhibitors & Electron Transport
• Respiratory inhibitors can be used to determine
the order of reactions in the electron transport chain
• Isolate intact _____________ from cells
• Provide an oxidizable substrate so that electron
transport can occur
• Add a respiratory inhibitor
• Measure relative amounts of oxidized and reduced
forms of various electron carriers
• Inhibitors have an effect on three sites in the electron
transport chain
The Effect of Respiratory Inhibitors
Sites of Action of Some Respiratory Inhibitors
Shuttle Mechanisms
• Shuttle mechanisms: transport metabolites
between _____________ and _____________
• Glycerol phosphate shuttle:
• We know glycolysis in the _____________ produces
NADH
• NADH does not cross the _____________________
membrane, but glycerol phosphate and
dihydroxyacetone phosphate do
• Through the glycerol phosphate shuttle, ____ ATP are
produced in the mitochondria for each cytosolic NADH
The Glycerol-Phosphate Shuttle
The Malate-Aspartate Shuttle
• The Malate-Aspartate Shuttle:
• Has been found in mammalian kidney, liver, and heart
• Malate crosses the mitochondrial membrane, while
oxaloacetate cannot
• The transfer of electrons from NADH in the cytosol
produces NADH in the mitochondria
• In the malate-aspartate shuttle, _____ mitochondrial
ATP are produced for each cytosolic NADH
The Malate-Aspartate Shuttle
Summary
• Shuttle mechanisms transfer ___________,
but not _____________, from the cytosol
across the mitochondrial membrane
• In the malate-aspartate shuttle, 2.5
molecules of ATP are produced for each
molecule of cytosolic NADH, rather than 1.5
ATP in the glycerol-phosphate shuttle…
• This affects the overall yield of ATP in these
tissues
ATP Yield from Complete Oxidation of Glucose
• In the complete oxidation of glucose, a total of
______ or ______ molecules of ATP are produced
for each molecule of glucose, depending on the
shuttle mechanism
The ATP Yield from Complete Oxidation of Glucose