Chapter 14 - Electron Transport and
Oxidative Phosphorylation
• The cheetah, whose capacity for aerobic
metabolism makes it one of the fastest animals
Oxidative Phosphorylation in Mitochondria
• Oxidative phosphorylation is the process by which NADH and
FADH2 are oxidized and ATP is formed
• NADH and FADH2 are reduced coenzymes from the oxidation of
glucose by glycolysis and the citric acid cycle
The Respiratory Electron-transport Chain (ETC) is a series of
enzyme complexes embedded in the inner mitochondrial
membrane, which oxidize NADH and QH2. Oxidation energy
is used to transport protons across the inner mitochondrial
membrane, creating a proton gradient
ATP synthase is an enzyme that uses the proton gradient energy
to produce ATP
Mitochondria are energy centers of a cell
Cytosol
Mitochondria
Fig 14.2
Fig 14.6
Structure of the
mitochondrion
• Final stages of aerobic oxidation of biomolecules in eukaryotes
occur in the mitochondrion
• Site of citric acid cycle and fatty acid oxidation which generate
reduced coenzymes
• Contains electron transport chain to oxidize reduced coenzymes
Overview of oxidative phosphorylation
Fig 14.1
Electron Flow in Oxidative Phosphorylation
• Five oligomeric assemblies of proteins associated with oxidative
phosphorylation are found in the inner mitochondrial membrane
• Complexes I-IV contain multiple cofactors, and are involved in
electron transport
• Electrons flow through complexes I-IV
• Complexes I, III and IV pump protons across the inner
mitochondrial membrane as electrons are transferred
• Mobile coenzymes: ubiquinone (Q) and cytochrome c serve as
links between electron transport complexes
• Complex IV reduces O2 to water
• Complex V is ATP synthase, which uses the generated proton
gradient across the membrane to make ATP
Cofactors in Electron Transport
• NADH donates electrons two at a time to complex I
of the electron transport chain
• Flavin coenzymes are then reduced
(Complex I) FMN
FMNH2
(Complex II) FAD
FADH2
• FMNH2 and FADH2 donate one electron at a time to
ubiquinone (U or coenzyme Q)
• All subsequent steps in electron transport proceed
by one electron transfers
Mobile electron carriers
1. Ubiquinone (Q)
Q is a lipid soluble molecule that diffuses within
the lipid bilayer, accepting electrons from
Complex I and Complex II and passing them to
Complex III.
2. Cytochrome c
Associated with the outer face of the inner
mitochondrial membrane. Transports electrons
from Complex III to Complex IV.
Iron in metalloenzymes
• Iron undergoes reversible oxidation and reduction:
Fe3+ + e- (reduced substrate)
Fe2+ + (oxidized substrate)
• Enzyme heme groups and cytochromes contain iron
• Nonheme iron exists in iron-sulfur clusters (iron is
bound by sulfide ions and S- groups from cysteines)
• Iron-sulfur clusters can accept only one e- in a reaction
Iron-sulfur clusters
• Iron atoms are complexed with an
equal number of sulfide ions (S2-) and
with thiolate groups of Cys side chains
• Heme consists of a
tetrapyrrole Porphyrin ring
system complexed with
iron
Heme Fe(II)-protoporphyrin IX
Complex I. NADH-ubiquinone oxidoreductase
• Transfers two electrons from NADH as a hydride ion (H:-) to
flavin mononucleotide (FMN), to iron-sulfur complexes, to
ubiquinone (Q), making QH2
• About 4 protons (H+) are translocated across the inner
mitochondrial membrane per 2 electrons transferred
Fig 14.9
Complex II. Succinate-ubiquinone oxidoreductase
• Also known as succinate dehydrogenase complex
• Transfers electrons from succinate to flavin adenine dinucleotide
(FAD) as a hydride ion (H:-), to an iron-sulfur complex, to
ubiquinone (Q), making QH2
• Complex II does not pump protons
Fig 14.11
Complex III. Ubiquinol-cytochrome c oxidoreductase
• Transfers electrons from QH2 to cytochrome c, mediated by ironsulfur and other cytochromes
• Electron transfer from
QH2 is accompanied
by the translocation
of 4 H+ across the
inner mitochondrial
membrane
Fig 14.14
Complex IV. Cytochrome c oxidase
• Uses four-electrons from the soluble electron carrier cytochrome
c to reduce oxygen (O2) to water (H2O)
• Uses Iron atoms (hemes of cytochrome a) and copper atoms
• Pumps two protons (H+) across the inner mitochondrial
membrane per pair of electrons, or four H+ for each O2 reduced
O2 + 4 e- + 4H+
Fig 14.19
2 H2O
Complex V: ATP Synthase
• F0F1 ATP Synthase uses the proton gradient energy for the
synthesis of ATP
• Composed of a
“knob-and-stalk”
structure
• F1 (knob) contains
the catalytic subunits
• F0 (stalk) has a
proton channel which
spans the membrane.
• Estimated passage of
3 protons (H+) per
ATP synthesized
Knob-and-stalk structure of ATP synthase
Prentice Hall c2002
Chapter 14
19
Mechanism of ATP Synthase
• There are 3 active sites, one in each b subunit
• Passage of protons through the Fo channel causes the c-e-g unit
to rotate inside the a3b3 hexamer, opening and closing the bsubunits, which make ATP
Prentice Hall c2002
Chapter 14
20
Fig 14.20 Transport of ATP, ADP and Pi
across the inner mitochondrial membrane
• Adenine nucleotide translocase: unidirectional
exchange of ATP for ADP (antiport)
• Symport of Pi and H+ is electroneutral
The P:O Ratio
molecules of ADP phosphorylated
P:O ratio = ----------------------------------------atoms of oxygen reduced
• Translocation of 3H+ required by ATP synthase for
each ATP produced
• 1 H+ needed for transport of Pi, ADP and ATP
• Net: 4 H+ transported for each ATP synthesized
Calculation of the P:O ratio
Complex I
III
IV
#H+ translocated/2e4
4
2
Since 4 H+ are required for each ATP synthesized:
For NADH: 10 H+ translocated / O (2e-)
P/O = (10 H+/ 4 H+) = 2.5 ATP/O
For succinate (FADH2) substrate = 6 H+/ O (2e-)
P/O = (6 H+/ 4 H+) = 1.5 ATP/O
Regulation of Oxidative Phosphorylation
• Overall rate of oxidative phosphorylation
depends upon substrate availability and cellular
energy demand
• Important substrates: NADH, O2, ADP
• In eukaryotes intramitochondrial ratio ATP/ADP
is a secondary control mechanism
• High ratio inhibits oxidative phosphorylation as
ATP binds to a subunit of Complex IV