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CHAPTER 20
The ElectronTransport Chain
The cheetah, whose capacity for aerobic metabolism
makes it one of the fastest animals
The role of Oxidative Phosphorylation and
Electron-transport in Mitochondria
-
Oxidative phosphorylation is the process by which NADH
and FADH2 (QH2) are oxidized and ATP is formed
-
Glycolysis and citric acid cycle are carried out to produce the reduced
forms of NAD+ and FAD (Q) from oxidation of glucose.
-
The membrane-associated electron transport system 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
-
The electrons from the oxidation NADH and QH2 are passed to
a terminal electron acceptor usually oxygen (O2) to produce water
-
ATP synthase (ATPase) is a key enzyme that used the proton gradient
energy to produce ATP
Figure 20.2
Structure of the
mitochondrion
-
Outer membrane has few proteins. Channels are present that allow
free diffusion of ions and water soluble metabolites.
-
Inner membrane is very rich in protein (protein : lipid ratio of 4:1)
- Permeable to neutral molecules (O2 and CO2)
- Is a barrier to protons and large polar and ionic substances
- Polar substances must be actively transported (pyruvate transferase)
-
Intermembrane space is where the protons are transported during the
membrane-associated electron transport process
-
Matrix contents include the enzymes associated with production of
acetyl-CoA and the citric acid cycle (except succinate dehydrogenase
complex). Protons are removed from the matrix during electron transport
Figure 20.15: The electron transport chain
4 H+ / 2e-
4 H+ / 2e-
2 H+ / 2e-
V
3 H+ / ATP
-
Complexes I, III, and IV pump protons across the inner membrane as electrons are transferred
Mobile coenzymes: ubiquinone (Q) and cytochrome c serve as links between complexes
Complex IV reduces O2 to water
Complex V is ATP synthase, which uses the proton gradient across the membrane to make ATP
Figure 20.15: The electron transport chain
4 H+ / 2e-
V
- NADH donates electrons two at a time to complex I
(NADH-Q reductase complex) of the electron transport chain
- 4 H+ are pumped across the inner mitochondrial membrane
Figure 20.15: The electron transport chain
4 H+ / 2e-
X
4 H+ / 2e-
V
- Complex I donates 2 e- to Ubiquinone (Q), forming QH2
- QH2 donates 2 e- to Complex III (cytochrome c reductase complex)
Mobile electron carrier 1
Ubiquinone (Q)
Q is a lipid soluble molecule that diffuses within
the lipid bilayer of the inner mitochondrial membrane,
accepting electrons from Complex I and Complex II
and passing them to Complex III
Figure 20.15: The electron transport chain
4 H+ / 2e-
4 H+ / 2e-
2 H+ / 2e-
V
-
Complex III donates one e- to cytochrome c
cytochrome c transfers one e- to Complex IV (cytochrome c oxidase complex)
This one e- transfer is repeated to transfer both electrons
Mobile electron carrier 2
Cytochrome c
A protein associated with the outer face of the inner
mitochondrial membrane. Transports electrons from complex
III to complex IV.
Figure 20.15: The electron transport chain
4 H+ / 2e-
4 H+ / 2e-
2 H+ / 2e-
V
A total of 10 H+ are pumped across the inner mitochondrial membrane for every two
electrons donated to Complex I and the electrons transferred to oxygen to make H2O.
Figure 20.15: The electron transport chain
X
V
- FADH2 is bound to Complex II (succinate dehydrogenase)
Complex II. Succinate dehydrogenase
- Transfers electrons from succinate to flavin adenine dinucleotide
(FAD) as a hydride ion (H:-), to an Fe-S complex (one electron
at a time), to ubiquinone (Q), making QH2
- Complex II does not pump protons
Structure of E. Coli succinate dehydrogenase complex
Figure 20.15: The electron transport chain
X
V
- Complex II donates 2 e- to Ubiquinone (Q), forming QH2
- QH2 donates 2 e- to Complex III (cytochrome c reductase complex)
Figure 20.15: The electron transport chain
4 H+ / 2e-
X
-
V
Complex III donates one e- to cytochrome c
cytochrome c transfers one e- to Complex IV (cytochrome c oxidase complex)
This one e- transfer is repeated to transfer both electrons
Figure 20.15: The electron transport chain
4 H+ / 2e-
X
2 H+ / 2e-
V
A total of 6 H+ are pumped across the inner mitochondrial membrane for every two
electrons of FADH2 and the two electrons transferred to oxygen to make H2O.
Electron Transport
involving Complexes I-IV
Figure 20.6
Iron and Copper in metalloenzymes are
important in electron transport
Iron can undergo reversible oxidation and reduction:
- Enzyme heme groups and cytochromes contain iron and are
important in the electron transport process
- Nonheme iron exists in iron-sulfur clusters.
iron is bound by sulfide ions and S- groups from cysteine
(iron-sulfur clusters can accept only one e- in a reaction)
- Copper (Cu) assists in the electron transport in Complex IV
Cu2+
Cu+
Figure 20.7 Iron – sulfur proteins
Iron atoms are complexed with
an equal number of sulfide ions
(S2-) and with thiolate groups
of Cys side chains
Each can undergo reduction-oxidation
reactions
Figure 20.8 Heme Fe(II)-protoporphyrin component of
cytochrome c oxidase
Heme consists of a tetrapyrrole
porphyrin ring system complexed
with iron
Figure 20.15: The electron transport chain
4 H+ / 2e-
4 H+ / 2e-
2 H+ / 2e-
V
3 H+ / ATP
Next: Use of proton gradient for synthesis of ATP by ATP synthase (Complex V).
Assignment
Read Chapter 20
Read Chapter 21
Topics not covered:
Standard Reduction Potentials (Fig 20.1)
Details of the inner workings of the Complexes