Download Electron Transport Chain

23.2 Electron Transport and ATP
The enzymes and
electron carriers for
electron transport are
located along the
inner membrane of
the mitochondria.
Learning Goal Describe the transfer of hydrogen ions and
electrons in electron transport and the process of oxidative
phosphorylation in ATP synthesis.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Electron Transport
The reduced coenzymes NADH and FADH2 produced from
glycolysis, oxidation of pyruvate, and the citric acid cycle are
oxidized to provide the energy for the synthesis of ATP.
In electron transport or the respiratory chain,
• hydrogen ions and electrons from NADH and FADH2 are
passed from one electron acceptor or carrier to the next
until they combine with oxygen to form H2O.
• energy released during electron transport is used to
synthesize ATP from ADP and Pi during oxidative
phosphorylation.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Glycolysis, Citric Acid Cycle Results
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Electron Transport System
In the electron transport system,
• there are five protein
complexes, which are numbered
I, II, III, IV, and V.
• two electron carriers, coenzyme
Q and cytochrome c, attached
to the inner membrane of the
mitochondrion, carry electrons
between these protein
complexes bound to the inner
membrane.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Electron Transport Chain
In electron transport, the oxidation of NADH and FADH2
provides hydrogen ions and electrons that eventually
react with oxygen to form water.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Complex I
In complex I,
• electron transport begins when hydrogen ions and
electrons are transferred from NADH to complex I.
• loss of hydrogen from NADH regenerates NAD+ to
oxidize more substrates in oxidative pathways such
as the citric acid cycle.
• hydrogen ions and electrons are transferred to the
mobile electron carrier CoQ, forming CoQH2.
• CoQH2 carries electrons from complexes I and II to
complex III.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Complex I, Electron Transfer
During electron transfer,
• H+ ions are pumped through complex I into the
intermembrane space, producing a reservoir of H+
(hydrogen ion gradient).
• for every two electrons that pass from NADH to CoQ,
4H+ are pumped across the mitochondrial membrane,
producing a charge separation on opposite sides of
the membrane.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Complex II
Complex II consists of the enzyme succinate
dehydrogenase from the citric acid cycle.
In complex II,
• CoQ obtains hydrogen and electrons directly from
FADH2. This produces CoQH2 and regenerates the
oxidized coenzyme FAD, which becomes available to
oxidize more substrates.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Complex III
Complex II consists of the enzyme succinate
dehydrogenase from the citric acid cycle.
In complex II,
• CoQ obtains hydrogen and electrons directly from
FADH2 and becomes CoQH2.
• two electrons are transferred from the mobile carrier
CoQH2 to a series of iron-containing proteins called
cytochromes.
• electrons are then transferred to two cytochrome c,
which can move between complexes III and IV.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Complex III, Cytochrome c
Cytochrome c
• contains Fe3+/Fe2+, which is
reduced to Fe2+ and oxidized
to Fe3+.
• generates energy from electron
transfer to pump 4H+ from the
matrix into the intermembrane
space, increasing the hydrogen
ion gradient.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Complex IV
At complex IV,
• four electrons from four cytochrome c are passed to
other electron carriers.
• electrons combine with hydrogen ions and oxygen
(O2) to form two molecules of water.
• energy is used to pump H+ from the mitochondrial
matrix into the intermembrane space, further
increasing the hydrogen ion gradient.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Oxidative Phosphorylation
Energy is coupled with the production of ATP in a process
called oxidative phosphorylation. In 1978, Peter
Mitchell theorized about a chemiosmotic model, which
• links the energy from electron transport to a hydrogen
ion gradient that drives the synthesis of ATP.
• allows complexes I, III, and IV to act as hydrogen ion
pumps, producing a hydrogen ion gradient.
• equalizes pH and electrical charge between the matrix
and intermembrane space that occurs when H+ must
return to the matrix.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Oxidative Phosphorylation, ATP
In the chemiosmotic model,
• H+ cannot move through the inner membrane but returns to
the matrix by passing through a fifth protein complex in the
inner membrane called ATP synthase (also called complex V).
• the flow of H+ from the intermembrane space through the ATP
synthase generates energy that is used to synthesize ATP
from ADP and Pi.
This process of oxidative phosphorylation couples the energy
from electron transport to the synthesis of ATP.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Electron Transport and ATP Synthesis
• When NADH enters electron transport at complex I, the
energy transferred can be used to synthesize 2.5 ATP.
• When FADH2 enters electron transport at complex II, it
provides energy for the synthesis of 1.5 ATP.
• Current research indicates that the oxidation of one
NADH yields 2.5 ATP and one FADH2 yields 1.5 ATP.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Regulation of Electron Transport and
Oxidative Phosphorylation
Electron transport
• is regulated by the availability of ADP, Pi, oxygen (O2),
and NADH.
• decreases with low levels of any of these compounds and
decreases the formation of ATP.
When a cell is active and ATP is consumed rapidly, the
elevated levels of ADP will activate the synthesis of ATP.
The activity of electron transport is strongly dependent on
the availability of ADP for ATP synthesis.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Study Check
Match each with its function:
CoQ
cyt c
A. a mobile carrier between complexes II and III
B. carries electrons from complexes I and II to
complex III
C. accepts H and electrons from FADH2
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Solution
Match each with its function:
CoQ
cyt c
A. a mobile carrier between complexes II and III
B. carries electrons from complexes I and II to
complex III
C. accepts H and electrons from FADH2
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
Cyt C
CoQ
CoQ
© 2016 Pearson Education, Inc.
Study Check
Classify each as a product of the
1. CO2
A. citric acid cycle
2. FADH2
A. citric acid cycle
3. NAD+
A. citric acid cycle
4. NADH
A. citric acid cycle
5. H2O
A. citric acid cycle
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
B. electron transport chain
B. electron transport chain
B. electron transport chain
B. electron transport chain
B. electron transport chain
© 2016 Pearson Education, Inc.
Solution
Classify each as a product of the
1.
2.
3.
4.
5.
CO2
FADH2
NAD+
NADH
H2O
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
A. citric acid cycle
A. citric acid cycle
B. electron transport chain
A. citric acid cycle
B. electron transport chain
© 2016 Pearson Education, Inc.