lOMoARcPSD|59658805
Oxidative Phosphorylation
Biochemistry (University of Oxford)
Scan to open on Studocu
Studocu is not sponsored or endorsed by any college or university
Downloaded by olinder seth ()
, lOMoARcPSD|59658805
Describe what is meant by Oxidative Phosphorylation, indicating experimental evidence to
support the chemiosmotic hypothesis. Describe the physiological significance of uncoupling.
Oxidative phosphorylation takes place in the mitochondria. It is the aerobic metabolic pathway
in which ATP is generated, as a result of the transfer of electrons from NADH or FADH2 to O2
by a series of electron carriers. NADH and FADH2 are formed in glycolysis, fatty acid oxidation
and the citric acid cycle. They are energy rich molecules, each containing pair of electrons with
high transfer potential. The flow of electrons from NADH and FADH2 to O2 through protein
complexes on the inner mitochondrial membrane leads to protons being pumped out of the
mitochondrial matrix, and the resulting electrochemical gradient generates a proton-motive
force. Flow of the protons back to the matrix through and enzyme complex generates 26 of the
total 30 possible ATP formed when a molecule of glucose is completely oxidized to CO2 and
H2O.
The notation E0’ is used to give the reduction potential of a reaction couple. This value can be
determined by measuring the electron motive force generated by a half-cell in connection with a
standard reference cell.
The coupled equation X- + H+ → X + 1/2H2 can be written as the two half equations X- → X
+ e- and H+ +e- → 1/2H2
A negative value for E0’ tells us that the reduced form of a substance has a lower affinity for
electrons than H2, since H2 is the standard by which all other substances are compared. (H2 is
defined as having an E0’ value of 0V) Therefore, a strong reducing agent such as NADH has a
negative reduction potential, and will readily donate electrons. In contrast, a strong oxidising
agent such as O2 has a positive reduction potential and is ready to accept electrons.
The following equation can be used to relate change in Gibbs free energy (ΔGº’) to the change
in reduction potential (ΔE0’).
ΔGº’ = -nFΔE0’
Where n=number of electrons transferred
F= proportionality constant (The Faraday) = 23.06kcalmol-1
There is a 1.14V potential difference between NADH and O2, which drives the transport of
electrons through the chain and favours the formation of a proton gradient across the inner
mitochondrial membrane. Therefore, we can say that the driving force of oxidative
phosphorylation is the electron transfer potential of NADH or FADH2 relative to that of O2.
The half equations for this process are:
1/2O2 + 2H+ + 2e- → H2O E0’ = +0.82V
NAD+ + H+ + 2e- → NADH E0’ = -0.32V
Combining these equations gives:
1/2O2 + NADH + H+ → NAD+
We can now calculate the standard free energy for this reaction.
Downloaded by olinder seth ()
, lOMoARcPSD|59658805
ΔGº’ = (-2 x 23.06kcalmol-1mol-1V-1 x +0.82V) – (-2 x 23.06kcalmol-1mol-1V-1 x -0.32V)
ΔGº’ = -37.8kcalmol-1 – 14.8kcalmol-1
ΔGº’ = -52.6kcalmol-1 (or -220.1kJmol-1)
When released, this energy is initially used to generate a proton gradient. We can calculate the
free-energy change for a species moving from one side of a membrane where it is at
concentration c1 to the other side at concentration c2 using the equation:
ΔG = RTln(c2/c1) + 2FΔV = 2.303RTlog10(c2/c1) +2FΔV
Where Z is the electrical charge of the transported species and ΔV is the potential across the
membrane.
So, for the inner mitochondrial membrane:
pH outside is 1.4 units lower than inside. Therefore log10(c2/c1) = 1.4. Z = +1 for protons.
Then ΔG = (2.303 x 1.98x10-3kcalmol-1K-1 x 310K x 1.4) + (1 x 23.06kcalmol-1V-1 x 0.14V)
ΔG = 5.2 kcalmol-1 (21.8kJmol-1)
We can see that the membrane potential corresponds to a free energy of 5.2kcal per mole of
protons transported out of the matrix to the cytosolic side.
The electron carrying groups found in the inner mitochondrial membrane include flavins, iron-
sulfur clusters, quinones, hemes and copper ions. Electrons can only be transferred between
groups that are in contact, and the closer the proximity of groups to each other, the faster the
electrons can be passed along the chain. As a result, the electron transport chain is protein
mediated, whereby transmembrane proteins hold electron carrying groups in the correct
positions. The rate of electron transfer is
also controlled by the driving force of the
free-energy change associated with the
reaction. Every electron-transfer reaction
has an optimal driving force, which
means that the rate of transfer reaches a
maximum, and decreases at very large
driving forces. (See Figure.1) This is
known as the ‘inverted region’.
Figure.1
Downloaded by olinder seth ()
Oxidative Phosphorylation
Biochemistry (University of Oxford)
Scan to open on Studocu
Studocu is not sponsored or endorsed by any college or university
Downloaded by olinder seth ()
, lOMoARcPSD|59658805
Describe what is meant by Oxidative Phosphorylation, indicating experimental evidence to
support the chemiosmotic hypothesis. Describe the physiological significance of uncoupling.
Oxidative phosphorylation takes place in the mitochondria. It is the aerobic metabolic pathway
in which ATP is generated, as a result of the transfer of electrons from NADH or FADH2 to O2
by a series of electron carriers. NADH and FADH2 are formed in glycolysis, fatty acid oxidation
and the citric acid cycle. They are energy rich molecules, each containing pair of electrons with
high transfer potential. The flow of electrons from NADH and FADH2 to O2 through protein
complexes on the inner mitochondrial membrane leads to protons being pumped out of the
mitochondrial matrix, and the resulting electrochemical gradient generates a proton-motive
force. Flow of the protons back to the matrix through and enzyme complex generates 26 of the
total 30 possible ATP formed when a molecule of glucose is completely oxidized to CO2 and
H2O.
The notation E0’ is used to give the reduction potential of a reaction couple. This value can be
determined by measuring the electron motive force generated by a half-cell in connection with a
standard reference cell.
The coupled equation X- + H+ → X + 1/2H2 can be written as the two half equations X- → X
+ e- and H+ +e- → 1/2H2
A negative value for E0’ tells us that the reduced form of a substance has a lower affinity for
electrons than H2, since H2 is the standard by which all other substances are compared. (H2 is
defined as having an E0’ value of 0V) Therefore, a strong reducing agent such as NADH has a
negative reduction potential, and will readily donate electrons. In contrast, a strong oxidising
agent such as O2 has a positive reduction potential and is ready to accept electrons.
The following equation can be used to relate change in Gibbs free energy (ΔGº’) to the change
in reduction potential (ΔE0’).
ΔGº’ = -nFΔE0’
Where n=number of electrons transferred
F= proportionality constant (The Faraday) = 23.06kcalmol-1
There is a 1.14V potential difference between NADH and O2, which drives the transport of
electrons through the chain and favours the formation of a proton gradient across the inner
mitochondrial membrane. Therefore, we can say that the driving force of oxidative
phosphorylation is the electron transfer potential of NADH or FADH2 relative to that of O2.
The half equations for this process are:
1/2O2 + 2H+ + 2e- → H2O E0’ = +0.82V
NAD+ + H+ + 2e- → NADH E0’ = -0.32V
Combining these equations gives:
1/2O2 + NADH + H+ → NAD+
We can now calculate the standard free energy for this reaction.
Downloaded by olinder seth ()
, lOMoARcPSD|59658805
ΔGº’ = (-2 x 23.06kcalmol-1mol-1V-1 x +0.82V) – (-2 x 23.06kcalmol-1mol-1V-1 x -0.32V)
ΔGº’ = -37.8kcalmol-1 – 14.8kcalmol-1
ΔGº’ = -52.6kcalmol-1 (or -220.1kJmol-1)
When released, this energy is initially used to generate a proton gradient. We can calculate the
free-energy change for a species moving from one side of a membrane where it is at
concentration c1 to the other side at concentration c2 using the equation:
ΔG = RTln(c2/c1) + 2FΔV = 2.303RTlog10(c2/c1) +2FΔV
Where Z is the electrical charge of the transported species and ΔV is the potential across the
membrane.
So, for the inner mitochondrial membrane:
pH outside is 1.4 units lower than inside. Therefore log10(c2/c1) = 1.4. Z = +1 for protons.
Then ΔG = (2.303 x 1.98x10-3kcalmol-1K-1 x 310K x 1.4) + (1 x 23.06kcalmol-1V-1 x 0.14V)
ΔG = 5.2 kcalmol-1 (21.8kJmol-1)
We can see that the membrane potential corresponds to a free energy of 5.2kcal per mole of
protons transported out of the matrix to the cytosolic side.
The electron carrying groups found in the inner mitochondrial membrane include flavins, iron-
sulfur clusters, quinones, hemes and copper ions. Electrons can only be transferred between
groups that are in contact, and the closer the proximity of groups to each other, the faster the
electrons can be passed along the chain. As a result, the electron transport chain is protein
mediated, whereby transmembrane proteins hold electron carrying groups in the correct
positions. The rate of electron transfer is
also controlled by the driving force of the
free-energy change associated with the
reaction. Every electron-transfer reaction
has an optimal driving force, which
means that the rate of transfer reaches a
maximum, and decreases at very large
driving forces. (See Figure.1) This is
known as the ‘inverted region’.
Figure.1
Downloaded by olinder seth ()
