10/02/22
Mitochondrial Electron Transport and ATP Synthesis
ETC basics (recap):
The ETC is located in the inner mitochondrial membrane.
The electron transfer components are arranged in order from most negative to most
positive standard reduction potential (ε°’).
Energy released by redox reactions within the ETC is used to move protons from the
matrix to the intermembrane space.
The proton gradient is used to drive ATP synthesis.
Flavin mononucleotide (FMN) -> when reduced acts as a hydrogen (proton + electron)
carrier
- Two electrons from NADH flow, initially via a FMN, through complex I
- They then flow one at a time via seven iron-sulphur clusters (Fe-S)
Complex I – NADH-CoQ Reductase
14 central and 26-32 accessory subunits
Ubiquinone redox chemistry induces conformational changes driving proton pumping 4H+
translocated per 2 electrons.
NAD+ -> exclusively a two-electron carrier, it accepts a pair of electrons simultaneously.
Reduction potentials:
Component ε°’ (V)
NADH -0.315
Succinate 0.031
, 10/02/22
Complex II – Succinate-CoQ Reductase
Succinate dehydrogenase -> the enzyme that oxidises a molecule of succinate to fumarate in
the citric acid cycle, is also one of the four subunits of complex II
- Thus, citric acid cycle is physically & functionally linked to the ETC
2 electrons released in conversion of succinate -> fumarate, transferred first to FAD in
succinate dehydrogenase, then to Fe-S clusters, and finally to CoQ.
Reduction potentials:
Component ε°’ (V)
FAD -0.040
CoQ 0.045
Note: both complex I and complex II reduce ubiquinone
Complex III – CoQH2-Cytochrome c Reductase
A CoQH2 generated by complex I or complex II donates 2 electrons to complex III.
Overall reaction:
- QH2 + 2 cyt cox + 2H+matrix → Q + 2 Cyt cred + 4H+cytosol
The Q cycle:
2 electrons from QH2 bound at the Q0 site take different routes:
1. One transferred via Fe-S centre and cytochrome c1 to cytochrome c
2. Other transferred via two cytochrome b molecules to second oxidised Q molecule at
Qi site
2 protons from QH2 are then released to cytosol, and Q at Q0 is replaced by a second QH2
molecule and the cycle repeated.
Mitochondrial Electron Transport and ATP Synthesis
ETC basics (recap):
The ETC is located in the inner mitochondrial membrane.
The electron transfer components are arranged in order from most negative to most
positive standard reduction potential (ε°’).
Energy released by redox reactions within the ETC is used to move protons from the
matrix to the intermembrane space.
The proton gradient is used to drive ATP synthesis.
Flavin mononucleotide (FMN) -> when reduced acts as a hydrogen (proton + electron)
carrier
- Two electrons from NADH flow, initially via a FMN, through complex I
- They then flow one at a time via seven iron-sulphur clusters (Fe-S)
Complex I – NADH-CoQ Reductase
14 central and 26-32 accessory subunits
Ubiquinone redox chemistry induces conformational changes driving proton pumping 4H+
translocated per 2 electrons.
NAD+ -> exclusively a two-electron carrier, it accepts a pair of electrons simultaneously.
Reduction potentials:
Component ε°’ (V)
NADH -0.315
Succinate 0.031
, 10/02/22
Complex II – Succinate-CoQ Reductase
Succinate dehydrogenase -> the enzyme that oxidises a molecule of succinate to fumarate in
the citric acid cycle, is also one of the four subunits of complex II
- Thus, citric acid cycle is physically & functionally linked to the ETC
2 electrons released in conversion of succinate -> fumarate, transferred first to FAD in
succinate dehydrogenase, then to Fe-S clusters, and finally to CoQ.
Reduction potentials:
Component ε°’ (V)
FAD -0.040
CoQ 0.045
Note: both complex I and complex II reduce ubiquinone
Complex III – CoQH2-Cytochrome c Reductase
A CoQH2 generated by complex I or complex II donates 2 electrons to complex III.
Overall reaction:
- QH2 + 2 cyt cox + 2H+matrix → Q + 2 Cyt cred + 4H+cytosol
The Q cycle:
2 electrons from QH2 bound at the Q0 site take different routes:
1. One transferred via Fe-S centre and cytochrome c1 to cytochrome c
2. Other transferred via two cytochrome b molecules to second oxidised Q molecule at
Qi site
2 protons from QH2 are then released to cytosol, and Q at Q0 is replaced by a second QH2
molecule and the cycle repeated.
