Oxidative Phosphorylation Notes

Oxidative Phosphorylation

Oxidative phosphorylation is the process by which most of the cell's ATP is produced.
It involves the oxidation of NADH and FADH2, with oxygen as the final electron acceptor.
The energy released during oxidation is used to phosphorylate ADP to ATP.

Stage 1: Breakdown of foods into simple subunits (amino acids, simple sugars, fatty acids and glycerol) in the cytosol.
Stage 2: Breakdown of simple subunits to acetyl CoA, producing limited amounts of ATP and NADH through glycolysis in the cytosol.
Stage 3: Complete oxidation of acetyl CoA to H2O and CO2 in the mitochondria, producing large amounts of ATP via oxidative phosphorylation.

Oxidative phosphorylation involves coupling an energetically unfavorable reaction (ADP + Pi → ATP) to an energetically favorable reaction (oxidation of NADH).
Overall reaction: NADH + H+ + \frac{1}{2} O2 → NAD+ + H2O; \Delta Go’ = -220 \text{ kJ mol}^{-1}
ADP + Pi → ATP; \Delta Go’ = +30.5 \text{ kJ mol}^{-1}

Chemiosmosis is the process by which the energy from electron transfer is used to pump protons (H+) across a membrane, creating an electrochemical gradient.
This gradient is called the proton motive force (PMF).
The PMF has two components:
- Chemical gradient (\DeltapH)
- Electrical gradient (membrane potential or \DeltaV)
In mitochondria, the PMF is generated across the inner membrane, with the intermembrane space having a lower pH (higher H+ concentration) than the matrix.
ATP synthase uses the energy stored in the PMF to synthesize ATP.

The electron transport chain is a series of membrane protein complexes in the inner mitochondrial membrane.
It couples electron transport to H+ movement, creating the PMF.
NADH and FADH2 from the citric acid cycle are electron donors.
Electrons are passed through the electron transport chain to O2, reducing it to H2O.
H+ are pumped from the matrix into the intermembrane space, generating the PMF.

The electron transport chain consists of 4 membrane protein complexes (Complexes I-IV).
Complex I (NADH dehydrogenase):
- Oxidizes NADH to NAD+.
- Transfers 2 electrons to ubiquinone (Q), forming ubiquinol (QH2).
- Pumps 4 H+ across the membrane per 2 electrons.
- Has a hydrophilic domain (in the matrix) and a membrane domain.
- The hydrophilic domain contains redox cofactors like FMN and Fe-S centers.
- Reduction by 2 electrons causes a conformational change, leading to H+ pumping.
Complex II (Succinate dehydrogenase):
- Oxidizes succinate to fumarate in the citric acid cycle.
- Electrons are passed to FAD, forming FADH2.
- FADH2 passes electrons to ubiquinone via Fe-S centers.
- Reduction of ubiquinone is accompanied by uptake of 2 H+ from the matrix, forming ubiquinol.
Complex III (bc1 complex):
- Passes electrons from ubiquinol (QH2) to cytochrome c (cyt c).
- Pumps 4 H+ across the membrane as 2 electrons are passed.
- Operates by the “Q cycle” mechanism.
Cytochrome c:
- Small water-soluble protein in the intermembrane space.
- Moves electrons from Complex III to Complex IV.
- Contains a haem cofactor.
Complex IV (cytochrome c oxidase):
- Receives electrons from cytochrome c and passes them to O2, reducing it to H2O.
- 4 electrons are needed for each O2 molecule.
- Therefore 2 electrons passed from NADH (or FADH2) reduce \frac{1}{2} O2.
- 2 H+ are pumped across the membrane for every 2 electrons from NADH/FADH2.

Flavin mononucleotide (FMN):
- Structure similar to FAD.
- Reduced by 2 e- + 2 H+.
Haem (heme):
- Fe atom in a porphyrin ring.
- Reduced by 1 e- from Fe3+ to Fe2+.
- Different types (a, b, c) have slightly different structures.
- Proteins containing haem are called cytochromes.
Iron-sulphur (Fe-S) centers:
- Fe bound to several S atoms.
- 1e- reduction from Fe3+ → Fe2+.
- Several different types with different structures.
Copper (Cu) ions:
- 1e- reduction from Cu2+ → Cu+.

Ubiquinone is popular as a dietary supplement due to its role in energy production.
However, there is no real scientific evidence that it improves health or fitness in healthy individuals.