Electron Transport Chain and Oxidative Phosphorylation

Electron Transport Chain

Steps in the Electron Transport Chain

1. Electrons and protons carried by NADH + H⁺ are transferred to FMN, reducing it.
2. FMN is oxidized, transferring its electrons to Coenzyme Q (CoQ), which is reduced. This allows FMN to accept more electrons and continue the chain.
3. CoQ is oxidized and passes its electrons to the next acceptor, a cytochrome. Cytochromes are dehydrogenases.
4. Cytochromes transport protons into the mitochondrial matrix. The chain continues with the electrons.
5. Cytochromes are iron-sulfur molecules. The iron is oxidized (ferric) or reduced (ferrous) Fe. Each iron atom carries one electron, so the process occurs twice.
6. Cytochromes following CoQ in the chain are Cyt b, Cyt c, and Cyt a3.
7. Electrons reach the end of the chain, where oxygen binds with protons from the matrix to form water. Molecular oxygen is essential as the final electron acceptor, allowing the chain to continue.
8. FAD can enter the chain at a lower level, at CoQ.

Oxidative Phosphorylation

Oxidative phosphorylation is the process where ATP is synthesized from ADP and inorganic phosphate (P_i) using energy from electrons moving to lower energy levels in the electron transport chain.

• For every intramitochondrial NADH, 9-10 protons are generated (yielding 3 ATP).
• Each extramitochondrial NADH (e.g., from glycolysis) and each FADH₂ generate 6 protons (yielding 2 ATP).

Chemiosmotic Coupling

Chemiosmotic coupling is a mechanism that relies on a proton gradient across the mitochondrial membrane.

• Electron acceptors form three enzyme complexes that act as proton pumps. They use energy from electron transfer to move protons from the matrix to the intermembrane space.
• For every intramitochondrial NADH, 9-10 protons are generated (yielding 3 ATP).
• Each extramitochondrial NADH (e.g., from glycolysis) and each FADH₂ generate 6 protons (yielding 2 ATP).
• The impermeable membrane prevents protons from returning, creating a gradient. This gradient forces protons through F-particles.
• F-particles (F₀, anchored to the membrane, and F₁, extending into the matrix) form a channel for proton movement.
• As protons pass through F-particles, ATP synthesis from ADP and P_i is catalyzed.