4 - ATP Synthesis of ET/ OxPhos

Which complex is responsible for ATP synthesis in the ETC/OxPhos pathway?

Complex V, also known as ATP Synthase or the F₀F₁ complex, is responsible for ATP synthesis.

How is the proton gradient utilized to synthesize ATP?

The electrochemical proton gradient drives protons through the F₀ channel of Complex V. This movement rotates the γ subunit, causing conformational changes in the β subunits of the F₁ portion that catalyze the formation of ATP from ADP and Pi.

What is the structure of Complex V (ATP Synthase)?

• F₀ portion

• F₁ portion

• The γ subunit

F₀ portion

Embedded in the inner mitochondrial membrane, composed of subunits a, b (2), and c (12); forms the proton channel.

F₁ portion

Projects into the matrix, composed of α (3), β (3), γ, δ, and ε subunits.

γ subunit

Acts as a rotating cam that forces β subunits to adopt conformations: Loose (binds ADP + Pi), Tight (synthesizes ATP), and Open (releases ATP).

What are uncouplers, and how do they elucidate the mechanism of ATP synthesis?

Uncouplers (e.g., 2,4-Dinitrophenol) are lipid-soluble weak acids that dissipate the proton gradient by allowing protons to flow back into the matrix independently of Complex V.

• They uncouple electron transport from ATP synthesis: ETC continues, but no ATP is produced.

• This supports Mitchell's Chemiosmotic Theory.

What is the Chemiosmotic Theory?

Proposed by Peter Mitchell, it states that ATP synthesis is driven by a proton-motive force across the inner mitochondrial membrane. Proton flow through ATP synthase induces conformational changes that catalyze ATP formation.

What is the mechanism of ATP synthesis in Complex V?

• Proton flow rotates the c-ring and γ subunit.

• Rotation of γ forces β subunits through three states:

  • Loose (βL): Binds ADP + Pi

  • Tight (βT): Forms ATP

  • Open (βO): Releases ATP

• One full γ rotation → 3 ATP synthesized.

How is oxidative phosphorylation different from substrate-level phosphorylation?

• Oxidative phosphorylation: ATP is produced indirectly using energy from the ETC-generated proton gradient (e.g., via ATP synthase).

• Substrate-level phosphorylation: ATP is formed directly via transfer of a phosphate group from a high-energy substrate to ADP (e.g., in glycolysis or the TCA cycle).