5.2.6 The Role of NAD & FAD

Roles of NAD and FAD in Respiration

1. During Glycolysis and the Krebs Cycle:

• NAD is reduced to NADH by accepting hydrogen atoms during dehydrogenation reactions.
• FAD is reduced to FADH₂ during the Krebs cycle, specifically when succinate is converted to fumarate.

2. In the Electron Transport Chain (ETC):

• NADH donates its hydrogen atoms (H⁺ and e⁻) to Complex I of the ETC:
• Electrons are passed along the chain, releasing energy used to pump protons (H⁺) into the intermembrane space.
• NAD⁺ is regenerated and returns to glycolysis or the Krebs cycle.
• FADH₂ donates its hydrogen atoms to Complex II of the ETC:
• Its electrons enter the chain at a slightly later stage than NADH, so FADH₂ generates less ATP per molecule.

3. Proton Gradient Formation:

• The protons (H⁺) released from NADH and FADH₂ contribute to the formation of a proton gradient across the inner mitochondrial membrane.
• This gradient drives ATP synthesis via the enzyme ATP synthase.

ATP Yield

• Each molecule of NADH contributes to the synthesis of 2.5 ATP molecules via oxidative phosphorylation.
• Each molecule of FADH₂ contributes to the synthesis of 1.5 ATP molecules because it enters the ETC later than NADH.

Key Differences Between NAD and FAD

Feature	NAD	FAD
Site of Reduction	Glycolysis, Link Reaction, Krebs Cycle	Krebs Cycle (succinate → fumarate)
Electron Entry Point	Complex I	Complex II
ATP Yield	2.5 ATP per molecule	1.5 ATP per molecule