FAD

FAD (flavin adenine dinucleotide) is a redox coenzyme that can accept electrons and protons. In its oxidized state (often notated just as FAD or sometimes FAD-), it can accept two electrons and two protons to become FADH- (its reduced form). FAD is a cofactor for several dehydrogenase enzymes; for example, it is bound to succinate dehydrogenase in the citric acid cycle. FAD is derived from riboflavin (vitamin B2).

• When FAD accepts electrons, it does so by taking on two hydrogens (H-), becoming FADH-. This typically happens in enzyme active sites. A key difference from NAD-: FAD is often tightly bound to its enzyme (a prosthetic group), whereas NAD- is usually a free-floating coenzyme.
• In metabolic pathways, whenever you see an enzyme that produces FADH-, that enzyme has FAD as a cofactor in its oxidized form. For instance, succinate dehydrogenase uses FAD to oxidize succinate to fumarate, reducing FAD to FADH- in the process. Later, FADH- is oxidized back to FAD as it passes electrons into the electron transport chain (at CoQ/Complex II).
• Because FAD is linked to vitamin B2, dietary riboflavin is required to maintain adequate FAD/FMN levels. This means riboflavin deficiency could impair any biochemical pathway requiring FAD. Fortunately, riboflavin is common in foods, and severe deficiency is uncommon.

• Enzyme cofactor questions: A question might ask something like 'Which coenzyme is required by succinate dehydrogenase-' The answer is FAD (which gets reduced to FADH-). Similarly, in questions about fatty acid oxidation, the first step (by acyl-CoA dehydrogenase) uses FAD as a cofactor, producing FADH-.
• NAD vs FAD: Sometimes exams compare these two coenzymes. Key points: NAD- is typically free in solution and picks up one proton (H-) when reduced, whereas FAD is often enzyme-bound and picks up two protons. NADH is produced in many steps (glycolysis, Krebs, etc.), while FADH- is produced in fewer steps (like one step in Krebs cycle). Recognize that both are vital electron carriers but have slightly different roles and yields (as discussed, FADH- yields less ATP).
• Vitamin B2 connection: If you see a question about a patient with ariboflavinosis (riboflavin deficiency) having issues in energy metabolism, connect it to FAD/FMN. The question might describe symptoms and ask which coenzyme is directly affected; the answer could be FAD (and FMN) because they come from riboflavin.