Flavin Adenine Dinucleotide, or FAD, is a coenzyme found in all living cells. It plays a crucial role in various biological reactions, particularly in the metabolism of carbohydrates and fats. FAD acts as an electron carrier, helping to transfer energy during cellular respiration, which is essential for producing ATP, the energy currency of the cell. FAD is derived from the vitamin riboflavin (vitamin B2) and consists of a riboflavin molecule linked to an adenine nucleotide. It exists in two forms: the oxidized form, FAD, and the reduced form, FADH2. The conversion between these forms is vital for many enzymatic reactions, including those in the Krebs cycle.