FAD-FAD is the abbreviation for flavin adenine dinucleotide. FAD is a coenzyme that functions as a prosthetic (helper) group in a number of enzymes, such as xanthine oxidase. FAD acts as an electron carrier in the electron transport chain in the mitonchondrion of cells. The mitochondrion are organelles (small organs in cells), which serve as energy "factories" and thus, FAD is an important component of this organelle. FAD is composed of riboflavin 5'-phosphate (FMN) and adenosine 4'-phosphate. See riboflavin.