FAD is reduced to FADH2 in mitochondria during the Krebs cycle, which is also known as citric acid cycle or TCA cycle. FADH2 is a reduced form of FAD (flavin adenine dinucleotide).
The Krebs cycle is a vital part of the aerobic cellular respiration process. Cellular respiration involves oxidation of foods to carbon dioxide and water in a multi-stage process. The potential energy that is released at each step of respiration is stored in the form of chemical bonds in FADH2, NADH2 and eventually ATP (adenosine triphosphate). The Krebs cycle consists of a of series of eight reactions resulting in the formation of FADH2 and NADH, which both serve as fuel for the final step of respiration, electron transport. After the respiration process is complete, oxygen is required to oxidize NADH and FADH2 back to NAD and FAD so they can be reused in future Krebs cycles. The energy released from oxidation of NADH and FADH2 is utilized for formation of ATP, the energy source for cells.