NADH and FADH2 are both examples of coenzymes.
Coenzymes are small, organic substances. Unlike enzymes, coenzymes are not proteins. Some coenzymes are vitamins. Although coenzymes are not catalytic by themselves, they can help enzymes increase the rate of reactions (“co” = together). When bound together, the coenzyme-enzyme complex forms an active enzyme called the holoenzyme. Although coenzymes bind with enzymes, coenzymes are not considered to be the substrate of the enzyme.
NADH and FADH2 are coenzymes that are both present during cellular respiration. NADH and FADH2 are both produced during the Krebs Cycle of cellular respiration. The electrons carried by NADH and FADH2 are then used during the electron transport chain of cellular respiration when ADP (adenosine diphosphate) is phosphorylated into ATP (adenosine triphosphate).