FADH2 (${\mathrm{FADH}}_2$)

FADH2 is the two-electron-reduced form of flavin adenine dinucleotide. The notation summarizes reduction of the flavin portion, while the surrounding enzyme controls how electrons and protons are transferred.

In succinate dehydrogenase, reduction and reoxidation occur with the flavin bound to the enzyme. Thus FADH2 should not always be pictured as a freely diffusing molecule that carries electrons between distant enzymes.