Wikipedia
In biochemistry, flavin adenine dinucleotide (FAD) is a redox cofactor, more specifically a prosthetic group, involved in several important reactions in metabolism. FAD can exist in three (or four: flavin-N(5)-oxide) different redox states, which are the quinone, semiquinone, and hydroquinone. FAD is converted between these states by accepting or donating electrons.
FAD, in its fully oxidized form, or quinone form, accepts two electrons and two protons to become FADH (hydroquinone form). The semiquinone (FADH) can be formed by either reduction of FAD or oxidation of FADH by accepting or donating one electron and one proton, respectively. See the mechanism section below for details.
A flavoprotein is a protein that contains a flavin moiety, this may be in the form of FAD or flavin mononucleotide (FMN). There are many flavoproteins besides components of the succinate dehydrogenase complex, including α-ketoglutarate dehydrogenase and a component of the pyruvate dehydrogenase complex, some examples are shown in section 6.