Reduction is made with the addition of hydrogen atoms to specific nitrogen atoms on the isoalloxazine ring system:

The electron acceptor - the isoalloxazine ring - of FMN is identical to that of FAD.

Flavin (from Latin flavus, "yellow") is the common name for a group of organic compounds based on pteridine, formed by the tricyclic heteronuclear organic ring isoalloxazine.

Sarcosine dehydrogenase contains a covalently bound FAD group " linked via the 8 alpha position of the isoalloxazine ring to an imidazole N(3) of a histidine residue".

Reduction of isoalloxazine ring (FAD, FMN oxidized form) yields the reduced forms of the flavoproteins (FMNH2 and FADH2).