Anaerobic sulfite reductase contains an FAD and NADPH binding module with structural similarity to ferredoxin reductase and sequence similarity to dihydroorotate dehydrogenases. Clostridium pasteurianum inducible dissimilatory type sulfite reductase is linked to ferredoxin and reduces NH2OH and SeO3 at a lesser rate than it's normal substate SO3(2-). Dihydroorotate dehydrogenases (DHODs) catalyze the only redox reaction in pyrimidine de novo biosynthesis. They catalyze the oxidation of (S)-dihydroorotate to orotate coupled with the reduction of NAD+.

FAD/NAD binding domain in the electron transfer subunit of dihydroorotate dehydrogenase. Dihydroorotate dehydrogenases (DHODs) catalyze the only redox reaction in pyrimidine de novo biosynthesis. They catalyze the oxidation of (S)-dihydroorotate to orotate coupled with the reduction of NAD+. In L. lactis, DHOD B (encoded by pyrDa) is co-expressed with pyrK and both gene products are required for full activity, as well as 3 cofactors: FMN, FAD, and an [2Fe-2S] cluster.

The oxygenase reductase FAD/NADH binding domain acts as part of the multi-component bacterial oxygenases which oxidize hydrocarbons using oxygen as the oxidant. Electron transfer is from NADH via FAD (in the oxygenase reductase) and an [2FE-2S] ferredoxin center (fused to the FAD/NADH domain and/or discrete) to the oxygenase. Dioxygenases add both atoms of oxygen to the substrate, while mono-oxygenases (aka mixed oxygenases) add one atom to the substrate and one atom to water. In dioxygenases, Class I enzymes are 2 component, containing a reductase with Rieske type [2Fe-2S] redox centers and an oxygenase. Class II are 3 component, having discrete flavin and ferredoxin proteins and an oxygenase. Class III have 2 [2Fe-2S] centers, one fused to the flavin domain and the other separate.

Nucleotide-diphospho-sugar transferase. Proteins in this family have been been predicted to be nucleotide-diphospho-sugar transferases.

Ferredoxin reductase (FNR), an FAD and NAD(P) binding protein, was intially identified as a chloroplast reductase activity, catalyzing the electron transfer from reduced iron-sulfur protein ferredoxin to NADP+ as the final step in the electron transport mechanism of photosystem I. FNR transfers electrons from reduced ferredoxin to FAD (forming FADH2 via a semiquinone intermediate) and then transfers a hydride ion to convert NADP+ to NADPH. FNR has since been shown to utilize a variety of electron acceptors and donors and has a variety of physiological functions including nitrogen assimilation, dinitrogen fixation, steroid hydroxylation, fatty acid metabolism, oxygenase activity, and methane assimilation in many organisms. FNR has an NAD(P)-binding sub-domain of the alpha/beta class and a discrete (usually N-terminal) flavin sub-domain which vary in orientation with respect to the NAD(P) binding domain. The N-terminal moeity may contain a flavin prosthetic group (as in flavoenzymes) or use flavin as a substrate. Because flavins such as FAD can exist in oxidized, semiquinone (one- electron reduced), or fully reduced hydroquinone forms, FNR can interact with one and 2 electron carriers. FNR has a strong preference for NADP(H) vs NAD(H).