FAD/NAD binding domain in the electron transfer subunit of dihydroorotate dehydrogenase-like proteins. 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 NAD binding. NAD(P) binding domain of ferredoxin reductase-like proteins catalyze electron transfer between an NAD(P)-binding domain of the alpha/beta class and a discrete (usually N-terminal) domain which vary in orientation with respect to the NAD(P) binding domain. The N-terminal domain may contain a flavin prosthetic group, as in flavoenzymes, or use flavin as a substrate. Ferredoxin is reduced in the final stage of photosystem I. The flavoprotein Ferredoxin-NADP+ reductase transfers electrons from reduced ferredoxin to FAD, forming FADH2 via a semiquinone intermediate, and then transfers a hydride ion to convert NADP+ to NADPH.
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