Nicotinamide adenine (F) dinucleotide (NAD+)
Figure 2 Structural relationship of vitamin-coenzyme for (A) thiamine (B^, (C) riboflavin (B2), and (E) niacin. In the left column is the structure of the vitamin, in the right column the coenzyme derived from the vitamin. Note the pyrophosphate group in the coenzyme derived from thiamine (B) and the prevalence of phosphate and adenyl groups in the coenzymes derived from riboflavin (D) and niacin (F), showing the necessity for ATP in their synthesis.
enzymes. The flavin cofactor is bound covalently to the structure preventing disengagement during purification procedures.
Reactions Flavin enzymes are designed to remove (and add) electrons to and from substrates. In general, flavin coenzymes are stronger oxidizing agents than the pyrimidine coenzymes (NAD+, NADP+) and tend to participate in more complex reactions. Also, flavin coenzymes can accept single electrons from a donor, forming a semiquinone and allowing flavoproteins to take part in reactions that form free radicals. Having a single electron also allows favins to bind molecular oxygen as a hydroperoxyl complex.
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