Flavin mononucleotide oxidation states

The oxidation state of thiazolines and oxazolines can be adjusted by additional tailoring enzymes. For instance, oxidation domains (Ox) composed of approximately 250 amino acids utilize the cofactor FMN (flavin mononucleotide) to form aromatic oxazoles and thiazoles from oxazolines and thiazolines, respectively. Such domains are likely utilized in the biosynthesis of the disorazoles, " diazonimides, bleomycin, and epothiolone. The typical domain organization for a synthetase containing an oxidation domain is Cy-A-PCP-Ox however, in myxothiazol biosynthesis one oxidation domain is incorporated into an A domain. Alternatively, NRPSs can utilize NAD(P)H reductase domains to convert thiazolines and oxazolines into thiazolidines and oxazolidines, respectively. For instance, PchC is a reductase domain from the pyochelin biosynthetic pathway that acts in trans to reduce a thiazolyinyl-Y-PCP-bound intermediate to the corresponding thiazolidynyl-Y-PCP. ... 

Figure 18.12. Oxidation States of Flavins. The reduction of flavin mononucleotide (FMN) to FMNH2 proceeds through a semiquinone intermediate.

Flavins (El) catalyze many different bioreactions of physiological importance [7-9]. Riboflavin, flavin mononucleotide (FMN), and flavin adenine dinucleotide (FAD) have the 7,8-dimethyl isoalloxazine ring in common but differ in the side chain attached to NIO. With their five redox states, fully oxidized, one-electron reduced semiquinoid (F1H and F1 ), and fully reduced hydroquinone (FIH2 and F1H ), flavins are involved in one-electron and two-electron transfer reactions [10]. 

Flavin catalysis may involve three oxidation levels of the ring, and each of these may exist as a cation, anion or neutral compound, giving a total of 9 forms. The oxidation levels are flavoquinone (oxidized) flavosemiquinone (half-reduced) flavohydroqui-none (reduced). Some catalytic mechanisms involve only one electron, i.e. the flavoquinone is converted to the semiquinone, or the semiquinone is converted to the hydroquinone. Other mechanisms involve two electrons, and the flavin shuttles back and forth between the quinone and the hydroquinone states. FAD is biosynthesized from flavin mononucleotide by the action of FAD pyrophosphatase (EC 3.6.1.8) FMN + ATPseFAD + PP,. 

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