Flavins Are Used in Reactions Involving One or Two Electron Transfers

Flavins Are Used in Reactions Involving One or Two Electron Transfers 

Flavin adenine dinucleotide (FAD) (fig. 10.8) and flavin mononucleotide (FMN) are the coenzymatically active forms of vitamin B2, riboflavin. Riboflavin is the NI0-ribityl isoalloxazine portion of FAD, which is enzymatically converted into its coenzymatic forms first by phosphorylation of the ribityl C-5 hydroxy group to FMN and then by ade-nylylation to FAD. FMN and FAD are functionally equivalent coenzymes, and the one that is involved with a given enzyme appears to be a matter of enzymatic binding specificity. 

The catalytically functional portion of the coenzymes is the isoalloxazine ring, specifically N-5 and C-4a (see fig. 10.8 ), which is thought to be the immediate locus of catalytic function, although the entire chromophoric system extending over N-5, C-4a, C-lOa, N-l, and C-2 should be 

Flavin-containing enzymes are known as flavoproteins and, when purified, normally contain their full complements of FAD or FMN. The bright yellow color of flavoproteins is due to the isoalloxazine chromophore in its oxidized form. In a few flavoproteins, the coenzyme is known to be covalently bonded to the protein by means of a sulfhydryl or imidazole group at the C-8 methyl group and in at least one case at C-6. In most flavoproteins, the coenzymes are tightly but noncovalently bound, and many can be resolved into apoenzymes that can be reconstituted to holoenzymes by readdition of FAD or FMN. 

Flavin coenzymes exist in three spectrally distinguishable oxidation states that account in part for their catalytic functions the yellow oxidized form, the red or blue one-electron reduced form, and the colorless two electron re- 

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