External Flavoprotein Dioxygenases

These are pyridine nucleotide-linked flavoprotein complexes that require ferrous ions for activity and which catalyze the double hydroxy-lation of aromatic nuclei in a highly stereoselective manner (190). Often the resulting diol undergoes dehydrogenation to the corresponding o-diphenol by the oxidized pyridine nucleotide (eq. 42) (2). Occasionally, the diol decomposes due to the inherent instability created by substituents. 

The most studied members of this class are listed below. 

I Ferredoxin )( Ferredoxin ]( Iron-sulfur 1 ( V Reductase / J Protein J  

There are two additional pyridine nucleotide-requiring flavo-enzymes for which the metal ion requirements are unknown 212) (see 9 and 10 below). Both cleave the pyridine nucleus at the bond adjacent to the hydroxyl substituent. However, the similarity with phenol-cleaving dioxygenases is fortuitous. The reaction course derives from the inherent instability of the intermediate cw-diols rather than from a similar catalytic mechanism. Two different mechanisms have been advanced. The first, which is based on kinetic studies 213, 214), consists of reduction of the substrate to its anion followed by its oxygenation (eq. 51). The 

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The external flavoprotein dioxygenases function by catalyzing the transfer of electrons from reduced nicotinamides to the aromatic nuclei of the substrate. The resulting radical anions would be sufficiently soft to react with molecular oxygen without requiring any particular assistance from the enzymes. 

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