 Two dioxygenases, 1H-3-hydroxy-4-oxo-quinoline 2,4-dioxygenase (QDO) and 1H-3-hydroxy-4-oxoquinaldine 2,4-dioxygenase (HDO), have been shown to require neither an organic or metal cofactor.These enzymes catalyze the degradation of quinolone heterocycles in a manner similar to quercetin dioxygenase, but are thought to mediate a radical reaction of a dioxygen molecule with a carbanion on the substrate (figure 5).Both HDO and QDO belong to the α/β hydrolase superfamily of enzymes, although the cataclytic residues in HDO and QDO do not seem to serve the same function as they do in the rest of the enzymes in the α/β hydrolase superfamily.