Batie et al. [Chemistry and Biochemistry of Flavoenzymes, 3, 543-556(1991)] proposed a classification system for ring-hydroxylating oxygenases in which the oxygenases are grouped into three classes in terms of the number of constituent components and the nature of th redox centers. But in recent years, many ring-hydroxylating oxygenases have been newly identified and characterized, and found dificult to classify into these three classes. Typical examples are carbazole 1,9a-dioxygenase and 2-oxo-1,2-dihydroquinoline 8-monooxygenase, which have been classified into class III and class IB, respectively, from biochemical characteristics. However, a phylogenetic study showed that the terminal oxygenases of both are closely related to class IA. Because this discrepancy derived from counting all the components together, here we proposed a new scheme based on the homology of the amino acid sequences of the α subunits of the terminal oxygenase components. This new scheme strongly reflects the actual phylogenetic affiation of the terminal oxygenase component. By comparing their sequences pairwise using the CLUSTAL W program, 54 oxygenase components were classified into 4 groups (groups I, II, III, and IV). While group I contains broad-range oxygenases sharing low homology, groups II, III, and IV contain some typical oxygenase: benzoate/toluate dioxygenases for group II, naphthalene/polycyclic aromatic hydrocarbon dioxygenases for group III, and benzene/toluene/biphenyl dioxygenases for group IV. Our new scheme is simple and powerful, since an oxygenase component can be nearly automatically grouped when the DNA sequence is available, and it fits very well with the phylogenetic affiiation.

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