A new regime of heme-dependent aromatic oxygenase superfamily

Two histidine-ligated heme-dependent monooxygenase proteins, TyrH and SfmD, have recently been found to resemble enzymes from the dioxygenase superfamily currently named after tryptophan 2,3-dioxygenase (TDO), that is, the TDO superfamily. These latest findings prompted us to revisit the structure and function of the superfamily. The enzymes in this superfamily share a similar core architecture and a histidine-ligated heme. Their primary functions are to promote O-atom transfer to an aromatic metabolite. TDO and indoleamine 2,3-dioxygenase (IDO), the founding members, promote dioxygenation through a two-step monooxygenation pathway. However, the new members of the superfamily, including PrnB, SfmD, TyrH, and MarE, expand its boundaries and mediate monooxygenation on a broader set of aromatic substrates. We found that the enlarged superfamily contains eight clades of proteins. Overall, this protein group is a more sizeable, structure-based, histidine-ligated heme-dependent, and functionally diverse superfamily for aromatics oxidation. The concept of TDO superfamily or heme-dependent dioxygenase superfamily is no longer appropriate for defining this growing superfamily. Hence, there is a pressing need to redefine it as a heme-dependent aromatic oxygenase (HDAO) superfamily. The revised concept puts HDAO in the context of thiol-ligated heme-based enzymes alongside cytochrome P450 and peroxygenase. It will update what we understand about the choice of heme axial ligand. Hemoproteins may not be as stringent about the type of axial ligand for oxygenation, although thiolate-ligated hemes (P450s and peroxygenases) more frequently catalyze oxygenation reactions. Histidine-ligated hemes found in HDAO enzymes can likewise mediate oxygenation when confronted with a proper substrate.