Heme Oxygenase–Like Metalloenzymes

Heme oxygenase (HO)-like metalloenzymes are an emerging protein superfamily diverse in reaction outcome and mechanism. Found primarily in bacterial biosynthetic pathways, members conserve a flexible protein scaffold shared with the heme catabolic enzyme, HO, and a set of metal-binding residues. Most HO-like metalloenzymes assemble a diiron cluster, although manganese-iron and mononuclear iron cofactors can also be accommodated. In the canonical HO-like diiron oxygenases/oxidases (HDOs), an Fe₂(II/II) complex reacts with O₂ to form a peroxo-Fe₂(III/III) intermediate (P), common to all HDOs studied to date. The HO-like scaffold confers both distinctive metal-binding properties, allowing for dynamic cofactor assembly and disassembly, and unusual reactivity to its associated metallocofactor. These features may prove to be important in HDO-mediated catalysis of the fragmentation and rearrangement reactions that remain unprecedented among other dinuclear iron enzymes. Much of the sequence space in the HO-like metalloenzyme superfamily remains unexplored, offering exciting opportunities for the discovery of new mechanisms and reactivities.