Structure and assembly of the diiron cofactor in the heme-oxygenase–like domain of the <i>N</i> -nitrosourea–producing enzyme SznF

Abstract

Significance The enzyme SznF assembles the N -nitrosourea pharmacophore of the drug streptozotocin. Its N -oxygenase domain resembles heme-oxygenase (HO) and belongs to an emerging superfamily of HO-like diiron enzymes (HDOs) with unstable metallocofactors that have resisted structural characterization. Insight into cofactor dynamics from our prior investigation of SznF’s N -oxygenation reactions suggested an approach that has yielded a structure of a functionally assigned HDO with its diiron cofactor intact. Conformational changes accompanying cofactor (dis)assembly explain its instability, and the observation of an unanticipated glutamate ligand that is conserved in only a subset of HDO sequences provides a potential basis for top-level assignment of enzymatic function. Our results thus provide a roadmap for structural and functional characterization of novel HDOs.