Thiooxazole Formation on a Nontypeable Haemophilus influenzae Virulence Factor Requires a Mixed-Valent Diiron...

The multinuclear nonheme iron-dependent oxidative enzyme (MNIO) family employs a multi-iron cofactor to catalyze a range of post-translational modifications (PTMs) in the biosynthesis of ribosomally synthesized, post-translationally modified peptide (RiPP) natural products. While significant progress has been made toward understanding the range of chemical transformations performed by MNIOs, the nature of the iron cofactor has only been investigated in one instance. Here, we examine the MNIO involved in oxazolin biosynthesis to gain further insight into the metallocofactors employed by this impressive family of enzymes. Oxazolin, a RiPP virulence factor from nontypeable Haemophilus influenzae , contains six copper-binding 5-thiooxazole groups installed by the MNIO HvfB. Weak interactions between HvfB and its required partner protein, HvfC, motivated genetic fusion of the two proteins, which yielded an effective mimic of the protein complex with high enzymatic activity. While HvfB binds up to three iron ions, concerted EPR, ENDOR, and Mössbauer spectroscopic characterization of the active protein reveals that accumulation of a mixed-valent diiron(II/III) cluster correlates with 5-thiooxazole product formation. This oxidation state is attained only in the presence of HvfC, revealing a new role for the partner protein in modulating the iron cofactor. Site-directed mutagenesis of metal-coordinating residues was used to probe the function of the third iron-binding site. This work clarifies the nature of the active iron cofactor for oxazolin maturation, providing a second example of a mixed-valent diiron oxidase in RiPP biosynthesis.