Insight into the Maturation Process of the Nitrile Hydratase Active Site

The metal binding motif of all nitrile hydratases (NHases, EC 4.2.1.84) is highly conserved (CXXCSCX) in the α-subunit. Accordingly, an eight amino acid peptide (VCTLCSCY), based on the metal binding motif of the Co-type NHase from Pseudonocardia thermophilia (PtNHase), was synthesized and shown to coordinate Fe(II) under anaerobic conditions. Parallel-mode EPR data on the mononuclear Fe(II)-peptide complex confirmed an integer-spin signal at g' ∼ 9, indicating an S = 2 system with unusually small axial ZFS, D = 0.29 cm^-1 Exposure to air yielded a transient high-spin EPR signal most consistent with an intermediate/admixed S = ³/₂ spin state, while the integer-spin signal was extinguished. Prolonged exposure to air resulted in the observation of EPR signals at g = 2.04, 2.16, and 2.20, consistent with the formation of a low-spin Fe(III)-peptide complex with electronic and structural similarity to the NHase from Rhodococcus equi TG328-2 (ReNHase). Coupled with MS data, these data support a progression for iron oxidation in NHases that proceeds from a reduced high spin to an oxidized high spin followed by formation of an oxidized low-spin iron center, something that heretofore has not been observed.