Identification of an Intermediate Species along the Nitrile Hydratase Reaction Pathway by EPR Spectroscopy

A new method to trap catalytic intermediate species was employed with Fe-type nitrile hydratase from Rhodococcus equi TG328-2 (ReNHase). ReNHase was incubated with substrates in a 23% (w/w) NaCl/H2O eutectic system that remained liquid at -20 degrees C, thereby permitting the observation of transient species that were present at electron paramagnetic resonance (EPR)-detectable levels in samples frozen while in the steady state. Fe-III-EPR signals from the resting enzyme were unaffected by the presence of 23% NaCl, and the catalytic activity was similar to 55% that in the absence of NaCl at the optimum pH of 7.5. The reaction of ReNHase in the eutectic system at -20 degrees C with the substrates acetonitrile or benzonitrile induced significant changes in the EPR spectra. A previously unobserved signal with highly rhombic g-values (g(1) = 2.31) was observed during the steady state but did not persist beyond the exhaustion of the substrate, indicating that it arises from a catalytically competent intermediate. Distinct signals due to product complexes provide a detailed mechanism for product release, the rate-limiting step of the reaction. Assignment of the observed EPR signals was facilitated by density functional theory calculations, which provided candidate structures and g-values for various proposed ReNHase intermediates. Collectively, these results provide new insights into the catalytic mechanism of NHase and offer a new approach for isolating and characterizing EPR-active intermediates in metalloenzymes.

Automated summary

A new method was employed to trap catalytic intermediate species in Fe-type nitrile hydratase ReNHase. The reaction induced significant changes in EPR spectra, with a previously unobserved signal indicating a catalytically competent intermediate. Density functional theory calculations facilitated the assignment of observed EPR signals, providing new insights into the catalytic mechanism of NHase.