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Nitrile hydratases (NHases) have an unusual iron or cobalt catalytic center with two oxidized cysteine ligands, cysteine-sulfinic acid and cysteine-sulfenic acid, catalyzing the hydration of nitriles to amides. Recently, we found that the NHase of Rhodococcus erythropolis N771 exhibited an additional catalytic activity, converting tert-butylisonitrile (tBuNC) to tert-butylamine. Taking advantage of the slow reactivity of tBuNC and the photoreactivity of nitrosylated NHase, we present the first structural evidence for the catalytic mechanism of NHase with time-resolved x-ray crystallography. By monitoring the reaction with attenuated total reflectance-Fourier transform infrared spectroscopy, the product from the isonitrile carbon was identified as a CO molecule. Crystals of nitrosylated inactive NHase were soaked with tBuNC. The catalytic reaction was initiated by photo-induced denitrosylation and stopped by flash cooling. tBuNC was first trapped at the hydrophobic pocket above the iron center and then coordinated to the iron ion at 120 min. At 440 min, the electron density of tBuNC was significantly altered, and a new electron density was observed near the isonitrile carbon as well as the sulfenate oxygen of alphaCys114. These results demonstrate that the substrate was coordinated to the iron and then attacked by a solvent molecule activated by alphaCys114-SOH.