New inhibitors of iron-containing nitrile hydratases

There is growing evidence in the literature emphasizing the significance of the post-translational modification of cysteine thiols to sulfenic acids (SOH), which have been found in a number of proteins. Crystallographic and mass spectrometric evidence has shown the presence of this group in an inactive form of the industrially important enzyme nitrile hydratase (NHase). This oxidized cysteine is unique in that it forms part of the coordination sphere of the low-spin iron III at the active site of the enzyme. The presence of this unstable sulfenic group in the active form of NHase is the subject of some controversy. To try to detect this function in NHase, we have studied the inhibitory effect on nitrile hydration of reagents known to react with sulfenic acids. Two NHases were studied, namely, Rhodococcus rhodochrous R312 NHase and Comamonas testosteroni NI1 NHase, and the reagents used were meta-chlorocarbonyldicyano-phenylhydrazone (m-ClCP), 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl), and 2-nitro-5-thiocyanato-benzoic acid (NTBA). Following this approach we report three novel inhibitors of NHases. In addition, we report thiocyanate reagents that can be used to monitor NHase activity spectroscopically.