Publications

• The active site sulfenic acid ligand in nitrile hydratases can function as a nucleophile.

  Martinez S., Wu R., Sanishvili R., Liu D., Holz R.

  Nitrile hydratase (NHase) catalyzes the hydration of nitriles to their corresponding commercially valuable amides at ambient temperatures and physiological pH. Several reaction mechanisms have been proposed for NHase enzymes; however, the source of the nucleophile remains a mystery. Boronic acids ... >> More

  Nitrile hydratase (NHase) catalyzes the hydration of nitriles to their corresponding commercially valuable amides at ambient temperatures and physiological pH. Several reaction mechanisms have been proposed for NHase enzymes; however, the source of the nucleophile remains a mystery. Boronic acids have been shown to be potent inhibitors of numerous hydrolytic enzymes due to the open shell of boron, which allows it to expand from a trigonal planar (sp(2)) form to a tetrahedral form (sp(3)). Therefore, we examined the inhibition of the Co-type NHase from Pseudonocardia thermophila JCM 3095 (PtNHase) by boronic acids via kinetics and X-ray crystallography. Both 1-butaneboronic acid (BuBA) and phenylboronic acid (PBA) function as potent competitive inhibitors of PtNHase. X-ray crystal structures for BuBA and PBA complexed to PtNHase were solved and refined at 1.5, 1.6, and 1.2 Å resolution. The resulting PtNHase-boronic acid complexes represent a "snapshot" of reaction intermediates and implicate the cysteine-sulfenic acid ligand as the catalytic nucleophile, a heretofore unknown role for the αCys(113)-OH sulfenic acid ligand. Based on these data, a new mechanism of action for the hydration of nitriles by NHase is presented. << Less

  J Am Chem Soc 136:1186-1189(2014) [PubMed] [EuropePMC]

• Full reaction mechanism of nitrile hydratase: a cyclic intermediate and an unexpected disulfide switch.

  Hopmann K.H.

  The full reaction mechanism of nitrile hydratase has remained elusive, despite extensive theoretical and experimental studies. A novel reaction mechanism for nitrile hydratase is proposed here, with remarkable features and very feasible barriers. Our results, obtained on the basis of large quantum ... >> More

  The full reaction mechanism of nitrile hydratase has remained elusive, despite extensive theoretical and experimental studies. A novel reaction mechanism for nitrile hydratase is proposed here, with remarkable features and very feasible barriers. Our results, obtained on the basis of large quantum-mechanical active site models, identify Cys-SO(-) as the nucleophile, performing a direct nucleophilic attack on the metal-coordinated nitrile. This implies the formation of an intriguing cyclic intermediate, which subsequently is cleaved through attack of the axial cysteine on the sulfenate, thereby forming a disulfide bond. In this mechanism, nitrile hydration occurs without directly involving a water molecule. Subsequent water-mediated disulfide cleavage regenerates the active site. This is the first example of a disulfide switch directly implicated in an enzymatic reaction mechanism. << Less

  Inorg Chem 53:2760-2762(2014) [PubMed] [EuropePMC]

• Thermal equilibrium of two conformations in photosensitive nitrile hydratase probed by the FTIR band of nitric oxide bound to the non-heme iron center.

  Suzuki H., Nojiri M., Kamiya N., Noguchi T.

  Nitrile hydratase (NHase) from Rhodococcus N-771 is a novel enzyme that is inactive in the dark due to an enodogenous nitric oxide (NO) molecule bound to the non-heme iron center, and is activated by its photodissociation. FTIR spectra in the NO stretching region of the dark-inactive NHase were re ... >> More

  Nitrile hydratase (NHase) from Rhodococcus N-771 is a novel enzyme that is inactive in the dark due to an enodogenous nitric oxide (NO) molecule bound to the non-heme iron center, and is activated by its photodissociation. FTIR spectra in the NO stretching region of the dark-inactive NHase were recorded in the temperature range of 270-80 K. Two NO peaks were observed at 1854 and 1846 cm-1 at 270 K, and both frequencies upshifted as the temperature was lowered, retaining the peak separation of 8-9 cm-1. The relative intensity of the lower-frequency peak increased with decreasing temperature up to ~120 K, whereas it was mostly unchanged below this temperature. This observation indicates that two distinct conformations with slightly different NO structures are thermally equilibrated in the dark-inactive NHase above ~120 K, and the interconversion is frozen-in at lower temperatures. The intensity ratio of the NO bands changed gradually upon increasing the pH from 5.5 to 11.0, but no specific pKa value was found. This result, together with the comparison of the light-induced FTIR difference spectra measured at pH 6.5 and 9.0, suggests that the protonation/deprotonation of a specific amino acid group in the active site of NHase is not a direct cause of the occurrence of the two conformations, although several protonatable groups in the protein may influence the energetics of the two conformers. From the previous observation that the isolated alpha subunit of NHase exhibited a single broad NO peak, it is suggested that interaction of the beta subunit forming the reactive cavity is essential for the double-minimum potential of the active-site structure. The frequencies and widths of the two NO bands changed upon addition of propionamide, 1,4-dioxane, and cyclohexyl isocyanide, indicating that these compounds are bound to the active pocket and change the interactions of the iron center or the dielectric environments around the NO molecule. Thus, the NO bands of NHase can also be a useful probe to monitor the binding of substrates and their analogues to the active pocket. << Less

  J Biochem 136:115-121(2004) [PubMed] [EuropePMC]