In enzymology, nitrile hydratases (NHases; EC 4.2.1.84) are mononuclear iron or non-corrinoid cobalt enzymes that catalyse the hydration of diverse nitriles to their corresponding amides

R-C≡N + H₂O → R-C(O)NH₂

In biochemistry, cobalt is in general found in a corrin ring, such as in vitamin B₁₂. Nitrile hydratase is one of the rare enzyme types that use cobalt in a non-corrinoid manner. The mechanism by which the cobalt is transported to NHase without causing toxicity is unclear, although a cobalt permease has been identified, which transports cobalt across the cell membrane. The identity of the metal in the active site of a nitrile hydratase can be predicted by analysis of the sequence data of the alpha subunit in the region where the metal is bound. The presence of the amino acid sequence VCTLC indicates a Co-centred NHase and the presence of VCSLC indicates Fe-centred NHase.

Nitrile hydratase and amidase are two hydrating and hydrolytic enzymes responsible for the sequential metabolism of nitriles in bacteria that are capable of utilising nitriles as their sole source of nitrogen and carbon, and in concert act as an alternative to nitrilase activity, which performs nitrile hydrolysis without formation of an intermediate primary amide. A sequence in genome of the choanoflagellate Monosiga brevicollis was suggested to encode for a nitrile hydratase. The M. brevicollis gene consisted of both the alpha and beta subunits fused into a single gene. Similar nitrile hydratase genes consisting of a fusion of the beta and alpha subunits have since been identified in several eukaryotic supergroups, suggesting that such nitrile hydratases were present in the last common ancestor of all eukaryotes.

NHases have been efficiently used for the industrial production of acrylamide from acrylonitrile on a scale of 600 000 tons per annum, and for removal of nitriles from wastewater. Photosensitive NHases intrinsically possess nitric oxide (NO) bound to the iron centre, and its photodissociation activates the enzyme. Nicotinamide is produced industrially by the hydrolysis of 3-cyanopyridine catalysed by the nitrile hydratase from Rhodococcus rhodochrous J1, producing 3500 tons per annum of nicotinamide for use in animal feed.

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