Nitrile hydratase

Nitrile hydratase and amidase are the two hydrolytic enzymes responsible for the sequential metabolism of nitriles in some bacteria and fungi which are capable of utilising aliphatic nitriles as the sole source of nitrogen and carbon. Nitrile hydratases (NHases; EC 4.2.1.84) are mononuclear iron or non-corrinoid cobalt enzymes that catalyse the hydration of a large number of diverse nitriles to their corresponding amides in a reaction

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R-C≡N + H₂O → R-C(O)NH₂

(note the direction of the reaction is the opposite of reaction written in EC 4.2.1.84). NHases have been efficiently used for the industrial production of acrylamide from acrylonitrile 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.

Structure

NHases are composed of two types of subunits, α and β, which are not related in amino acid sequence. NHases exist as αβ dimers or α₂β₂ tetramers and bind one metal atom per αβ unit. The 3-D structures of a number of NHases have been determined. The α subunit consists of a long extended N-terminal "arm", containing two α-helices, and a C-terminal domain with an unusual four-layered structure (α-β-β-α). The β subunit consists of a long N-terminal loop that wraps around the α subunit; a helical domain that packs with N-terminal domain of the α subunit; and a C-terminal domain consisting of a β-roll and one short helix. The metal centre is located in the central cavity at the interface between two subunits. All protein ligands to the metal atom are provided by the α subunit. The protein ligands to the iron are the sidechains of the three cysteine (Cys) residues and two mainchain amide nitrogens. The metal ion is octahedrally coordinated, with the protein ligands at the five vertices of an octahedron. The sixth position, accessible to the active site cleft, is occupied either by NO or by a solvent-exchangeable ligand (hydroxide or water). The two Cys residues coordinated to the metal are post-translationally modified to Cys-sulfinic (Cys-SO₂H) and -sulfenic (Cys-SOH) acids.