• Prosthetic group features
• Nitrile hydratase reaction
• Active site residues
• NHase in enzyme databases
• NHase in alignment databases
• NHase in 3­D databases
• References

Mononuclear iron centre		Iron ligands	Formal iron oxidation/spin states
Activated	Resting
(SCys)3(N)2OH	(SCys)3(N)2NO	3 × SCys; 2 × N; OH¯ or ·NO	FeIII (S=1/2)

Nitrile hydratase and amidase are the two hydrolytic enzymes responsible for the sequential metabolism of nitrile compounds in some bacteria and fungi which are capable of utilising aliphatic nitriles as the sole source of nitrogen and carbon [1-3]. Nitrile hydratases (NHases; EC 4.2.1.84) are mononuclear iron or (non­corrinoid) cobalt enzymes that catalyse the hydration of a large number of diverse nitriles to their corresponding amides:

+

H2O

(1)

The 3­D structures of photoactivated NHase from Rhodococcus sp. R312 [5] and nitrosylated NHase from Rhodococcus sp. N­771 [6] have been determined. The enzyme exists as an ß dimer. The subunit consists of a long extended N­terminal `arm' (residues 10-52), containing two ­helices, and a C­terminal domain with an unusual four­layered structure (­ß­ß­). The ß subunit consists of a long 30­residue N­terminal loop that wraps around the subunit; a helical domain (residues 30-112) that packs with N­terminal domain of the subunit; and a C­terminal domain consisting of a ß­roll and one short helix.

The metal centre is located in the central cavity at the interface between two subunits. All protein ligands to the iron are provided by the subunit. The protein ligands to the iron are the sidechains of the three Cys residues and two mainchain amide nitrogens. The low­spin Fe^III 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) [7]. In Rhodococcus sp. N­771 NHase, two Cys residues coordinated to the iron were found to be post­translationally modified to Cys-sulphinic (Cys-SO₂H) and -sulphenic (Cys-SOH) acids. Together with oxygen of the Ser residue, these modifications induced a `claw' setting of oxygen atoms capturing an NO molecule [6]. A role for the iron centre in catalysis remains unclear. Mechanistic proposals were made which all suggest that the metal ion acts as a Lewis acid [5]. The table below lists the mononuclear iron environment residues in known 3­D structures.

Enzyme	Mononuclear iron environment residues ( subunit)				Fe sixth ligand	PDB code	Ref.
Rhodococcus sp. R312	Cys­110 (S)	Cys­113 (S, N)	Ser­114 (N)	Cys­115 (S)	OH¯	1ahj	[5]
Rhodococcus sp. N­771	Cys­109 (S)	Cys-SO2H­112 (S, N)	Ser­113 (N)	Cys-SOH­114 (S)	·NO	-	[6]

NHase in enzyme databases

ENZYME	LIGAND	BRENDA	UMBBD	Official name	Alternative names
4.2.1.84	4.2.1.84	4.2.1.84	e0067	Nitrile hydratase	Acrylonitrile hydratase; NHase; nitrilase

NHase in alignment databases

Protein Family	Pfam	LPFC 3­D alignment
20343; nitrile hydratase chain	-	-
80462; nitrile hydratase ß chain	-	-

PDB	scop	BSM	RELI Base	Header	¹
1ahj	-	1ahj	1ahj	Nitrile hydratase; Rhodococcus sp. R312	-

¹ Macromolecular Structures abstract. Full text is available to BioMedNet Members

References

1. Yamada, H. and Kobayashi, M. (1996) Nitrile hydratase and its application to industrial production of acrylamide. Biosci. Biotechnol. Biochem. 60, 1391-1400.
2. Nawaz, M.S., Heinze, T.M. and Cerniglia, C.E. (1992) Metabolism of benzonitrile and butyronitrile by Klebsiella pneumoniae. Appl. Environ. Microbiol. 58, 27-31.
3. Linardi, V.R., Dias, J.C. and Rosa, C.A. (1996) Utilization of acetonitrile and other aliphatic nitriles by a Candida famata strain. FEMS Microbiol. Lett. 144, 67-71.
4. Wyatt, J.M. and Knowles, C.J. (1995) The development of a novel strategy for the microbial treatment of acrylonitrile effluents. Biodegradation 6, 93-107.
5. Huang, W., Jia, J., Cummings, J., Nelson, M.J., Schneider, G. and Lindqvist, Y. (1997) Crystal structure of nitrile hydratase reveals a novel iron centre in a novel fold. Structure 5, 691-699.
6. Nagashima, S., Nakasako, M., Dohmae, N., Tsujimura, M., Takio, K., Odaka, M., Yohda, M., Kamiya, N. and Endo, I. (1998) Novel non­heme iron center of nitrile hydratase with a claw setting of oxygen atoms. Nature Struct. Biol. 5, 347-352.
7. Scarrow, R.C., Brennan, B.A., Cummings, J.G., Jin, H., Duong, D.J., Kindt, J.T. and Nelson, M.J. (1996) X­ray spectroscopy of nitrile hydratase at pH 7 and 9. Biochemistry 35, 10078-10088.

Bibliography on structural studies of nitrile hydratase

Reviews on nitrile hydratase