Nitrile hydratase
Encyclopedia
In enzymology, nitrile hydratases (NHases; ) are mononuclear iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

 or non-corrinoid cobalt
Cobalt
Cobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....

 enzymes that catalyse the hydration of diverse nitriles to their corresponding amides

R-C≡N + H2O → R-C(O)NH2

Metal cofactor

In biochemistry, cobalt
Cobalt
Cobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....

 is in general found in a corrin
Corrin
Corrin is an heterocyclic compound. It is the parent macrocycle related to substituted derivative that is found in vitamin B12. Its name reflects that it is the "core" of vitamin B12 .-Coordination chemistry:...

 ring, such as in vitamin B12
Vitamin B12
Vitamin B12, vitamin B12 or vitamin B-12, also called cobalamin, is a water-soluble vitamin with a key role in the normal functioning of the brain and nervous system, and for the formation of blood. It is one of the eight B vitamins...

. 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
Permease
The permeases are membrane transport proteins, a class of multipass transmembrane proteins that facilitate the diffusion of a specific molecule in or out of the cell by passive transport...

 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.

Metabolic pathway

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
Nitrilase
Nitrilase enzymes catalyse the hydrolysis of nitriles to carboxylic acids and ammonia, without the formation of "free" amide intermediates. Nitrilases are involved in natural product biosynthesis and post translational modifications in plants, animals, fungi and certain prokaryotes. Nitrilases can...

 activity, which performs nitrile hydrolysis without formation of an intermediate primary amide. A sequence in genome of the eukaryote Monosiga brevicollis that has been suggested to encode for a nitrile hydratase has been identified.. This is the first hint that nitrile hydratase activity might also occur outside of prokaryotic organisms.

Industrial applications

NHases have been efficiently used for the industrial production of acrylamide
Acrylamide
Acrylamide is a chemical compound with the chemical formula C3H5NO. Its IUPAC name is prop-2-enamide. It is a white odourless crystalline solid, soluble in water, ethanol, ether, and chloroform. Acrylamide is incompatible with acids, bases, oxidizing agents, iron, and iron salts...

 from acrylonitrile
Acrylonitrile
Acrylonitrile is the chemical compound with the formula C3H3N. This pungent-smelling colorless liquid often appears yellow due to impurities. It is an important monomer for the manufacture of useful plastics. In terms of its molecular structure, it consists of a vinyl group linked to a nitrile...

 and for removal of nitriles from wastewater. Photosensitive NHases intrinsically possess nitric oxide
Nitric oxide
Nitric oxide, also known as nitrogen monoxide, is a diatomic molecule with chemical formula NO. It is a free radical and is an important intermediate in the chemical industry...

 (NO) bound to the iron centre, and its photodissociation
Photodissociation
Photodissociation, photolysis, or photodecomposition is a chemical reaction in which a chemical compound is broken down by photons. It is defined as the interaction of one or more photons with one target molecule....

 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.

Enzymatic mechanism

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
Cysteine
Cysteine is an α-amino acid with the chemical formula HO2CCHCH2SH. It is a non-essential amino acid, which means that it is biosynthesized in humans. Its codons are UGU and UGC. The side chain on cysteine is thiol, which is polar and thus cysteine is usually classified as a hydrophilic amino acid...

 (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
Sulfenic acid
A sulfenic acid is an organosulfur compound and oxoacid with the general formula RSOH, where R ≠ H. Simple sulfenic acids, such as methanesulfenic acid, CH3SOH, are highly reactive and cannot be isolated in solution. In the gas phase the lifetime of methanesulfenic acid is about one minute...

(Cys-SOH) acids.
The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
x
OK