Acetylene Hydratase from Pelobacter acetylenicus

Acetylene hydratase (, AH) is a bacterial enzyme, originally discovered in the anaerobic microorganism '' Pelobactor acetylenicus'', that catalyzes the non-redox hydration of

acetylene Acetylene (Chemical nomenclature, systematic name: ethyne) is a chemical compound with the formula and structure . It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is u ...

to form

acetaldehyde Acetaldehyde (IUPAC systematic name ethanal) is an organic compound, organic chemical compound with the chemical formula, formula , sometimes abbreviated as . It is a colorless liquid or gas, boiling near room temperature. It is one of the most ...

. :C₂H₂ + H₂O → CH₃CHO The mechanism is thought to involve attachment of acetylene to the metal followed by

nucleophilic attack In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they a ...

of water. Because acetylene binding to the Mo in

nitrogenase Nitrogenases are enzymes () that are produced by certain bacteria, such as cyanobacteria (blue-green bacteria) and rhizobacteria. These enzymes are responsible for the reduction of nitrogen (N2) to ammonia (NH3). Nitrogenases are the only fa ...

lends some support that the mechanism involves a Mo→CH2=CH2 bond. Acetylene inhibits several microbial transformations where it interacts with the active site of the metal-dependent enzymes including

hydrogenase A hydrogenase is an enzyme that Catalysis, catalyses the reversible Redox, oxidation of molecular hydrogen (H2), as shown below: Hydrogen oxidation () is coupled to the reduction of electron acceptors such as oxygen, nitrate, Ferric, ferric i ...

and nitrogenase. This

enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...

relies on

tungsten Tungsten (also called wolfram) is a chemical element; it has symbol W and atomic number 74. It is a metal found naturally on Earth almost exclusively in compounds with other elements. It was identified as a distinct element in 1781 and first ...

as the metal center and is the heaviest metal that plays a prominent part in the

nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...

sulfur Sulfur ( American spelling and the preferred IUPAC name) or sulphur ( Commonwealth spelling) is a chemical element; it has symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms ...

and

carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...

metabolic processes. The Fe-4S

cubane Cubane is a synthetic hydrocarbon compound with the Chemical formula, formula . It consists of eight carbon atoms arranged at the corners of a Cube (geometry), cube, with one hydrogen atom attached to each carbon atom. A solid crystalline substanc ...

keeps the W in the reduced W(IV) state, the most stable reduced oxidation state, while W(VI) is the other stable oxidation state (2nd and 3rd row transition metals are usually most stable in their highest oxidation state). Mo and W enzymes ubiquitously involve W(IV)/W(VI) in the catalysis, however AH is unique since the tungstoenzyme stays as W(IV) in the catalysis. The tungstoenzyme stays as W(IV) throughout the catalysis because the enzyme catalyzes a non-redox reaction described as the hydration of acetylene to acetaldehyde. The

active site In biology and biochemistry, the active site is the region of an enzyme where substrate molecules bind and undergo a chemical reaction. The active site consists of amino acid residues that form temporary bonds with the substrate, the ''binding s ...

tungsten has a distorted octahedral geometry that is coordinated by

molybdopterin Molybdopterins are a class of cofactors found in most molybdenum-containing and all tungsten-containing enzymes. Synonyms for molybdopterin are: MPT and pyranopterin-dithiolate. The nomenclature for this biomolecule can be confusing: Molybdopte ...

co-factors along with a cysteine residue coordinated by a water molecule as the sixth ligand. The active site residues are Asp13, Cys12, Trp179, Arg606, Met140 and Ile142. Asp13 plays an important role in assisting the catalysis where the active site residue deprotonates the water hydroxide making it a better nucleophile.

References

EC 4.2.1 {{4.2-enzyme-stub