enzymology Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...

, an ethylbenzene hydroxylase () is an

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...

that

catalyzes Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...

the

chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the pos ...

:ethylbenzene + H₂O + acceptor

\rightleftharpoons

(S)-1-phenylethanol + reduced acceptor The 3 substrates of this enzyme are

ethylbenzene Ethylbenzene is an organic compound with the formula . It is a highly flammable, colorless liquid with an odor similar to that of gasoline. This monocyclic aromatic hydrocarbon is important in the petrochemical industry as an reaction intermediat ...

, H₂O, and

acceptor Acceptor may refer to: * Acceptor (accounting), the addressee of a bill of exchange * In the Indian Contract Act of 1872, the acceptor is the person to whom a proposal is made, and who has communicated his or her acceptance of the said proposal ...

, whereas its two products are (S)-1-phenylethanol and

reduced acceptor An electron acceptor is a chemical entity that accepts electrons transferred to it from another compound. It is an oxidizing agent that, by virtue of its accepting electrons, is itself reduced in the process. Electron acceptors are sometimes mista ...

. This enzyme belongs to the family of

oxidoreductase In biochemistry, an oxidoreductase is an enzyme that catalyzes the transfer of electrons from one molecule, the reductant, also called the electron donor, to another, the oxidant, also called the electron acceptor. This group of enzymes usually ut ...

s, specifically those acting on CH or CH₂ groups with other acceptors. The systematic name of this enzyme class is ethylbenzene:acceptor oxidoreductase. Other names in common use include ethylbenzene dehydrogenase, and ethylbenzene:(acceptor) oxidoreductase. This enzyme participates in ethylbenzene degradation by ''Aromatoleum aromaticum'', a denitrifying bacterium related to the genera ''Azoarcus'' and ''Thauera''. It is a molybdenum enzyme belonging to the

DMSO reductase DMSO reductase is a molybdenum-containing enzyme that catalyzes reduction of dimethyl sulfoxide (DMSO) to dimethyl sulfide (DMS). This enzyme serves as the terminal reductase under anaerobic conditions in some bacteria, with DMSO being the ter ...

family. Molybdenum enzymes are distinguished by the presence of a unique active site containing molybdenum atom, one or two molybdopterins and additional ligands (i.e. aminoacid residue of Ser, Cys, SeCys or Asp and very often oxygen Mo=O ligand). EBDH is synthesized exclusively in cells grown anaerobically on ethylbenzene and has been identified as a soluble periplasmic protein.

Structural studies

As of late 2007, only one

structure A structure is an arrangement and organization of interrelated elements in a material object or system, or the object or system so organized. Material structures include man-made objects such as buildings and machines and natural objects such as ...

has been solved for this class of enzymes, with the PDB accession code . EBDH consists of three subunits of 96, 43, and 23 kDa, and contains a molybdenum cofactor and a heme b559 cofactor linked by a linear row of five iron-sulfur clusters. 2ivf

Mechanism

The reaction is catalyzed by the enzyme using a molybdenum cofactor (MoCo), which in the native state consists of a molybdenum (VI) nucleus ligated by two molybdopterin guanine dinucleotide (MGD) ligands and an aspartic acid residue. Two electrons acquired by the cofactor during the reaction, i.e., the hydroxylation of the hydrocarbon, are then transferred via a chain of iron-sulfur clusters connecting the molybdenum with a heme b cofactor in the alpha subunit, from which the electrons are donated to a yet-unknown acceptor. Notably, EBDH exhibits in vitro activity only with artificial electron acceptors of high redox potential, like the ferricenium ion (E0’= +380 mV). This suggests that its natural electron acceptor may be a periplasmic cytochrome c of similarly high potential, which would couple the ethylbenzene oxidation to the nitrate respiration of ''A. aromaticum''. The EBDH catalytic cycle has two parts: i) oxidation part, where substrate is oxidized to alcohols and the enzyme is reduced to its catalytically inactive form, and ii) enzyme re-oxidation part, where EBDH active site (MoCo) is oxidized and restored to its catalytically active form. Recent theoretical and experimental studies point toward radical C-H activation as the initial reaction and rate limiting step. A possible alternative hydride transfer seems to be less likely. The mechanism concludes with conversion of the hydrocarbon to a carbocation intermediate and rebound of a hydroxide to form the hydroxylated product. Moreover, a histidine residue (His192) of the active site seems to be involved in the reaction mechanism.

References

* * * * {{Portal bar, Biology, border=no EC 1.17.99 Enzymes of known structure