Ethylbenzene Dehydrogenase and Related Molybdenum Enzymes Involved in Oxygen-Independent Alkyl Chain Hydroxylation

Abstract

Ethylbenzene dehydrogenase initiates the anaerobic bacterial degradation of ethylbenzene and propylbenzene. Although the enzyme is currently only known from a few closely related denitrifying bacterial strains affiliated to the Rhodocyclaceae, it clearly marks a universally occurring mechanism used for attacking recalcitrant substrates in the absence of oxygen. Ethylbenzene dehydrogenase belongs to subfamily 2 of the DMSO reductase-type molybdenum enzymes together with paralogous enzymes involved in the oxygen-independent hydroxylation of p-cymene, the isoprenoid side chains of sterols and even possibly n-alkanes; the subfamily also extends to dimethylsulfide dehydrogenases, selenite, chlorate and perchlorate reductases and, most significantly, dissimilatory nitrate reductases. The biochemical, spectroscopic and structural properties of the oxygen-independent hydroxylases among these enzymes are summarized and compared. All of them consist of three subunits, contain a molybdenum-bis-molybdopterin guanine dinucleotide cofactor, five Fe-S clusters and a heme b cofactor of unusual ligation, and are localized in the periplasmic space as soluble enzymes. In the case of ethylbenzene dehydrogenase, it has been determined that the heme b cofactor has a rather high redox potential, which may also be inferred for the paralogous hydroxylases. The known structure of ethylbenzene dehydrogenase allowed the calculation of detailed models of the reaction mechanism based on the density function theory as well as QM-MM (quantum mechanics - molecular mechanics) methods, which yield predictions of mechanistic properties such as kinetic isotope effects that appeared consistent with experimental data.

Anaerobic Microbial Degradation of Hydrocarbons: From Enzymatic Reactions to the Environment

“…styrene) (de)carboxylation via 1,3-dipolar cycloaddition [Payne et al, 2015]. In case of 2-methylnaphthalene, fumarate-dependent activation leads to (2-naphthylmethyl)succinyl-CoA, which is converted via a β-oxidation-like reaction sequence to naphthoyl-CoA and succinate, as known 14 from anaerobic toluene degradation [Heider et al, 2016c] ( fig. 5 ).…”

“…1 b) vated to the respective succinate derivatives (see below section 'Stereochemistry of anaerobic activation of n-alkanes and co-metabolic capacities of anaerobic n-alkane degraders'). Recent homology modeling on the basis of the BSS X-ray crystal structure [Funk et al, 2015] Heider et al [2016c]. The soluble heterotrimeric EBDH contains a Mo-bis-MGD cofactor (MoCo) in the α-subunit and is closely related to membrane-anchored nitrate reductase from Escherichia coli (NarGHI) of the DMSO reductase subfamily II.…”

Multi-element isotope fractionation concepts to characterize the biodegradation of hydrocarbons — from enzymes to the environment

“…Under anoxic conditions, three principal hydrocarbon activation steps were identified: carboxylation [13], fumarate addition [14] and hydroxylation [15]. Notably, the low solubility of oxygen in water combined with its rapid consumption usually leads to the quick establishment of anoxic conditions in hydrocarbon-contaminated environments, resulting in the dominance of anaerobic biodegradation reactions in hydrocarbon loaded systems.…”

“…Two anaerobic ethylbenzene activation reactions are known: firstly, benzylic ethylbenzene hydroxylation catalyzed by ethylbenzene dehydrogenase (EBDH) [15] reported for nitrate, iron(III) and manganese(IV) as TEAs [29,31 ]; and finally, fumarate addition to the alkyl side chain reported for sulfate as TEA [32]. In the oxygenindependent hydroxylation reaction, a hydroxyl group stemming from water is inserted [21], a reaction which is linked to strong carbon and tremendous hydrogen isotope fractionation preventing a calculation of isotope fractionation factors and L values using the conventional Rayleigh approach [33 ] as illustrated in Figure 1.…”

Optically Active 1‐Deuterio‐1‐phenylethane – Preparation and Proof of Enantiopurity

“…[9] Furthermore, stereospecific formation of (S)-1phenylethanol by ethylbenzene dehydrogenase has been demonstrated. [11] However, it is unclear whether the reaction proceeds with retention, inversion or racemization at the methylene group of ethylbenzene. [11] However, it is unclear whether the reaction proceeds with retention, inversion or racemization at the methylene group of ethylbenzene.…”