Biotransformation of Hexabromocyclododecanes (HBCDs) with LinB—An HCH-Converting Bacterial Enzyme

Abstract

Hexabromocyclododecanes (HBCDs) and hexachlorocyclohexanes (HCHs) are polyhalogenated hydrocarbons with similar stereochemistry. Both classes of compounds are considered biologically persistent and bioaccumulating pollutants. In 2009, the major HCH stereoisomers came under regulation of the Stockholm convention. Despite their persistence, HCHs are susceptible to bacterial biotransformations. Here we show that LinB, an HCH-converting haloalkane dehalogenase from Sphingobium indicum B90A, is also able to transform HBCDs. Racemic mixtures of α-, β-, and γ-HBCDs were exposed to LinB under various conditions. All stereoisomers were converted, but (-)α-, (+)β-, and (+)γ-HBCDs were transformed faster by LinB than their enantiomers. The enantiomeric excess increased to 8 ± 4%, 27 ± 1%, and 20 ± 2% in 32 h comparable to values of 7.1%, 27.0%, and 22.9% as obtained from respective kinetic models. Initially formed pentabromocyclododecanols (PBCDOHs) were further transformed to tetrabromocyclododecadiols (TBCDDOHs). At least, seven mono- and five dihydroxylated products were distinguished by LC-MS so far. The widespread occurrence of HCHs has led to the evolution of bacterial degradation pathways for such compounds. It remains to be shown if LinB-catalyzed HBCD transformations in vitro can also be observed in vivo, for example, in contaminated soils or in other words if such HBCD biotransformations are important environmental processes.

Advances in enantioselective analysis of chiral brominated flame retardants. Current status, limitations and future perspectives

“…With the respect of BFRs, most of the enantioselective biodegradation studies were focused on HBCDs (Heeb et al, 2014;Heeb et al, 2015;Heeb et al, 2012b;Heeb et al, 2013) which has already been added in 2009, in the Stockholm Convention for Persistent Organic Pollutants, because of its harmful effect on environment and wildlife, and due to its bioaccumulation potential, process that can be also enantioselective (Eljarrat et al, 2009;Guerra et al, 2008). HBCDs are produced industrially by bromination of cyclododeca-1,5,9-triene (CDT), creating six stereo centers at positions 1, 2, 5, 6, 9, and 10 of the formed reaction products.…”

“…Like many other halogenated chiral pollutants, HCBCDs follow different degradation pathways in anaerobic conditions comparing with aerobic pathways and the enantioselectivity of the reactions which depends on transporter-driven cell uptake or on enzymatic reactions. (Heeb et al, 2014;Heeb et al, 2015;Heeb et al, 2012b;Heeb et al, 2013) investigated extensively the aerobic metabolism of HBCDs (Fig. 5).…”

“…Firstly, (Heeb et al, 2012a) suggested that pentabromocyclododecanols (PBCDOHs) are potential environmental transformation products of hexabromocyclododecanes (HBCDs), and the enantioselective separation of 8 diastereoisomers namely α-, β-, γ-, δ-, ε-, ζ-, η-, and θ-PBCDOHs were separated with a combination of normal (SiO 2 , F60, 230-400 mesh) and reversed-phase LC (C18-RP, 125 mm × 4 mm, 5 μm, 100 Å, Nucleosil 100-5, Macherey-Nagel, Oensingen, Switzerland), followed by of enantiomeric separation a permethylated-β-cyclodextrin chiral-phase column (PM-β-CP). In the next study, (Heeb et al, 2012b) purified the HCH-converting haloalkane dehalogenases LinB, from Sphigobium indicum B90A and used this enzyme to investigate the enantioselective degradation of HBCD, similarly to the aerobic transformation of α-HCH. It was found that among the enantiomers, (−) α-, (+) β-, and (+) γ-HBCD are preferentially transformed by LinB, while at least seven PBCDOHs and five TBCDDOHs were tentatively identified by LC-MS-MS (triple quadrupole mass analyzer).…”

“…The recorded enantiomeric enrichment of HBCD (calculated as enantiomeric excess (EE) (Patterson and Schnell, 2014;Shi et al, 2006)) was 8 ± 4% for (+) α-, 27 ± 1% for (−) β-, and 20 ± 2% for (−) γ-HBCD respectively in 32 h comparable to values of 7.1%, 27.0%, and 22.9% as obtained from respective kinetic models. Since this preliminary study (Heeb et al, 2012b) highlighted the fastest transformations of (+) β-and (+) γ-HBCDs under given conditions, in the next study (Heeb et al, 2013) focused on LinB-transformation of achiral δ-HBCDs into chiral metabolites, namely to two pentabromocyclododecanols (PBCDOHs) and two tetrabromocyclododecadiols (TBCDDOHs). In these reactions only the stereocenter from C6 is converted to an alcohol with inversion from S-to R-configuration in a nucleophilic, S N 2-like substitution reaction, while the other five stereocenters of δ-HBCD remained unchanged.…”

Microbial Degradation of Some Halogenated Compounds: Biochemical and Molecular Features

“…Heeb et al purified the HCH-converting haloalkane dehalogenase LinB, from Sphigobium indicum B90A and applied the enzyme for HBCD degradation. LinB transforms HBCD into pentabromocyclododecanols (PBCDOHs) and further tetrabromocyclododecadiols (TBCDDOHs) [59]. Whether LinB or other haloalkane dehalogenase are the de novo HBCD degradation enzyme is unknown.…”