title: Stereochemistry of enzymatic transformations of (+)β- and (-)β-HBCD with LinA2--a HCH-degrading bacterial enzyme of Sphingobium indicum B90A.
abstract: LinA2, a bacterial enzyme expressed in various Sphingomonadaceae, catalyzes the elimination of HCl from hexachlorocyclohexanes (HCHs) and, as discussed here, the release of HBr from certain hexabromocyclododecanes (HBCDs). Both classes of compounds are persistent organic pollutants now regulated under the Stockholm Convention. LinA2 selectively catalyzes the transformation of β-HBCDs; other stereoisomers like α-, γ-, and δ-HBCDs are not converted. The transformation of (-)β-HBCD is considerably faster than that of its enantiomer. Here, we present the XRD crystal structure of 1E,5S,6S,9R,10S-pentabromocyclododecene (PBCDE) and demonstrate that its enantiomer with the 1E,5R,6R,9S,10R-configuration is the only metabolite formed during LinA2-catalyzed dehydrobromination of (-)β-HBCD. Formation of this product can be rationalized by HBr elimination at C5 and C6. A reasonable enzyme-substrate complex with the catalytic dyad His-73 and Asp-25 approaching the hydrogen at C6 and a cationic pocket of Lys-20, Try-42 and Arg-129 binding the leaving bromine at C5 was found from in silico docking experiments. A second PBCDE of yet unknown configuration was obtained from (+)β-HBCD. We predicted its stereochemistry to be 1E,5S,6S,9S,10R-PBCDE from docking experiments. The enzyme-substrate complex obtained from LinA2 and an activated conformation of (+)β-HBCD allows the HBr elimination at C9 and C10 leading to the predicted product. Both modeled enzyme-substrate complexes are in line with 1,2-diaxial HBr eliminations. In conclusion, LinA2, a bacterial enzyme of the HCH-degrading strain Sphingobium indicum B90A was able to stereoselectively convert β-HBCDs. Configurations of both PBCDE metabolites were predicted by molecular docking experiments and confirmed in one case by XRD data.
authors.affiliation: Empa, Swiss Federal Laboratories for Materials Testing and Research, Laboratory for Analytical Chemistry, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland. Electronic address: norbert.heeb@empa.ch.
authors.forename: Norbert V
authors.initials: NV
authors.lastname: Heeb
authors.affiliation: Empa, Swiss Federal Laboratories for Materials Testing and Research, Laboratory for Analytical Chemistry, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland; ZHAW, Zurich University of Applied Sciences, Institute of Chemistry and Biological Chemistry, Reidbach, CH-8820 Wädenswil, Switzerland.
authors.forename: Simon A
authors.initials: SA
authors.lastname: Wyss
authors.affiliation: Eawag, Swiss Federal Institute of Aquatic Science and Technology, Überlandstrasse 133, CH-8600 Dübendorf, Switzerland.
authors.forename: Birgit
authors.initials: B
authors.lastname: Geueke
authors.affiliation: Eawag, Swiss Federal Institute of Aquatic Science and Technology, Überlandstrasse 133, CH-8600 Dübendorf, Switzerland.
authors.forename: Thomas
authors.initials: T
authors.lastname: Fleischmann
authors.affiliation: Eawag, Swiss Federal Institute of Aquatic Science and Technology, Überlandstrasse 133, CH-8600 Dübendorf, Switzerland.
authors.forename: Hans-Peter E
authors.initials: HE
authors.lastname: Kohler
authors.affiliation: ETH, Swiss Federal Institute of Technology, Laboratory for Organic Chemistry, Hönggerberg, CH-8093 Zürich, Switzerland.
authors.forename: W
authors.initials: W
authors.lastname: Bernd Schweizer
authors.affiliation: Empa, Swiss Federal Laboratories for Materials Testing and Research, Laboratory for Analytical Chemistry, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland; ZHAW, Zurich University of Applied Sciences, Institute of Chemistry and Biological Chemistry, Reidbach, CH-8820 Wädenswil, Switzerland.
authors.forename: Heidi
authors.initials: H
authors.lastname: Moor
authors.affiliation: ZHAW, Zurich University of Applied Sciences, Institute of Chemistry and Biological Chemistry, Reidbach, CH-8820 Wädenswil, Switzerland.
authors.forename: Peter
authors.initials: P
authors.lastname: Lienemann
mesh_terms: D001426:Bacterial Proteins / Q000378:metabolism*; D002384:Catalysis; D001556:Hexachlorocyclohexane / Q000378:metabolism; D006842:Hydrocarbons, Brominated / Q000737:chemistry* / Q000378:metabolism*; D008968:Molecular Conformation; D042301:Sphingomonadaceae / Q000201:enzymology*; D013237:Stereoisomerism
chemical_list: D001426:Bacterial Proteins; D006842:Hydrocarbons, Brominated; C521198:pentabromocyclododecene; D001556:Hexachlorocyclohexane; C089796:hexabromocyclododecane
keywords: Brominated flame retardants; Crystal structure analysis; HBCD biotransformation; LinA2 metabolites; Pentabromocyclododecenes