LinA2, a HCH-converting bacterial enzyme that dehydrohalogenates HBCDs

Hexabromocyclododecanes (HBCDs) and hexachlorocyclohexanes (HCHs) share similar chemical structures and environmental persistence. Bacteria in HCH-contaminated soils evolved to degrade these pollutants using specialized enzymes. Prior research has shown that Sphingobium species possess dehalogenases like LinB that transform HCHs and HBCDs. However, the specificity and mechanisms of these enzymes remain partially unclear. No prior work had resolved how LinA2 interacts with HBCD stereoisomers. That uncertainty drove this study to investigate LinA2’s role in HBCD transformation. Researchers wanted to determine if LinA2 could convert HBCDs and how its activity compared to LinB. This gap motivated a detailed kinetic and stereochemical analysis of LinA2’s dehalogenation activity.

The study aimed to explore LinA2’s ability to dehalogenate HBCDs and compare its specificity to LinB. Researchers focused on how LinA2 interacts with different HBCD stereoisomers. They hypothesized that LinA2 might show distinct substrate preferences compared to LinB. The study sought to quantify the enzyme’s kinetic parameters and identify transformation products. They also wanted to determine if LinA2 could convert HBCDs to PBCDEs. The researchers proposed that structural similarities between HCHs and HBCDs might influence enzyme activity. This work aimed to clarify how LinA2 contributes to HBCD degradation. The findings could help refine bioremediation strategies targeting brominated pollutants.

The study used LinA2 from Sphingobium indicum B90A to test its dehalogenation activity on various HBCD stereoisomers. Researchers exposed racemic mixtures of α-, β-, γ-, and δ-HBCDs to LinA2 and monitored conversion. They measured enantiomeric excess (EE) to assess stereoselectivity. Substrate conversion was tracked using second-order kinetic models. Mass spectrometry identified transformation products like PBCDEs. The team compared LinA2’s activity to LinB’s broader substrate range. They calculated Michaelis-Menten parameters (KM and vmax) for LinA2. The study focused on β-HBCD conversion and product formation.

LinA2 converted (-)β-HBCD to PBCDEs but left other HBCD stereoisomers largely unchanged. The enantiomeric excess of β-HBCD increased up to 60% in 32 hours. α- and γ-HBCD stereoisomers showed no significant transformation. Kinetic analysis revealed KM=0.47 ± 0.07 μM and vmax=0.17 ± 0.01 μmol·L⁻¹·h⁻¹. One major (P1β) and two minor (P2β, P3β) metabolites were detected. Mass spectra confirmed the isotope patterns of PBCDEs. LinA2 showed higher substrate specificity than LinB, which converted all tested HBCDs. These findings suggest LinA2’s activity is limited to certain HBCD stereoisomers.

The study found that LinA2 selectively dehalogenates (-)β-HBCD but not other stereoisomers. The enzyme’s activity is stereospecific and substrate-dependent. LinA2’s kinetic parameters suggest it is less efficient than LinB but more specific. The transformation products confirm HBr elimination from HBCDs. The authors propose that structural similarities between HCHs and HBCDs influence enzyme-substrate interactions. This work highlights how bacterial enzymes adapt to environmental pollutants. The findings may inform strategies for bioremediation of brominated compounds. The study supports the idea that HCH and HBCD degradation share mechanistic pathways.