Insight into the Mechanism Underlying Dehalococcoides mccartyi Strain CBDB1-Mediated B₁₂-Dependent Aromatic Reductive...

Area of Science:

• Biochemistry
• Environmental Microbiology
• Quantum Chemistry

Background:

• Anaerobic bacteria detoxify aromatic halides via reductive dehalogenation.
• This process is catalyzed by cob(I)alamin, the active form of vitamin B12, within reductive dehalogenases.
• The precise inner-sphere electron transfer (ET) mechanism remains controversial.

Purpose of the Study:

• To investigate and elucidate the inner-sphere electron transfer mechanisms in reductive dehalogenation.
• To analyze the energetics of various potential ET pathways involving cobalamin and halogenated benzenes.
• To computationally predict the activity and regioselectivity of different aromatic halides.

Main Methods:

• Quantum chemical density functional theory (DFT) calculations.
• Analysis of 36 chloro-, bromo-, and fluorobenzenes with full-size cobalamin.
• Calculation of reaction free energies for proposed inner-sphere ET mechanisms.

Main Results:

• Most inner-sphere pathways were energetically ruled out.
• A proton-coupled two-ET (PC-TET) mechanism involving a B12 side-chain tyrosine (phenol) was identified as the only feasible route.
• The PC-TET mechanism accurately predicted the activity and regioselectivity for 16 out of 17 active chlorobenzene and bromobenzene substrates.
• Fluorobenzenes were computationally predicted to be recalcitrant, aligning with experimental observations.

Conclusions:

• The study proposes a novel PC-TET mechanism for reductive dehalogenation catalyzed by vitamin B12.
• This computational approach provides significant mechanistic insights and aligns with experimental data.
• The findings offer a predictive tool for assessing the feasibility of reductive aromatic dehalogenation, aiding in bioremediation strategies.