Cobamide-mediated enzymatic reductive dehalogenation via long-range electron transfer.

Area of Science:

• Biochemistry
• Enzymology
• Microbiology

Background:

• Metal-containing porphyrinoids are crucial for reductive dehalogenation mediated by cobamide-containing reductive dehalogenases (RDases).
• RDases function as terminal reductases in organohalide-respiring microbes, enabling the use of halogenated compounds as electron acceptors.
• The precise reaction mechanism of RDases remains an area of active investigation and debate.

Purpose of the Study:

• To investigate substrate-enzyme interactions in tetrachloroethene RDase (PceA), which also processes aryl halides.
• To elucidate the catalytic mechanism of PceA, particularly concerning electron transfer and halogen elimination.

Main Methods:

• Characterization of substrate binding within PceA's active site using structural analysis.
• Electron paramagnetic resonance (EPR) spectroscopy to probe cobalt-substrate interactions.
• Tracing the pathway of the leaving halide during reductive elimination.

Main Results:

• PceA's active site directs bromophenol binding away from the cobalt center, with the hydroxyl group oriented towards the metal.
• EPR spectroscopy did not detect direct, close cobalt-substrate interaction.
• Reductive elimination of a para-substituted halogen was observed, and its pathway was structurally mapped.

Conclusions:

• The findings support an enzymatic mechanism involving long-range electron transfer in PceA catalysis.
• This mechanism is novel within vitamin B12-dependent biochemistry.
• The study highlights an effective catalytic mode for RDases.