Structural and functional relations in cobamide-containing reductive dehalogenases from anaerobic bacteria
Biological degradation of persistent natural and synthetic halogenated hydrocarbons (organohalides) in anoxic environments is a vital part of the global halogen cycle. Cobamide-containing reductive dehalogenases (RDases) from organohalide-respiring bacteria play a key role in this process, since they utilize these compounds as terminal electron acceptors in a respiratory chain. In this study, RDases could be unambiguously differentiated from other cobamide-containing biological catalysts like adenosycobalamin-dependent enzymes and methyltransferases. The structure of a tetrachloroethene-converting RDase (PceA) was elucidated by X-ray crystallography. Its comparison with the crystal structure of a non-respiratory ortho-dibromophenol RDase revealed two highly conserved elements, the topology of the intramolecular electron transfer chain and a Tyr-Arg/Lys pair in the active site involved in proton transfer to the substrate during halogen substitution reactions. The catalytically active cobamide cofactor was shown to be non-covalently bound in a permanent base-off conformation with a weak water/hydroxyl group as upper axial ligand, thus facilitating the effective reduction of the cobalt ion in the course of the reaction. Thereby, a variability in the utilization of different cobamides without the loss of dehalogenating ativity in PceA was observed. Furthermore, the absence of an intimate cobalt-substrate interaction tracked with X-ray crystallography and electron paramagnetic resonance spectroscopy suggested a dissociative long-range electron transfer mechanism directly from the cobalt in the center of the enzyme's cofactor, which represents an unprecedented way of utilizing a cobamide in the multifarious chemistry of cobamide-containing enzymes. The catalytic cycle of dihaloeliminating RDases, which mediate a distinct mode of reductive dehalogenation, could finally be clearly distinguished from the halogen substitution mechanism by the need of proton transfer steps.