ABSTRACT
I. Introduction ...................................................................................................................... 875
A. Dehalogenation — Environmental Perspective....................................................... 875
B. Chlorine Kinetic Isotope Effects.............................................................................. 876
C. Calculations of Chlorine KIEs................................................................................. 876
D. Chlorine Isotopic Ratio Measurements ................................................................... 877
II. Chlorine Isotopic Fractionation in Microbial Processes ................................................. 878
A. Chlorine KIEs on Microbial Degradation of Chlorinated Compounds .................. 878
1. Reduction of Perchlorate................................................................................... 878
2. Reduction of Chlorinated Aliphatic Hydrocarbons .......................................... 879
B. Chlorine KIEs on Reactions Catalyzed by Dehalogenases..................................... 879
1. Haloalkane Dehalogenases................................................................................ 880
2. DL-2-Haloacid Dehalogenase ............................................................................ 883
3. Fluoroacetate Dehalogenase.............................................................................. 885
4. 4-Chlorobenzoil-CoA Dehalogenase ................................................................ 885
C. Chlorine KIEs on Enzymatic Halogenation ............................................................ 886
III. Future Perspectives .......................................................................................................... 888
Acknowledgments ........................................................................................................................ 888
References..................................................................................................................................... 888
Halogenated compounds constitute the most widespread class of chemicals in the present
ecosystem. They originate from anthropogenic sources because of extensive use of halogenated
organic compounds in dyes, pesticides, fire retardants, polymeric materials, and solvents. However,
they do not come from xenobiotic sources only. For example some fungi are estimated to emit
1,60,000 tons of chloromethane to the atmosphere each year. Nevertheless, industrial activities
have significantly disturbed the natural balance between production and consumption of certain
halogenated compounds. A number of pathways have evolved that aim at regaining the natural
balance by facilitating degradation of halogenated compounds. The key reaction of this degradation
is the actual dehalogenation during which the halogen substituent, which is usually responsible for
the toxic and xenobiotic character of the compound, is removed. There is a plethora of processes
that lead to dehalogenation. The diversity is illustrated in Figure 35.1 using the example of
chlorobenzoates for which at least three different dechlorination pathways are found in nature.
