Mechanistic Study of the Enzymatic Reaction of Chlorinated Organic Compounds with Environmental Pollutants

Organohalogen compounds are extensively distributed in the environment, and many of them are used as pesticides, but they cause some degree of pollution to the environment. Many other compounds are pollutants of the soil, water resources, or atmosphere. So, it is essential to understand the sources of these chemicals in the environment, their transformation processes, and their degradation pathways, such as the mechanism of dehalogenation reactions, which are important to maintain the stability of the ecosystem. Many scientists have extensively studied the enzymes involved in some of the dehalogenation reactions. This work has chosen halide methyltransferase (HMT) as the subject of study. Under the catalysis of HMT, halide anions such as chlorine can accept methyl groups from S-adenosylmethionine (AdoMet) to form monohalomethanes (chloromethane) and S-adenosylhomocystine (AdoHcy). HMT is widespread in terrestrial and marine plants, which release large amounts of monohalogenated methane into the atmosphere each year. Monohalogenated methane destroys the ozone layer that protects the Earth. Therefore, it is necessary to study the catalytic mechanism of the HMT reaction. We have determined the value of the kinetic isotope effect (KIE) for HMT-catalyzed Cl- using high-performance liquid chromatography-mass spectrometry of 0.9823 ± 0.0010. This demonstrates that the chemical bond of the isotope is not broken during the reaction, but the hybridization is converted from sp2 to sp3. The halide ion is not tightly bound to the substrate in the transition state reaction. In this review, we focused on the determination of natural stable isotope ratios of chlorine, methods for Compound-specific Isotope Analysis, and related applications of kinetic isotope effects of chlorine, using chlorinated organic compounds as examples.