Degradation of the halogenated organic compound poses ecological, sanitary, as well as synthetic, and other problems. Enzymatic degradation of these compounds seems to be a promising option. There are many natural sources of the chlorinated organic compounds, as well as those connected with human activity. The first source dominates. However, human production cannot be ignored, however, due to the considerable toxicity of the compounds produced and their dissemination. The mechanisms, by which enzymes called dehalogenases catalyze halogen-carbon bond breaking varies. Dehalogenases following different pathways were isolated from many microorganisms. Their substrates are usually involved in halorespiration processes, so dehalogenation is involved in carbon source and energy supply, as well. Moreover, cometabolic dehalogenation is possible. Dehalogenases are also present in mammals. For example, iodine dehalogenases (so called deiodinases) are necessary for thyroid hormones homeostasis. Hydrolytic dehalogenases are currently the most studied among all classes of dehalogenases. They include such unusual enzymes as 4-chlorobenzoil-CoA dehalogenase, which is capable of breaking chlorine - aromatic carbon bond under room temperature and neutral pH, and fluoroacetate dehalogenase, which cleaves the carbon-fluorine bond. For some of these enzymes three-dimensional structures are available now and mechanistic details have been studied. This contribution summarizes the present knowledge of enzymatic catalysis of this unusual chemical processes with special consideration on reaction mechanism of the best known hydrolytic dehalogenases.
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