Microbial Dehalogenation of Halogenated Alkanoic Acids, Alcohols and Alkanes

Publisher Summary This chapter focuses on the catabolism of halogenated aliphatic compounds in the series: alkanoic acids, alcohols, and alkanes. Microorganisms remove halogens from halogenated aliphatic compounds by the activity of enzymes generally called “dehalogenases.” A scheme for classifying dehalogenases is provided to accommodate the current range of known enzymes and their properties. The classification and naming of dehalogenases are based on catalytic properties, with subgroups based on other factors, such as substrate specificities, and nucleotide or amino-acid sequence information. On this basis four classes of 2HAA (halogenated alkanoic acids substituted in the C2 position) hydrolytic dehalogenases are recognized: Class IL 2HAA, Class ID 2HAA, Class 2I 2HAA, and Class 2R 2HAA. Dehalogenation of haloalcohols showed that 3-bromopropanol was biodegradable. Two main pathways for the mineralization of haloalcohols are discussed. The first pathway is mediated by enzymes that accept halosubstituted molecules as their substrates, with dehalogenation taking place only after the haloalcohols have been oxidized to the corresponding haloalkanoic acids. In the complete pathway, the epoxide resulting from the dehalogenation reaction is hydrolyzed by an epoxide hydrolase to the corresponding alcohol. Haloalcohol dehalogenases showed a restricted range of activities being active only when the halogen is vicinal to a hydroxyl group or to a keto group, such as in chloroacetone. Haloalkanes are significant environmental compounds, occurring as both natural products and as xenobiotic compounds. There seems to be a much wider diversity of systems and mechanisms involved in haloalkane dehalogenation than for either halogenated alkanoic acids or alcohols. Haloalkane degradation involving NADH-linked reactions, oxygenases, glutathione (GSH)-dependent reactions, as well as hydrolytic mechanisms have been proposed and demonstrated.