Phylogenetic Analyses of HCH Dehydrochlorinase, Enzyme Involved in Degradation of Hexachlorocyclohexane (HCH), a Persistent Organic Pollutant.

Hexachlorocyclohexane (HCH) is one of the most widely used insecticides, which is included in the list of persistent organic pollutants. The genes involved in HCH degradation are known as lin genes. linA codes for the enzyme HCH dehydrochlorinase, which is a type of lyase dehalogenase and catalyzes the first step of dehydrochlorination of α-, γ- and δ-HCH. It is important to understand the phylogeny of lyase dehalogenases, as they are involved in the degradation of various xenobiotics. Lyase dehalogenases are underrepresented in the database. Homology search suggested that HCH dehydrochlorinase shares similarities with bile-acid 7-alpha dehydratase, scytalone dehydratase, ketosteroid isomerase, nuclear transport factor 2, and biphenyl 2,3 dioxygenase. This similarity is supported by secondary structure, homology modeling, and catalytic motif prediction. The catalytic site residues of HCH dehydrochlorinase are similar to bile-acid 7-alpha dehydratase and scytalone dehydratase. Dichloromethane dehalogenase shares homology with glutathione S-transferases and maleylacetoacetate isomerase. This similarity is further supported by the presence of similar glutathione binding sites and similar secondary structures. This study provides evidence that HCH dehydrochlorinase may have evolved from the enzymes having dehydratase activity and dichloromethane dehalogenase from the theta class of glutathione S-transferase. This study also identified key catalytic residues that can be mutated to make more efficient HCH and dichloromethane-degrading enzymes that can be used for enzymatic bioremediation. [ABSTRACT FROM AUTHOR]