Biochemistry of Microbial Degradation of Hexachlorocyclohexane and Prospects for Bioremediation

Abstract

SUMMARY Lindane, the γ-isomer of hexachlorocyclohexane (HCH), is a potent insecticide. Purified lindane or unpurified mixtures of this and α-, β-, and δ-isomers of HCH were widely used as commercial insecticides in the last half of the 20th century. Large dumps of unused HCH isomers now constitute a major hazard because of their long residence times in soil and high nontarget toxicities. The major pathway for the aerobic degradation of HCH isomers in soil is the Lin pathway, and variants of this pathway will degrade all four of the HCH isomers although only slowly. Sequence differences in the primary LinA and LinB enzymes in the pathway play a key role in determining their ability to degrade the different isomers. LinA is a dehydrochlorinase, but little is known of its biochemistry. LinB is a hydrolytic dechlorinase that has been heterologously expressed and crystallized, and there is some understanding of the sequence-structure-function relationships underlying its substrate specificity and kinetics, although there are also some significant anomalies. The kinetics of some LinB variants are reported to be slow even for their preferred isomers. It is important to develop a better understanding of the biochemistries of the LinA and LinB variants and to use that knowledge to build better variants, because field trials of some bioremediation strategies based on the Lin pathway have yielded promising results but would not yet achieve economic levels of remediation.

Acinetobacter indicus sp. nov., isolated from a hexachlorocyclohexane dump site

“…So far we have characterized 15 novel species from this HCH dump site and these studies indicate that over the past few years the microbial community at the dump site has been drastically altered as a result of the presence of HCH waste, giving rise to bacterial strains that either degrade or tolerate very high levels of a-and b-HCH isomers. Both these HCH degraders and non-degraders seem to play an important role in depleting the HCH isomer levels at these sites (Lal et al, 2010).…”

Complete Genome Sequence of the Representative γ-Hexachlorocyclohexane-Degrading Bacterium Sphingobium japonicum UT26

“…␥-Hexachlorocylohexane (␥-HCH) is a man-made insecticide that has caused serious environmental problems worldwide (9). Although only several decades have passed since the initial release of ␥-HCH into the environment, many ␥-HCHdegrading bacterial strains have been isolated (9), suggesting that ␥-HCH-degrading ability was acquired by such strains within a short period (14).…”

“…Although only several decades have passed since the initial release of ␥-HCH into the environment, many ␥-HCHdegrading bacterial strains have been isolated (9), suggesting that ␥-HCH-degrading ability was acquired by such strains within a short period (14). Sphingobium japonicum UT26 was isolated from upland ␥-HCH-contaminated soil in Japan and utilizes ␥-HCH as its sole source of carbon and energy (8,16).…”

Genome Sequence of Sphingobium indicum B90A, a Hexachlorocyclohexane-Degrading Bacterium

“…Since then, B90A has been the focus of many studies leading to novel discoveries, such as the association of lin genes with IS6100 (3, 7), enantioselective transformation of chiral ␣-HCH by the linA1 and linA2 genes (15), the presence of efficient HCH dehydrochlorinase (4) and haloalkane dehalogenase systems (13), evolution of isomer-specific (␣-, ␤-, ␥-, ␦-, and ε-HCH) degradation pathways (2,6,11), and potential application of B90A in HCH bioremediation (11).…”