Reaction Mechanism and Stereochemistry of g-hexachlorocyclohexane Dehydrochlorinase LinA
Trantirek, L., Hynkova, K., Nagata, Y., Murzin, A., Ansorgova, A., Sklenar, V., Damborsky, J.
JOURNAL OF BIOLOGICAL CHEMISTRY 276: 7734-7740 (2001)
g-hexachlorocyclohexane dehydrochlorinase (LinA) catalyses the initial steps in the biotransformation of the important insecticide g-hexachlorocyclohexane (g-HCH) by the soil bacterium Sphingomonas paucimobilis UT26. Stereochemical analysis of the reaction products formed during conversion of g-HCH by LinA was investigated by GC-MS, NMR, CD and molecular modeling. The NMR spectra of 1,3,4,5,6-pentachlorocyclohexene (PCCH) produced from g-HCH using either enzymatic dehydrochlorination or alkaline dehydrochlorination were compared and found to be identical. Both enantiomers present in the racemate of synthetic g-PCCH were converted by LinA, each at a different rate. 1,2,4-trichlorobenzene (1,2,4-TCB) was detected as the only product of the biotransformation of biosynthetic g-PCCH. 1,2,4-TCB and 1,2,3-TCB were identified as the dehydrochlorination products of racemic g-PCCH. d-PCCH was detected as the only product of dehydrochlorination of d-HCH. LinA requires the presence of a 1,2-biaxial HCl pair on a substrate molecule. LinA enantiotopologically differentiates two 1,2-biaxial HCl pairs present on g-HCH and gives rise to a single PCCH enantiomer 1,3(R),4(S),5(S),6(R)-PCCH. Furthermore, LinA enantiomerically differentiates 1,3(S),4(R),5(R),6(S)-PCCH and 1,3(R),4(S),5(S),6(R)-PCCH. The proposed mechanism of enzymatic biotransformation of g-HCH to 1,2,4-TCB by LinA consists of two 1,2-anti conformationally dependent dehydrochlorinations followed by 1,4-anti dehydrochlorination.