Evidence for the involvement of corrinoids and factor F430 in the reductive dechlorination of 1,2-dichloroethane by Methanosarcina barkeri

Abstract

Cobalamin and the native and diepimeric forms of factor F430 catalyzed the reductive dechlorination of 1,2-dichloroethane (1,2-DCA) to ethylene or chloroethane (CA) in a buffer with Ti(III) citrate as the electron donor. Ethylene was the major product in the cobalamin-catalyzed transformation, and the ratio of ethylene to CA formed was 25:1. Native F430 and 12,13-di-epi-F430 produced ethylene and CA in ratios of about 2:1 and 1:1, respectively. Cobalamin dechlorinated 1,2-DCA much faster than did factor F430. Dechlorination rates by all three catalysts showed a distinct pH dependence, correlated in a linear manner with the catalyst concentration and doubled with a temperature increase of 10°C. Crude and boiled cell extracts of Methanosarcina barkeri also dechlorinated 1,2-DCA to ethylene and CA with Ti(III) citrate as the reductant. The catalytic components in boiled extracts were heat and oxygen stable and had low molecular masses. Fractionation of boiled extracts by a hydrophobic interaction column revealed that part of the dechlorinating components had a hydrophilic and part had a hydrophobic character. These chemical properties of the dechlorinating components and spectral analysis of boiled extracts indicated that corrinoids or factor F430 was responsible for the dechlorinations. The ratios of 3:1 to 7:1 of ethylene and CA formed by cell extracts suggested that both cofactors were concomitantly active.Chlorinated aliphatic C1 and C2 hydrocarbons, widespread contaminants in different environments, are found to be reductively dechlorinated by pure cultures of methanogens, sulfate reducers, homoacetogens, and other anaerobic bacteria (4,11,13,16,18,22,26,33). Tetrachloromethane (CCd4) was found to be transformed to lower-chlorinated methanes and CO2 by native or autoclaved cell suspensions of methanogens, Acetobacterium woodii, and Desulfobacterium autotrophicum (12,13,26). These anaerobic bacteria fix CO2 via the acetyl-coenzyme A pathway (Wood pathway) or degrade acetate via a reversed acetyl-coenzyme A pathway (17,32,42,44). Carbon monoxide dehydrogenase and a corrinoid-iron-sulfur protein are central enzymes of this pathway. It has been suggested that the anaerobic transformation of CCl4 could be a cometabolic activity of Wood pathway enzymes (12, 13). Free cobalamin catalyzed reductive dechlorination of CCl4 in a buffer reduced with Ti(III) citrate as the electron donor (27). In addition to lowerchlorinated methanes, cobalamin produced CO from CCl4 (28), a compound which is oxidized to CO2 by carbon monoxide dehydrogenase (17,32,42,44). In methanogenic bacteria, factor F430 may also be involved in reductive dechlorination. Factor F430 is a cofactor of methyl-coenzyme M reductase, the enzyme which catalyzes the last step in methanogenesis from methyl-coenzyme M to CH4 (14). This cofactor reductively dechlorinated CCl4 to lower-chlorinated methanes (26). All Cell suspensions of methanogens reductively dechlorinated 1,2-dichloroethane (1,2-DCA) to ethylene and chloroethane (CA) (22). We demonstrate here th...

Methyl-coenzyme M reductase of Methanobacterium thermoautotrophicum delta H catalyzes the reductive dechlorination of 1,2-dichloroethane to ethylene and chloroethane

“…3). In contrast to results obtained with 1,2-DCA, in which dechlorination by cobalamin and factor F430 took place at different rates (27), the formation of ethylene from BrES occurred at about the same rate (500 pmol/min/ nmol of catalyst) with either catalyst. Purified component C was also able to transform its own inhibitor BrES to ethylene (Table 3).…”

“…When boiled cell extract was omitted, slow or no dechlorination was observed. The omission of component C already indicated that component A was not able to reduce free corrinoids and factor F430 present in boiled cell extract, which could also act as catalysts for dechlorination (27). Controls in which additional cobalamin or factor F430 was added in the absence of component C showed no dechlorination at all.…”

“…The oxygen concentration in the anaerobic glove box was kept low with RO-20 palladium catalyst provided by BASF (Arnhem, The Netherlands). Ti(III) citrate was prepared from TiCl3 and sodium citrate as previously described (27). CoM-S-S-HTP and H-S-HTP isolated from M. thermoautotrophicum AH and CH3-SCoM, prepared by methylation of H-S-CoM with dimethylsulfate (28), were generous gifts of J. T. Keltjens (University of Nijmegen, Nijmegen, The Netherlands).…”

“…The analyses of ethylene and CA were done as described previously (27). Methane was analyzed with the same gas chromatograph used for the analysis of ethylene.…”

“…3-Bromopropanesulfonate (BrPS), synthesized as described by Ellermann et al (16), was a generous gift of R. K. Thauer (Marburg, Germany). Factor F430 was isolated from M. thermoautotrophicum AH as described previously (27).…”