Degradation of 3-chloro-2-methylpropionic acid by Xanthobacter sp. CIMW 99

Both stereoisomers of 3-chloro-2-methylpropionic acid (CMPA) and its methyl esters (MeCMPA) serve as growth substrates for a bacterial isolate (Xanthobacter sp. CIMW 99) when supplied as sole source of carbon and energy. Biodegradation of DL-CMPA and DL-MeCMPA was shown to be via a common pathway; an initial, constitutive, esterase converted the methyl ester to the corresponding carboxylic acid. Further metabolism required the activation of CMPA involving a CoA-, ATP-, Mg(2+)-dependent chloroacyl-CoA synthetase. Most noteworthy, it was the product of this reaction (3-chloro-2-methylpropionyl-CoA) that underwent hydrolytic dehalogenation to give 3-hydroxy-2-methylpropionyl-CoA (3-hydroxyisobutyryl-CoA). Further biodegradation proceeded by the action of a dehydrogenase on the CoA derivative to give methylmalonate-CoA-semialdehyde. Cells of CIMW 99 also contained a stable, constitutive, highly active 3-hydroxyisobutyrate dehydrogenase that was specific for the L(+) isomer. However, evidence is presented suggesting that this enzyme was not involved in the catabolism of the chlorinated substrates.