Stereospecificity of 2-monochloropropionate dehalogenation by the two dehalogenases of Pseudomonas putida PP3: evidence for two different dehalogenation mechanisms

Abstract

Pseudomonas putida PP3 grew on DL-2-monochloropropionate (2MCPA) with a release of chloride ions consistent with the dechlorination of both isomers. The organism grew on either D- or L-2MCPA. Dehalogenase activity in cell-free extracts showed that both D- and L-2MCPA were dehalogenated. Pseudomonas putida PP3 contains two dehalogenases, and studies with the separated enzymes revealed that the fraction I enzyme used both D- and L-2MCPA, the rate of dechlorination of L-2MCPA being 80% of the rate of D-2MCPA dechlorination. The product of the reaction, lactate, retained the same optical configuration as the substrate provided. The fraction II dehalogenase also dechlorinated D- and L-2MCPA, with the same difference in rates as for the fraction I dehalogenase, but the lactates produced were of the opposite configuration to their precursors. The two dehalogenases showed further differences with respect to inhibition by two sulphydryl-blocking agents, N-ethylmaleimide and p-chloromercuribenzoate. Fraction I dehalogenase was considerably more sensitive to these two reagents compared with the fraction II dehalogenase. Dithiothreitol partially protected the fraction I dehalogenase from N-ethylmaleimide inhibition. The results are discussed in terms of the possible evolutionary relationships of the two dehalogenases.