New Activity, Old Enzyme

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Science  11 Nov 2005:
Vol. 310, Issue 5750, pp. 945
DOI: 10.1126/science.310.5750.945c

Ever since we realized that chemicals introduced into the environment for the control of agricultural pests can persist for uncomfortably long periods, there has been an interest in microbes that are able to adapt to living off of (metabolizing) these synthetic carbon sources. In the case of the nematocide 1,3-dichloropropene, its degradation product, trans-3-chloroacrylic acid, undergoes hydrolytic decomposition with a half-life of 24,000 years at 19°C, roughly equal to the half-life for 239Pu decay. Fortunately, the soil bacterium Pseudomonas pavonaceae expresses the enzyme CaaD, which Horvat and Wolfenden show accelerates hydrolysis, yielding malonate semialdehyde through addition of water and loss of HCl, by a factor of 1012. They argue that this impressive rate enhancement is due largely to chemical transformations taking place in the active site (as opposed to substrate binding or product release) and that CaaD appears to be a considerably more proficient enzyme than its structural cousin 4-oxalocrotonate tautomerase, all of which provides support for the proposal that the degradation of 3-chloro-acrylate may be a recently acquired activity of a relatively ancient and catalytically sophisticated enzyme. — GJC

Proc. Natl. Acad. Sci. U.S.A. 102, 16199 (2005).

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