Swit_4259, an acetoacetate decarboxylase‐like enzyme from Sphingomonas wittichii RW1

The Gram‐negative bacterium Sphingomonas wittichii RW1 is notable for its ability to metabolize a variety of aromatic hydrocarbons. Not surprisingly, the S. wittichii genome contains a number of putative aromatic hydrocarbon‐degrading gene clusters. One of these includes an enzyme of unknown function, Swit_4259, which belongs to the acetoacetate decarboxylase‐like superfamily (ADCSF). Here, it is reported that Swit_4259 is a small (28.8 kDa) tetrameric ADCSF enzyme that, unlike the prototypical members of the superfamily, does not have acetoacetate decarboxylase activity. Structural characterization shows that the tertiary structure of Swit_4259 is nearly identical to that of the true decarboxylases, but there are important differences in the fine structure of the Swit_4259 active site that lead to a divergence in function. In addition, it is shown that while it is a poor substrate, Swit_4259 can catalyze the hydration of 2‐oxo‐hex‐3‐enedioate to yield 2‐oxo‐4‐hydroxyhexanedioate. It is also demonstrated that Swit_4259 has pyruvate aldolase‐dehydratase activity, a feature that is common to all of the family V ADCSF enzymes studied to date. The enzymatic activity, together with the genomic context, suggests that Swit_4259 may be a hydratase with a role in the metabolism of an as‐yet‐unknown hydrocarbon. These data have implications for engineering bioremediation pathways to degrade specific pollutants, as well as structure–function relationships within the ADCSF in general.