To distinguish two substrates that differ by just one atom, a bacterial enzyme takes an unusual tack, chemists report (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.1212591109). The enzyme, a thioesterase, subjects the two compounds to different reaction pathways, in contrast to the typical strategy of relying on subtle differences in binding. The work could hold broader lessons for biocatalysis. In the course of forging carbon-fluorine bonds, the microbe Streptomyces cattleya makes the toxic compound fluoroacetyl-CoA. Its thioesterase must dismantle the toxin while sparing acetyl-CoA, an essential metabolite. Intrigued by the enzyme’s selectivity, University of California, Berkeley, chemists Amy M. Weeks and Michelle C. Y. Chang performed kinetics experiments. They found that the enzyme subjects acetyl-CoA to a conventional hydrolysis reaction at the carbonyl group. But the enzyme selects for fluoroacetyl-CoA by hydrolyzing it far more quickly and through deprotonation at the carbon adjacent to the carbonyl. The researchers think the rapid pathway proceeds through a ketene intermediate (shown). If follow-up experiments with isotope labels confirm that suspicion, it would be among the first reported examples of a ketene intermediate in enzyme catalysis with a natural substrate, Chang says.