Catalytic control of enzymatic fluorine specificity

The investigation of unique chemical phenotypes has led to the discovery of enzymes with interesting behaviors that allow us to explore unusual function. The organofluorine-producing microbe Streptomyces cattleya has evolved a fluoroacetyl-CoA thioesterase (FlK) that demonstrates a surprisingly high level of discrimination for a single fluorine substituent on its substrate compared with the cellularly abundant hydrogen analog, acetyl-CoA. In this report, we show that the high selectivity of FlK is achieved through catalysis rather than molecular recognition, where deprotonation at the C_α position to form a putative ketene intermediate only occurs on the fluorinated substrate, thereby accelerating the rate of hydrolysis 10⁴-fold compared with the nonfluorinated congener. These studies provide insight into mechanisms of catalytic selectivity in a native system where the existence of two reaction pathways determines substrate rather than product selection.