Eliminylation, a type of posttranslational protein modification discovered recently, most likely proceeds via a carbanion intermediate, according to Hua Guo and coworkers at the University of New Mexico and New York University (J. Am. Chem. Soc., DOI: 10.1021/ja204378q). Some bacteria use this modification to disrupt signaling in host cells, which usually involves reversible phosphorylation and dephosphorylation. In eliminylation, bacterial phosphothreonine lyase enzymes catalyze the irreversible removal of a phosphate group from phosphorylated threonine. Unlike phosphatase enzymes, which reversibly catalyze a phosphoryl transfer reaction, the lyases catalyze breakage of a C–O bond and formation of a C=C double bond via β-elimination, which could proceed by any of three mechanisms. Using computational methods, Guo and coworkers find that the most likely mechanism is one in which a lysine residue initiates the reaction by abstracting a proton to form an enolate carbanion intermediate, which is stabilized by an oxyanion hole in the enzyme. Then, a histidine in the enzyme protonates the phosphate leaving group, which is eliminated to generate a double bond.