Mechanistic crystallography. Mechanism of inactivation of gamma-aminobutyric acid aminotransferase by (1R,3S,4S)-3-amino-4-fluorocyclopentane-1-carboxylic acid as elucidated by crystallography

(1R,3S,4S)-3-Amino-4-fluorocyclopentane-1-carboxylic acid (7) was previously shown to be a mechanism-based inactivator of gamma-aminobutyric acid aminotransferase (GABA-AT) [Qiu, J. and Silverman, R. B. (2000) J. Med. Chem. 43, 706-720]. Two mechanisms were considered as reasonable possibilities, a Michael addition mechanism and an enamine mechanism. On the basis of a variety of chemical studies, including tedious radiolabeling experiments, it was concluded that inactivation by 7 proceeds by a Michael addition mechanism. Here, a crystal structure of 7 bound to pig liver GABA-AT is reported, which clearly demonstrates that the adduct formed is derived from an enamine mechanism. This represents another example of how crystallography is an important tool for elucidation of inactivation mechanisms.