Stereoselective esterase from Pseudomonas putida IFO12996 reveals α/β hydrolase folds for D-β-acetylthioisobutyric acid synthesis

Esterase (EST) from Pseudomonas putida IFO12996 catalyzes the stereoselective hydrolysis of methyl DL-beta-Pacetylthioisobutyrate (DL-MATI) to produce D-beta-acetylthioisobutyric acid (DAT), serving as a key intermediate for the synthesis of angiotensin-converting enzyme inhibitors. The EST gene was cloned and expressed in Escherichia coli; the recombinant protein is a non-disulfide-linked homotrimer with a monomer molecular weight of 33,000 in both solution and crystalline states, indicating that these ESTs function as trimers. EST hydrolyzed DL-MATI to produce DAT with a degree of conversion of 49.5% and an enantiomeric excess value of 97.2% at an optimum pH of about 8 to 10 and an optimum temperature of about 57 to 67 degrees C. The crystal structure of EST has been determined by X-ray diffraction to a resolution of 1.6 angstrom, confirming that EST is a member of the alpha/beta hydrolase fold superfamily of enzymes and includes a catalytic triad of Ser97, Asp227, and His256. The active site is located approximately in the middle of the molecule at the end of a pocket similar to 12 angstrom deep. EST can hydrolyze the methyl ester group without affecting the acetylthiol ester moiety in DL-NUTI. The examination of substrate specificity of EST toward other linear esters revealed that the enzyme showed specific activity toward methyl esters and that it recognized the configuration at C-2.