Benzoylformate decarboxylase from Pseudomonas putida as stable catalyst for the synthesis of chiral 2-hydroxy ketones

The thiamin diphosphate- and Mg2+-dependent enzyme benzoylformate decarboxylase (BFD) from Pseudomonas putida was characterized with respect to its suitability to catalyze the formation of chiral 2-hydroxy ketones in a benzoin-condensation type reaction. Carboligation constitutes a side reaction of BFD, whereas the predominant physiological task of the enzyme is the non-oxidative decarboxylation of benzoylformate. For this purpose the enzyme was obtained in sufficient purity from Pseudomonas putida cells in a one-step purification using anion-exchange chromatography. To facilitate the access to pure BFD for kinetical studies, stability investigations, and synthetical applications, the coding gene was cloned into a vector allowing the expression of a hexahistidine fusion protein. The recombinant enzyme shows distinct activity maxima for the decarboxylation and the carboligation beside a pronounced stability in a broad pH and temperature range. The enzyme accepts a wide range of donor aldehyde substrates which are ligated to acetaldehyde as an acceptor in mostly high optical purities. The enantioselectivity of the carboligation was found to be a function of the reaction temperature, the substitution pattern of the donor aldehyde and, most significantly, of the concentration of the donor aldehyde substrate. Our data are consistent with a mechanistical model based on the X-ray crystallographic data of BFD. Furthermore we present a simple way to increase the enantiomeric excess of (S)-2-hydroxy-1-phenyl-propanone from 90% to 95% by skillful choice of the reaction parameters. Enzymatic synthesis with BFD are performed best in a continuously operated enzyme membrane reactor. Thus, we have established a new enzyme tool comprising a vast applicability for stereoselective synthesis.