Highly enantioenriched mixed benzoins are obtained selectively through a biocatalytical cross-coupling reaction of aromatic aldehydes using ThDP-dependent enzymes.

Pyruvate decarboxylase from Zymomonas mobilis (PDC) and benzoylformate decarboxylase from Pseudomonas putida (BFD) are thiamine diphosphate-dependent enzymes that decarboxylate 2-keto acids. Although they share a common homotetrameric structure they have relatively low sequence similarity and different substrate spectra. PDC prefers short aliphatic substrates whereas BFD favours aromatic 2-keto acids. These preferences are also reflected in their carboligation reactions. PDC catalyses the conversion of benzaldehyde and acetaldehyde to (R)-phenylacetylcarbinol and predominantly (S)-acetoin, whereas (R)-benzoin and mainly (S)-2-hydroxypropiophenone are the products of BFD catalysis. Comparison of the X-ray structures of both enzymes identified two residues in each that were likely to be involved in determining substrate specificity. Site-directed mutagenesis was used to interchange these residues in both BFD and PDC. The substrate range and kinetic parameters for the decarboxylation reaction were studied for each variant. The most successful variants, PDCI472A and BFDA460I, catalysed the decarboxylation of benzoylformate and pyruvate, respectively, although both variants now preferred the long-chain aliphatic substrates, 2-ketopentanoic and 2-ketohexanoic acid. With respect to the carboligase activity, PDCI472A proved to be a real chimera between PDC and BFD whereas BFDA460I/F464I provided the most interesting result with an almost complete reversal of the stereochemistry of its 2-hydroxypropiophenone product.

From a screening on agar plates with bis(benzoyloxyethyl) terephthalate (3PET), a Bacillus subtilis p-nitrobenzylesterase (BsEstB) was isolated and demonstrated to hydrolyze polyethyleneterephthalate (PET). PET-hydrolase active strains produced clearing zones and led to the release of the 3PET hydrolysis products terephthalic acid (TA), benzoic acid (BA), 2-hydroxyethyl benzoate (HEB), and mono-(2-hydroxyethyl) terephthalate (MHET) in 3PET supplemented liquid cultures. The 3PET-hydrolase was isolated from non-denaturating polyacrylamide gels using fluorescein diacetate (FDA) and identified as BsEstB by LC-MS/MS analysis. BsEstB was expressed in Escherichia coli with C-terminally fused StrepTag II for purification. The tagged enzyme had a molecular mass of 55.2 kDa and a specific activity of 77 U/mg on p-nitrophenyl acetate and 108 U/mg on p-nitrophenyl butyrate. BsEstB was most active at 40°C and pH 7.0 and stable for several days at pH 7.0 and 37°C while the half-life times decreased to 3 days at 40°C and only 6 h at 45°C. From 3PET, BsEstB released TA, MHET, and BA, but neither bis(2-hydroxyethyl) terephthalate (BHET) nor hydroxyethylbenzoate (HEB). The kcat values decreased with increasing complexity of the substrate from 6 and 8 (s-1) for p-nitrophenyl-acetate (4NPA) and p-nitrophenyl-butyrate (4NPB), respectively, to 0.14 (s-1) for bis(2-hydroxyethyl) terephthalate (BHET). The enzyme hydrolyzed PET films releasing TA and MHET with a concomitant decrease of the water-contact angle (WCA) from 68.2°±1.7° to 62.6°±1.1° due to formation of novel hydroxyl and carboxyl groups. These data correlated with a fluorescence emission intensity increase seen for the enzyme treated sample after derivatization with 2-(bromomethyl)naphthalene.

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.