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Screening for stereoselective cyanohydrin synthesis in 96-well plates was employed in the development of an efficient, pH-stable hydroxynitrile lyase for the conversion of sterically hindered aliphatic aldehydes. Site-saturation mutagenesis (SSM) resulted in a powerful catalyst for the stereoselective conversion of hydroxypivalaldehyde and pivalaldehyde to their corresponding (R)-cyanohydrins (ee > 97%) which are used as chiral building blocks (e.g., for pantothenic acid production). Furthermore, redesigning the PaHNL5 gene and improving its expression by Pichia pastoris with the help of a new P AOX1 promoter variant and the helper protein PDI (protein disulfide isomerase) led to elevated amounts of todays most efficient biocatalyst for vitamin B 5 synthesis.

3-Tetrahydrothiophenone (4) and 4-phenylthiobutan-2-one (7) were used as masked 2-butanone equivalents to give the corresponding cyanohydrins 5 (79 % yield, 91 % ee) and 8 (95 % yield, 96 % ee) in an enzymatic cyanohydrin reaction applying the hydroxynitrile lyase (HNL) from Hevea brasiliensis. After hydrolysis and desulphurisation the desired intermediate (S)-2-hydroxy-2-methylbutyric acid (10) was obtained with 99 % ee. Interestingly, when applying (R)-selective HNL from Prunus amygdalus again the (S)-cyanohydrin 5 was formed (62 % ee). The absolute configuration of 5 was verified by crystal structure determination of the corresponding hydrolysis derived carboxylate. The fact that both enzymes yield the same enantiomer was analysed and interpreted by molecular modelling calculations.

Hydroxynitrile lyases (HNLs) catalyze the cleavage of cyanohydrins. In the reverse reaction, they catalyze the formation of carbon-carbon bonds by enantioselective condensation of hydrocyanic acid with carbonyls. In this study, we describe two proteins from endophytic bacteria that display activity in the cleavage and the synthesis reaction of (R)-mandelonitrile with up to 74% conversion of benzaldehyde (enantiopreference ee 89%). Both showed high similarity to proteins of the cupin superfamily which so far were not known to exhibit HNL activity. Hydroxynitrile lyases (HNLs) catalyze the reversible cleavage of cyanohydrins, yielding the respective carbonyl compound and HCN. They are of great relevance in organic synthesis, in which the ability of the enzymes to catalyze the reverse reaction forming C-C bonds in a stereoselective manner is of substantial industrial relevance (9,11,19). Hydroxynitrile lyases are quite heterogeneous exhibiting remarkable diversity with respect to their substrate specificity, mass, glycosylation, and amino acid sequence, and their similarity to oxidoreductases, ␣/␤-hydrolases, carboxypeptidases, or Zn 2ϩ -containing alcohol dehydrogenases has been reported (3,4,8,17,18,19).Hydroxynitrile lyases are widespread in plants playing a major role in disease suppression, and only recently a bacterial protein with HNL activity in the cyanohydrin cleavage reaction was reported (16). Usually plants are colonized by a range of different bacteria (14). Endophytes in particular live in close association with their host and promote plant growth and health by various mechanisms, including the production of substances with phytohormonal activity or antimicrobial substances such as antibiotics (13,15,21). The production of HCN in pseudomonads has been reported (5), for example, but it has also been reported in other bacteria. In contrast to plants, cyanogenesis in bacteria usually follows a completely different biosynthetic pathway involving a HCN synthase (2,20). In this work, we present the discovery of a new bacterial enzyme class with hydroxynitrile lyase activity initially identified by function-based screening of a gene library of an isolated bacterial endophyte related to Pseudomonas mephitica and further investigated with a second highly similar protein from Burkholderia phytofirmans strain PsJN.Gene libraries of different endophytic bacteria isolated from potato were constructed in the pZero-2 vector by standard procedures (see supplemental material) and screened for HNL activity toward (R/S)-mandelonitrile using a colony-based colorimetric filter assay [67 mM (R/S)-mandelonitrile in 30 mM citratephosphate buffer (pH 3.5) at room temperature (RT)] (10). In the gene library of a strain related to Pseudomonas mephitica, HNL activity was detected. After subcloning and rescreening, the transformant exhibiting the strongest activity toward mandelonitrile was selected for further characterization. Sequencing revealed that the plasmid contained a 1,659-bp insert, which carried two open reading fram...

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Even if biocatalysis is finding increasing application, it still has to gain widespread use in synthetic chemistry. Reasons for this are limitations that enzymes have with regard to substrate range, reaction scope, and insufficient selectivity with unnatural compounds. These shortcomings can be challenged by enzyme and/or substrate engineering, which are employed to alter substrate specificity and enhance the enzyme selectivity toward unnatural substrates. Herein, these two approaches are coupled to improve the hydroxynitrile lyase catalyzed synthesis of 2-hydroxy-(4'-oxocyclohexyl)acetonitrile (4). The ketone functionality is masked as an enol ether, and the oxynitrilase of Hevea brasiliensis is engineered towards this masked substrate to give the product with a high optical purity and to drastically lower the amount of enzyme needed.

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Enzymes of the non-conventional yeast Yarrowia lipolytica seem to be tailor-made for the conversion of lipophilic substrates. Herein, we cloned and overexpressed the Zn-dependent alcohol dehydrogenase ADH2 from Yarrowia lipolytica in Escherichia coli. The purified enzyme was characterized in vitro. The substrate scope for YlADH2 mediated oxidation and reduction was investigated spectrophotometrically and the enzyme showed a broader substrate range than its homolog from Saccharomyces cerevisiae. A preference for secondary compared to primary alcohols in oxidation direction was observed for YlADH2. 2-Octanone was investigated in reduction mode in detail. Remarkably, YlADH2 displays perfect (S)-selectivity and together with a highly (R)-selective short chain dehydrogenase/ reductase from Yarrowia lipolytica it is possible to access both enantiomers of 2-octanol in >99% ee with Yarrowia lipolytica oxidoreductases.