Conversion of mandelonitrile and phenylglycinenitrile by recombinant E. coli cells synthesizing a nitrilase from Pseudomonas fluorescens EBC191

The enzymatic conversion of (chiral) α-amino- and α-hydroxynitriles is hampered by the low stability of these compounds under neutral conditions. In contrast, these substrates are much more stable at acidic pH-values. Therefore, the spontaneous decomposition of the model compounds mandelonitrile and phenylglycinenitrile was analysed in acid aqueous buffers at different pH-values. Both compounds showed at 4 °C half-lives of more than 24 h in Na-citrate buffer with pH-values of 5 and lower. Phenylglycinenitrile demonstrated a higher stability at all pH-values tested than mandelonitrile. The enzymatic conversion of mandelontrile was analysed at pH-values of 4–7 using a recombinantly produced nitrilase from Pseudomonas fluorescens EBC191. These experiments demonstrated that cell extracts and resting cells of the recombinant organism (E. coli JM109(pIK9)) showed at pH 5 the highest conversion rates for mandelonitrile. This was explained by the higher bioavailability of mandelonitrile in combination with significant residual enzyme activities at this pH. The resting cell system was superior to the cell-free system in respect to conversion rates, acid stability, and tolerance against high substrate concentrations. The enantioconservative conversion of the pure enantiomers of mandelonitrile to the respective mandelic acid enantiomers was achieved with resting cells at pH 5, but was not possible at pH 7. Significantly more mandeloamide as side-product was formed from (S)-mandelonitrile than from (R)-mandelonitrile by resting cells. The higher stability of phenylglycinenitrile in combination with the high enzyme activities allowed an efficient conversion of this substrate already at neutral pH-values.