The nitrilase from Pseudomonas fluorescens EBC191 converted 2-methyl-2-phenylpropionitrile, which contains a quaternary
carbon atom in the -position toward the nitrile group, and also similar sterically demanding substrates, such as 2-hydroxy-2-
phenylpropionitrile (acetophenone cyanohydrin) or 2-acetyloxy-2-methylphenylacetonitrile. 2-Methyl-2-phenylpropionitrile
was hydrolyzed to almost stoichiometric amounts of the corresponding acid. Acetophenone cyanohydrin was transformed to the
corresponding acid (atrolactate) and amide (atrolactamide) at a ratio of about 3.4:1. The (R)-acid and the (S)-amide were formed
preferentially from acetophenone cyanohydrin. A homology model of the nitrilase suggested that steric hindrance with amino
acid residue Tyr54 could impair the binding or conversion of sterically demanding substrates. Therefore, several enzyme variants
that carried mutations in the respective residues were generated and subsequently analyzed for the substrate specificity and
enantioselectivity of the reactions. Enzyme variants that demonstrated increased relative activities for the conversion of acetophenone
cyanohydrin were identified. The chiral analysis of these reactions demonstrated peculiar reaction kinetics, which suggested
that the enzyme variants converted the nonpreferred (S)-enantiomer of acetophenone cyanohydrin with a higher reaction
rate than that of the (preferred) (R)-enantiomer. Recombinant whole-cell catalysts that simultaneously produced the nitrilase
from P. fluorescens EBC191 and a plant-derived (S)-oxynitrilase from cassava (Manihot esculenta) converted acetophenone plus
cyanide at pH 4.5 to (S)-atrolactate and (S)-atrolactamide. These recombinant cells are promising catalysts for the synthesis of
stable chiral quaternary carbon centers from ketones.