Rationally re-designed mutation of NAD-independent L-lactate dehydrogenase: high optical resolution of racemic mandelic acid by the engineered Escherichia coli.

NAD-independent L-lactate dehydrogenase (L-iLDH) from Pseudomonas stutzeri SDM can potentially be used for the kinetic resolution of small aliphatic 2-hydroxycarboxylic acids. However, this enzyme showed rather low activity towards aromatic 2-hydroxycarboxylic acids. Val-108 of L-iLDH was changed to Ala by rationally site-directed mutagenesis. The L-iLDH mutant exhibited much higher activity than wide-type L-iLDH towards L-mandelate, an aromatic 2-hydroxycarboxylic acid. Using the engineered Escherichia coli expressing the mutant L-iLDH as a biocatalyst, 40 g·L(-1) of DL-mandelic acid was converted to 20.1 g·L(-1) of D-mandelic acid (enantiomeric purity higher than 99.5%) and 19.3 g·L(-1) of benzoylformic acid. A new biocatalyst with high catalytic efficiency toward an unnatural substrate was constructed by rationally re-design mutagenesis. Two building block intermediates (optically pure D-mandelic acid and benzoylformic acid) were efficiently produced by the one-pot biotransformation system.