A high-throughput screening assay for the directed evolution-guided discovery of halohydrin dehalogenase mutants for epoxide ring-opening reaction.

Halohydrin dehalogenases (HHDHs) are valuable biocatalysts involved in the synthesis of β-substituted alcohols via their nucleophile-mediated ring-opening activity. To use directed evolution to unleash the latent potential of HHDHs for the synthesis of β-substituted alcohols, we report a high-throughput assay for screening HHDHs mutagenesis libraries. The assay is performed in a 96-well microtiter plate format using a cell-free extract or whole-cells in the presence of the desired nucleophile. The developed method relies upon the color change of bromothymol blue, due to the pH change caused by HHDH-catalyzed ring-opening of the epoxide substrate. The assay was validated using gas chromatography and subsequently applied to high-throughput screening of halohydrin dehalogenase HheC mutagenesis library. Active mutants were found for the tested substrates. Due to its simplicity and flexibility towards the use of nucleophiles and epoxides, the method is an attractive alternative to the existing assays for HHDH epoxide ring-opening reaction and could be helpful in the rapid discovery of industrial biocatalysts.