Biocatalytic Preparation of Chiral Hydroxy Esters using Entomogenous Fungi: Bio-reduction of Keto Esters by Tochukaso and Related Species

To research the potential ability of tochukaso and related species to act as biocatalysts, we screened 10 entomogenous fungal strains. The recommended medium (Potato Dextrose broth) and a synthetic medium (PGO medium) were tested as the liquid media for culturing these fungi. Four strains (NBRC33061, 100684, 103832, and 109003) cultured using the PGO medium showed a good growth. The stereoselective reduction of ?- and ?-keto esters by these four strains was also investigated, and they were found to be able to reduce various ?-keto esters. Specifically, the reduction of ?-keto esters by the PGO-cultivated Isaria cicadae NBRC33061 strain in the presence of L-glutamate as an additive yielded corresponding ?-hydroxy esters with a high conversion ratio and in excellent enantioselectivity. Furthermore, it was found that the PGO-cultivated NBRC109003 strain in the presence of L-glutamate stereospecifically reduced ethyl 2-methyl-3-oxobutanoate to (2S, 3S)-2-methyl-3-hydroxybutanoate, i.e., only one of the four theoretically possible isomers. Overall, tochukaso and related species were shown to have a great potential for application as biocatalysts for the stereoselective reduction of carbonyl compounds.