Newly isolated Streptomyces spp. as enantioselective biocatalysts: hydrolysis of 1,2-O-isopropylidene glycerol racemic esters

Aims:To identify microbial strains with esterase activity able to enantioselectively hydrolyse esters of (R,S)-1,2-O-isopropylidene glycerol. Methods and Results:The microbial hydrolysis of various racemic esters of 1,2-O-isopropylidene glycerol (IPG) was attempted by screening among Streptomyces spp. previously selected on the basis of their carboxylesterase activity. The best results were observed in the hydrolysis of butyrate ester and two strains appeared promising as they showed opposite enantioselectivity: Streptomyces sp. 90852 gave predominantly (S)-IPG, while strain 90930 mostly gave the R-alcohol. Streptomyces sp. 90930 was identified as Streptomyces violaceusniger, whereas Streptomyces sp. 90852 is a new species belonging to the Streptomyces violaceus taxon. The carboxylesterase belonging to strain 90852 gave a maximum value of enantiomeric ratio (E) of 14-16. This strain was lyophilized and used as dry mycelium for catalysing the synthesis of isopropylidene glycerol butyrate in heptane showing reaction rate and enantioselectivity (E = 6.6) lower than what observed for the hydrolysis. Conclusions:A new esterase with enantioselective activity towards (R,S)-IPG butyrate has been selected. The best enantioselectivity is similar or even better than the highest reported value in the literature with commercial enzymes. The enzyme is produced by a new species belonging to the S. violaceus taxon. Significance and Impact of the Study:New esterases from streptomycetes can be employed for the enantioselective hydrolysis of chiral esters derived from primary alcohols, not efficiently resolved with commercial enzymes.