Purification and characterization of a novel enantioselective hydrolase from Bacillus subtilis

Screening of the micro-organisms from an in-house microbial culture repository, identified a bacterial strain bearing membrane-bound, inducible ester hydrolase activity. The strain designated as RRL-BB1 has been identified as Bacillus subtilis by 16 S rRNA typing. Its application in the kinetic resolution of several racemates, including drug intermediates, showed moderate to high enantioselectivity. The enzyme, designated as BBL, exhibited high enantioselectivity (ee (almost equal) 99%) with acyl derivatives of unsubstituted and substituted 1-(phenyl)ethanols and 1-(6-methoxy-2-naphthyl)ethanols. With acyl derivatives of 2-(6-methoxy-2-naphthyl)propan-1-ol, moderate enantioselectivity was observed. The enzyme also showed moderate enantioselectivity with alkyl esters of carboxylic acids i.e. 2-bromopropanoic acid and 2-hydroxy-4-phenylbutanoic acid. The enzyme was purified to > 90% purity from cell-free extract of RRL-BB1 with 26% overall yield. The purified enzyme exhibited hydrolase activity without any noticeable decrease in the rate of hydrolysis or the enantioselectivity profile. A specific activity of 450 units/mg protein resulted after at least a 200-fold purification of the crude cell-free extract. The key purification step was the irreversible adsorption of the salt-precipitated crude enzyme on hydrophobic resin, in the presence of a low salt concentration, and desorption of the enzyme with a linear gradient of 1% sodium cholate. The purified enzyme was a 45 kD monomer as shown by SDS/PAGE. The N-terminal amino acid sequence of the purified enzyme was determined as Thr-Lys-Leu-Thr-Val-Gln-Thr-Arg-Asp-Gly-Ala-Leu-Arg-Gly-Thr. The N-terminal sequence did not bear any homology with other known bacterial lipases. BBL is maximally active at 37 °C, pH 8.0 and fairly stable up to 40 °C, pH 6-10. The enzyme is insensitive to EDTA but inhibited by serine protease inhibitor PMSF. Its activity (72%) was retained in the presence of the anionic detergent SDS at a concentration of 0.2% (w/v).