A new esterase gene abmbh, encoding a benzoate hydrolase which can enantioselectively hydrolyze l-menthyl benzoate to l-menthol, was recently identified from the genomic library of a soil isolate Acinetobacter sp. ECU2040. The abmbh gene contains a 1080-bp open reading frame encoding a protein of 360 amino acids with a calculated molecular mass of 40.7 kDa. The corresponding enzyme AbMBH was functionally expressed in Escherichia coli BL21 (DE3), purified, and characterized. The AbMBH displayed the maximum activity towards p-nitrophenyl butyrate at 50 degrees C, and an optimum pH of 8.5. A K M of 2.6 mM and a k cat of 0.26 s(-1) were observed towards dl-menthyl benzoate. The AbMBH exhibited a moderate enantioselectivity (E = 27.5) towards dl-menthyl benzoate. It can also catalyze the enantioselective hydrolysis of a variety of racemic menthyl esters, including dl-menthyl acetate, dl-menthyl chloroacetate, and dl-menthyl butyrate.
The stereoselective synthesis of L-menthol is an attractive process in the flavor and fragrance industry. One promising way to obtain optically pure menthol is the enantioselective hydrolysis of menthol esters under enzymatic catalysis. We developed an effective and highly enantioselective method for the synthesis of L-(-)-menthol (>99% EE) by hydrolyzing the key industrial starting compound, d, l-menthyl benzoate. The enzyme of choice was the lipase from Candida rugosa (CRL). While commercially available preparations of this lipase showed only minor selectivity (E=15), excellent enantiomeric purity (E>100) was achieved using the heterologously expressed isoenzyme LIP1.