Improvements of enzyme activity and enantioselectivity via combined substrate engineering and covalent immobilization

Esterases, lipases, and serine proteases have been applied as versatile biocatalysts for preparing a variety of chiral compounds in industry via the kinetic resolution of their racemates. In order to meet this requirement, three approaches of enzyme engineering, medium engineering, and substrate engineering are exploited to improve the enzyme activity and enantioselectivity. With the hydrolysis of (R,S)-mandelates in biphasic media consisting of iso-octane and pH 6 buffer at 55 degrees C as the model system, the strategy of combined substrate engineering and covalent immobilization leads to an increase of enzyme activity and enantioselectivity from V-S/(E-t) = 1.62 mmol/h g and V-S/V-R = 43.6 of (R,S)-ethyl mandelate (1) for a Klebsiella oxytoca esterase (named as SNSM-87 from the producer) to 16.7 mmol/h g and 867 of (R,S)-2-methoxyethyl mandelate (4) for the enzyme immobilized on Eupergit C 250L. The analysis is then extended to other (R,S)-2-hydroxycarboxylic acid esters, giving improvements of the enzyme performance from V-S/(E-t) = 1.56 mmol/h g and V-S/V-R = 41.9 of (R,S)-ethyl 3-chloromandelate (9) for the free esterase to 39.4 mmol/h g and 401 of (R,S)-2-methoxyethyl 3-chloromandelate (16) for the immobilized enzyme, V-S/(E-t) = 5.46 mmol/h g and V-S/V-R = 8.27 of (R,S)-ethyl 4-chloromandelate (10) for free SNSM-87 to 33.5 mmol/hg and 123 of (R,S)-methyl 4-chloromandelate (14) for the immobilized enzyme, as well as V-S/(E-t) = 3.0 mmol/h g and V-S/V-R = 7.94 of (R,S)-ethyl 3-phenyllactate (11) for the free esterase to 40.7 mmol/hg and 158 of (R,S)-2-methoxyethyl 3-phenyllactate (18) for the immobilized enzyme. The great enantioselectivty enhancement is rationalized from the alteration of ionization constants of imidazolium moiety of catalytic histidine for both enantiomers and conformation distortion of active site after the covalent immobilization, as well as the selection of leaving alcohol moiety via substrate engineering approach.