Enzymes for the resolution of α-tertiary-substituted carboxylic acid esters

Abstract

Aromatic α-amino-α-methyl acids and α-hydrazino-α-methyl acids are known aromatic amino acid decarboxylase inhibitors. Specific derivatives such as 2-amino-2-methyl-3-(3,4-dihydroxyphenyl)propanoate, Aldomet®, and 2-hydrazino-2-methyl-3-(3,4-dihydroxypheny Lodosyn®, have been developed as therapeutic agents to treat hypertension and Parkinson's disease, respectively. We recently reported a method for the kinetic resolution of the racemic esters of such compounds using a crude preparation of a novel enzyme catalyst from the yeast Candida lipolytica (Yee, C.; Blythe, T. A.; McNabb, T. J.; Walts, A. E. J. Org. Chem. 1992, 57, 3525-3527). Here we report the purification and initial characterization of the active enzyme component, an enzyme given the name Candida lipolytica ester hydrolase (CLEH). CLEH was purified to > 95% homogeneity by chromatography on Matrex TM Blue B resin. The enzyme was found to be a glycoprotcin with M r = 80,000–300,000. In addition to esterolytic activity, the enzyme was found to catalyze the hydrolysis of amides, anilides and peptides. Sequence analysis of internal peptides of CLEH revealed striking homology to a number of enzymes belonging to the group of serine carboxypeptidases (E.G. 3.4.16.1). One peptide aligned with the canonical serine carboxypeptidase active site sequence, GESYAG. Based on the structural relationship of CLEH to serine carboxypeptidases, three representative serine carboxypeptidases were evaluated for their utility in resolving racemic α-tertiary ester substrates and compared with the activity of CLEH. All enzymes revealed similarly high activity and enantioselectivity towards the α-hydrazino-α-methyl ester precursor of the Parkinson-drug Carbidopa. However, differences in enantioselectivity were observed with other α-tertiary-substituted ester substrates. Serine carboxypeptidase-catalyzed ester resolutions thus offer a new route to many sterically hindered homochiral α-amino, α-hydrazino and α-hydroxy carboxylic acids.