title: Directed evolution of a para-nitrobenzyl esterase for aqueous-organic solvents.
abstract: Through sequential generations of random mutagenesis and screening, we have directed the evolution of an esterase for deprotection of an antibiotic p-nitrobenzyl ester in aqueous-organic solvents. Because rapid screening directly on the desired antibiotic (loracarbef) nucleus p-nitrobenzyl ester was not feasible, the p-nitrophenyl ester was employed. Catalytic performance on the screening substrate was shown to reasonably mimic enzyme activity toward the desired ester. One p-nitrobenzyl esterase variant performs as well in 30% dimethylformamide as the wildtype enzyme in water, reflecting a 16-fold increase in esterase activity. Random pairwise gene recombination of two positive variants led to a further two-fold improvement in activity. Considering also the increased expression level achieved during these experiments, the net result of four sequential generations of random mutagenesis and the one recombination step is a 50-60-fold increase in total activity. Although the contributions of individual effective amino acid substitutions to enhanced activity are small (< 2-fold increases), the accumulation of multiple mutations by directed evolution allows significant improvement of the biocatalyst for reactions on substrates and under conditions not already optimized in nature. The positions of the effective amino acid substitutions have been identified in a pNB esterase structural model developed based on its homology to acetylcholinesterase and triacylglycerol lipase. None appear to interact directly with the antibiotic substrate, further underscoring the difficulty of predicting their effects in a 'rational' design effort.
authors.affiliation: Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena 91125, USA.
authors.forename: J C
authors.initials: JC
authors.lastname: Moore
authors.forename: F H
authors.initials: FH
authors.lastname: Arnold
mesh_terms: D019943:Amino Acid Substitution; D000900:Anti-Bacterial Agents / Q000096:biosynthesis; D001412:Bacillus subtilis / Q000201:enzymology / Q000235:genetics; D001483:Base Sequence; D001665:Binding Sites / Q000235:genetics; D001709:Biotechnology; D002265:Carboxylic Ester Hydrolases / Q000737:chemistry* / Q000235:genetics* / Q000378:metabolism; D017931:DNA Primers / Q000235:genetics; D004274:DNA, Recombinant / Q000235:genetics; D019020:Directed Molecular Evolution; D004926:Escherichia coli / Q000235:genetics; D005798:Genes, Bacterial; D007700:Kinetics; D008958:Models, Molecular; D016296:Mutagenesis; D016133:Polymerase Chain Reaction; D011487:Protein Conformation; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D011995:Recombination, Genetic; D012997:Solvents; D014867:Water
chemical_list: D000900:Anti-Bacterial Agents; D017931:DNA Primers; D004274:DNA, Recombinant; D011994:Recombinant Proteins; D012997:Solvents; D014867:Water; C091617:4-nitrobenzyl esterase; D002265:Carboxylic Ester Hydrolases