Molecular cloning and characterization of two thermostable carboxyl esterases from Geobacillus stearothermophilus.

Screening of the genomic libraries of Geobacillus stearothermophilus ATCC12980 and ATCC7954 for esterase/lipase activity led to the isolation of two positive clones. The results of subclonings and sequence analyses identified two genes, est30 and est55, encoding two different carboxylesterases, and genetic rearrangement in the est55 locus was revealed from genomic comparison. The est30 gene encodes a polypeptide of 248 amino acids with a calculated molecular mass of 28338 Da, and the est55 gene encodes a polypeptide of 499 amino acids with a calculated molecular mass of 54867 Da. Both enzymes were purified to near homogeneity from recombinant strains of Escherichia coli. The results of enzyme characterization showed that while both enzymes possess optimal activities with short chain acyl derivatives, Est55 has a broader pH tolerance (pH 8-9) and optimal temperature range (30-60 degrees C) than Est30. The activation energy of Est55 (35.7 kJ/mol) was found to be significantly lower than that of Est30 (101.9 kJ/mol). Both enzymes were stable at 60 degrees C for more than 2 h; at 70 degrees C, the half-life for thermal inactivation was 40 and 180 min for Est55 and Est30, respectively. With p-nitrophenyl caproate as the substrate and assayed at 60 degrees C, Est55 had K(m) and k(cat) values of 0.5 microM and 39758 s(-1) while Est30 exhibited values of 2.16 microM and 38 s(-1). Inhibition studies indicated that both Est30 and Est55 were strongly inhibited by phenylmethanesulfonyl fluoride, p-hydroxymercuribenzoate, and tosyl-l-phenylalanine, consistent with the proposed presence of Ser-His-Glu catalytic triad of the alpha/beta hydrolase family. The enzymatic properties of Est30 and Est55 reported here warrant the potential applications of these enzymes in biotechnological industries.