Mutational analysis of Thermus caldophilus GK24 beta-glycosidase: role of His119 in substrate binding and enzyme activity.

Three amino acid residues (His119, Glu164, and Glu338) in the active site of Thermus caldophilus GK24 β-glycosidase (Tca β-glycosidase), a family 1 glycosyl hydrolase, were mutated by site-directed mutagenesis. To verify the key catalytic residues, Glu164 and Glu338 were changed to Gly and Gin, respectively. The E164G mutation resulted in drastic reductions of both β-galactosidase and β-glucosidase activities, and the E338Q mutation caused complete loss of activity, confirming that the two residues are essential for the reaction process of glycosidic linkage hydrolysis. To investigate the role of His119 in substrate binding and enzyme activity, the residue was substituted with Gly. The H119G mutant showed 53-fold reduced activity on 5 mM p-nitrophenyl β-D-galactopyranoside, when compared with the wild type; however, both the wild-type and mutant enzymes showed similar activity on 5 mM p-nitrophenyl β-D-glucopyranoside at 75°C. Kinetic analysis with p-nitrophenyl β-D-galactopyranoside revealed that the k cat value of the H119G mutant was 76.3-fold lower than that of the wild type, but the K m of the mutant was 15.3-fold higher than that of the wild type owing to the much lower affinity of the mutant. Thus, the catalytic efficiency (k cat /K m ) of the mutant decreased to 0.08% to that of the wild type. The k cat value of the H119G mutant forp-nitrophenyl β-D-glucopyranoside was 5.1-fold higher than that of the wild type, but the catalytic efficiency of the mutant was 2.5% of that of the wild type. The H119G mutation gave rise to changes in optima pH (from 5.5-6.5 to 5.5) and temperature (from 90°C to 80-85°C). This difference of temperature optima originated in the decrease of H119G's thermostability. These results indicate that His119 is a crucial residue in β-galactosidase and β-glucosidase activities and also influences the enzyme's substrate binding affinity and thermostability.