Purification and properties of recombinant β-glucosidase of the hyperthermophilic bacterium Thermotoga maritima

Abstract

A β-glucosidase of the hyperthermophilic bacterium Thermotoga maritima has been purified from a recombinant Escherichia coli clone expressing the corresponding gene. The enzyme was found to be a dimer with an apparent molecular mass of approximately 95 kDa as determined by size exclusion chromatography. It was composed of two apparently identical subunits of about 47 kDa (determined by sodium dodecyl sulphate-polyacrylamide gel electrophoresis). The enzyme had a bbroadsubstrate specificity and attacked β-glucoside, β-galactoside, β-fucoside, and, to a very small extent, also β-xyloside substrates. α-Glycosidic bonds were not hydrolysed. Kinetic measurement of the hydrolysis of o-nitrophenyl-β-d-glucopyranoside (oNPGlc) and o-nitrophenyl-β-d-galactopyranoside (oNPGal) in the concentration ranges 0.05–20 mm and 0.1–10 mm, respectively, at 75°C resulted in non-linear Lineweaver-Burk and Eadie-Hofstee 3lots whereas cellobiose and lactose did not induce this type of effect. Lactose caused substrate inhibition above 350 mm. The enzyme was optimally active at about pH 6.1. The T. maritima β-glucosidase represents the most thermostable β-glucosidase described to date. In 50 mm sodium phosphate buffer, pH 6.2, at an enzyme concentration of 50 μg/ml, the pure enzyme without additives retained more than 60% of its initial activity after a 6-h incubation at 95°C.