Characterization of a new thermostable esterase from the moderate thermophilic bacterium Bacillus circulans

The new moderate thermophilic bacterial strain MAS2, previously isolated at 50°C on a mineral medium containing triolein as sole carbon and energy source has been characterized and identified as Bacillus circulans. This strain does not produced lipase but an esterase activity which production was not enhanced by addition of Tween 80 or triolein in the culture medium. The reaction rate was maximum at 60°C. After 1 h incubation at 70° and 85°C the remaining activities were 100% and 50%, respectively, showing the high thermostability of this esterase activity. Optimum pH was in the range 8.5–9.5 with an important (60%) activity retained at pH 10.0. The kinetic constants for p-nitrophenyl caprylate (pNPC8) hydrolysis were KM=0.24 mM and Vm=4.3 nmol/min mg. Using fatty acids with different chain lengths, the highest activity was observed on pNPC2 which confirmed the esterase nature of the enzyme. A significant activity remained on mid-chain-length fatty acids such as pNPC6 and pNPC8. Because of its high thermostability, activity at alkaline pH and broad specificity range for fatty acids, this enzyme showed high potential for use in biocatalysis.