Expression and characterization of an extremely thermophilic 1,4-α-glucan branching enzyme from Rhodothermus obamensis STB05

A gene encoding 1,4-alpha-glucan branching enzyme (GBE, EC 2.4.1.18) from the extremely thermophilic bacterium Rhodothermus obamensis STB05 was successfully cloned and expressed in Escherichia colt. Extracellular expression of the recombinant enzyme (R. o-GBE) was achieved with a yield of 1080 mg/L. Then it was purified and further characterized biochemically. R.o-GBE was optimally active at pH 7.0 and 65 degrees C. It remained stable at temperatures up to 80 degrees C and had a half-life at 85 degrees C of approximately 31 min. Far-UV circular dichroism and intrinsic fluorescence analyses revealed that high temperatures reduced its activity by changing the secondary and tertiary structure of R. o-GBE. The enzyme had broad pH stability between pH 3.0 and 11.0 at 4 degrees C, and preferred weakly acidic conditions at high temperatures. None of the metal ions enhanced the activity of R.o-GBE, but Ca2+ may be required for its activity. Its specific activity with amylopectin was 6651 U/mg, which is much higher than that reported for other GBEs. Its excellent thermostability, broad pH stability, and high specific activity make R.o-GBE highly suitable for industrial applications.