Isolation and characterization of a thermostable dextranase

Abstract

A thermostable dextranase has been isolated from an anaerobic thermophilic bacterium, Rt364, collected from a New Zealand thermal spring. The enzyme was purified by ammonium sulfate precipitation and successive ion exchange, hydrophobic interaction, and size exclusion chromatographies. The enzyme exhibited an apparent molecular weight of ∼140 kDa, a temperature optimum of 80°C, and a pH optimum of ∼5.5. The enzyme was extremely stable. No activity was lost over 12 h at 75°C. It is more thermostable than the dextranase from Chaetomium gracile, the most thermostable dextranase previously characterized; however, the Rt364 dextranase has a much lower specific activity, 10 U mg −1, compared to 2,750 U mg −1 for the fungal enzyme at their respective temperature optima. The enzyme from Rt364 hydrolyzes dextran, starch, amylose, and amylopectin with approximately the same catalytic efficiencies but does not hydrolyze pullulan. It has therefore been designated an amylodextranase which is analogous to the recently characterized amylopullulanase.