4-alpha-glucanotransferase from the hyperthermophilic archaeon Thermococcus litoralis--enzyme purification and characterization, and gene cloning, sequencing and expression in Escherichia coli

Abstract

4-Alpha-Glucanotransferase was purified from cells of Thermococcus litoralis, a hyperthermophilic archaeon. The molecular mass of the enzyme was estimated to be approximately 87 kDa by gel filtration. The optimal temperature for its activity was 90 degrees C. The enzyme catalyzed the transglycosylation of maltooligosaccharides, yielding maltooligosaccharides of various lengths and glucose. When maltoheptaose was used as the substrate, glucoamylase-resistant and glucoamylase-sensitive saccharides were produced. On incubation of amylose with the T. litoralis enzyme, glucoamylase-resistant but alpha-amylase-sensitive molecules were produced, but the amount of reducing sugar showed only slight increases. These results indicate that the T. litoralis enzyme catalyzes not only intermolecular transglycosylation to produce linear alpha-1,4-glucan, but also intramolecular transglycosylation to produce cyclic alpha-1,4-glucan (cycloamylose), similarly to potato 4-alpha-glucanotransferase (called disproportionating enzyme). The gene encoding the T. litoralis 4-alpha-glucanotransferase was cloned, sequenced and expressed in Escherichia coli. The nucleotide sequence of the gene encoded a 659-amino acid protein with a calculated molecular mass of 77,883 Da. The amino acid sequence of the T. litoralis enzyme showed high similarity with those of alpha-amylases of Pyrococcus furiosus, a hyperthermophilic archaeon, and Dictyoglomus thermophilum, an extremely thermophilic bacterium, but little similarity with those of other known 4-alpha-glucanotransferases.