A cycloamylose-forming hyperthermostable 4-α-glucanotransferase of Aquifex aeolicus expressed in Escherichia coli

The gene (malM) encoding for 4-α-glucanotransferase (4αGTase) from the hyperthermophilic bacterium Aquifex aeolicus was cloned and expressed in Escherichia coli. The recombinant enzyme was purified to homogeneity and its behavior towards various substrates was examined. The enzyme was hyperthermostable, exhibiting maximal activity at 90 °C and it retained 70% of its original activity after incubation at 90 °C for 30 min. A low affinity was observed for the enzyme towards maltose (Km=71 mM) and the kcat/Km value for maltotriose was approximately 166 times greater than that observed for maltose but the values did not change significantly with larger maltooligosaccharides. The A. aeolicus 4αGTase produced cycloamylose with a minimum degree of polymerization (DP) of 16, whereas the cycloamylose produced by the amylomaltase from the thermophilic bacterium Thermus aquaticus, which is also a Type II 4αGTase, produced a cycloamylose with a minimum DP of 22. These findings indicate that the A. aeolicus 4αGTase differs from the T. aquaticus amylomaltase and the glucanotransferases produced by other hyperthermophilic organisms.