Critical factors to high thermostability of an alpha-amylase from hyperthermophilic archaeon Thermococcus onnurineus NA1.

Genomic analysis of a hyperthermophilic archaeon, Thermococcus onnurineus NA1 [1], revealed the presence of an open reading frame consisting of 1,377 bp similar to alpha-amylases from Thermococcales, encoding a 458-residue polypeptide containing a putative 25-residue signal peptide. The mature form of the alpha-amylase was cloned and the recombinant enzyme was characterized. The optimum activity of the enzyme occurred at 80 degrees C and pH 5.5. The enzyme showed a liquefying activity, hydrolyzing maltooligosaccharides, amylopectin, and starch to produce mainly maltose (G2) to maltoheptaose (G7), but not pullulan and cyclodextrin. Surprisingly, the enzyme was not highly thermostable, with half-life (t(1/2)) values of 10 min at 90 degrees C, despite the high similarity to alpha-amylases from Pyrococcus. Factors affecting the thermostability were considered to enhance the thermostability. The presence of Ca2+ seemed to be critical, significantly changing t(1/2) at 90 degrees C to 153 min by the addition of 0.5 mM Ca2+. On the other hand, the thermostability was not enhanced by the addition of Zn2+ or other divalent metals, irrespective of the concentration. The mutagenetic study showed that the recovery of zinc-binding residues (His175 and Cys189) enhanced the thermostability, indicating that the residues involved in metal binding is very critical for the thermostability.