Thermostability of three alpha-amylases of Streptomyces sp IMD 2679

The amylolytic system of Streptomyces sp IMD 2679 is composed of three a-am ylases, amylase I, II and III, with temperature maxima of 60, 60-65 and 65 degrees C, respectively, Although each a-amylase displayed higher stability in the pH range 6.0-8.5 than at pH 5.0-5.5, differences in their thermosta bilities were more evident as the pH increased from pH 6.0 to 8.5, There wa s a 14-min difference in half-lives between amylase III, the most thermosta ble enzyme and amylase II at pH 6.0, and a 46-min difference in the half-li ves of amylase III and the least thermostable enzyme, amylase I at pH 6.5. In addition, the alpha-amylases underwent a pH-dependent monomer-dimer tran sformation. Increased thermostability of the alpha-amylases was reflected i n the variable contents of amino acids (Arg, His, Ser) responsible for elec trostatic interactions, and in the levels of aliphatic and bulky hydrophobi c amino acids. There was a two-fold reduction in Cys levels in amylase III relative to amylase I and II.