Abstract
The amylolytic system of Streptomyces sp IMD 2679 is composed of three α-amylases, amylase I, II and III, with temperature maxima of 60, 60–65 and 65°C, respectively. Although each α-amylase displayed higher stability in the pH range 6.0–8.5 than at pH 5.0–5.5, differences in their thermostabilities were more evident as the pH increased from pH 6.0 to 8.5. There was a 14-min difference in half-lives between amylase III, the most thermostable enzyme and amylase II at pH 6.0, and a 46-min difference in the half-lives of amylase III and the least thermostable enzyme, amylase I at pH 6.5. In addition, the α-amylases underwent a pH-dependent monomer-dimer transformation. Increased thermostability of the α-amylases was reflected in the variable contents of amino acids (Arg, His, Ser) responsible for electrostatic interactions, and in the levels of aliphatic and bulky hydrophobic amino acids. There was a two-fold reduction in Cys levels in amylase III relative to amylase I and II.
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