Characterization of a thermostable alpha-amylase from a thermophilic Streptomyces megasporus strain SD12

An extracellular alpha-amylase (1,4-alpha D-glucan glucan hydrolase; EC 3.2.1.1) was isolated from the cell free broth of Streptomyces megasporus SD12 grown in glucose, soluble starch and raw starch. The enzyme was purified 55-fold with a specific activity of 847.33 U mg-1 of protein and with a yield of 36% activity. The apparent molecular mass of the enzyme was 97 kDa, as estimated by SDS-PAGE. The pI of the enzyme was 5.4 and it was stable at a pH range of 5.5 to 8.5 with an optimum pH 6. The enzyme was stable upto 85 degrees C with a half life of 60 min. With soluble starch as substrate the enzyme exhibited a K(m) and kcat value of 4.4 mg ml-1 and 2335 U min-1 mg-1 of protein respectively. The major end products of starch hydrolysis were maltotriose and maltose depending on the incubation period. The production of the enzyme with agricultural wastes as substrates was 643 to 804 U min-1 mg-1 of protein in submerged fermentation whereas solid state fermentation could produce only 206 U min-1 mg-1 of protein.