Glucoamylase activity from the thermophilic fungus Scytalidium thermophilum. Biochemical and regulatory properties

A glucoamylase activity produced by the thermophilic fungus Scytalidium thermophilum was purified 6.8‐fold by ion exchange chromatography. The protein exhibited a molecular mass of about 86 kDa (7% PAGE and SDS‐PAGE), or 68.5 kDa (Bio Sil SEC‐400 FPLC). The pI of the enzyme was 8.4. Optima of pH and temperature, with starch or maltose as substrates were, 6.5/60 °C and 5.0/55 °C, respectively. The enzyme had a half‐life of 22 min at 55 °C with starch as substrate, and it was fully stable with maltose. Maltase activity was activated by 10 mM Ba⁺⁺ (7.8%); Mn⁺⁺ (12.4%) or Mg⁺⁺ (28%). The enzyme contained approximately 25.5% carbohydrate. K_m and V_max values for starch and maltose were 0.28 mg/ml, 67.2 U/mg protein and 1.40 mg/ml, 5.61 U/mg protein, respectively. The products of hydrolysis of starch, detected by thin layer chromatography, showed only glucose after 30 min, indicating a glucoamylase activity. The amino terminal sequence of the purified protein showed 93% homology with a glucoamylase activity purified from Humicola grisea var. thermoidea.