Purification and characterization of a thermostable glucoamylase from Chaetomium thermophilum

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Thermostable amylolytic enzymes are currently being investigated to improve industrial processes of starch degradation. A thermostable extracellular glucoamylase (exo-1, 4-α-D-glucanohydrolase, E.C.3.2.1.3) from the culture supernatant of a thermophilic fungus Chaetomium thermophilum was purified to sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) homogeneity by using ammonium sulfate fraction, DEAE-Sepharose Fast Flow chromatography, and Phenyl-Sepharose Fast Flow chromatography. SDS-PAGE of the purified enzyme showed a single protein band of molecular weight 64 kDa. The glucoamylase exhibited optimum catalytic activity at pH 4.0 and 65°C. It was thermostable at 50°C and 60°C, and retained 50% activity after 60 min at 65°C. The half-life of the enzyme at 70°C was 20 min. N-terminal amino acid sequencing (15 residues) was AVDSYIERETPIAWN. Different metal ions showed different effects on the glucoamylase activity. Ca²⁺, Mg²⁺, Na⁺, and K⁺ enhanced the enzyme activity, whereas Fe²⁺, Ag⁺, and Hg²⁺ cause obvious inhibition. These properties make it applicable to other biotechnological purposes.