Expression of Thermobifida fusca thermostable raw starch digesting alpha-amylase in Pichia pastoris and its application in raw sago starch hydrolysis

A gene encoding the thermostable raw starch digesting alpha-amylase in Thermobifida fusca NTU22 was amplified by PCR, sequenced and cloned into Pichia pastoris X-33 host strain using the vector pGAPZ alpha A, allowing constitutive expression and secretion of the protein. Recombinant expression resulted in high levels of extracellular amylase production, as high as 510 U/l in the Hinton flask culture broth. The purified amylase showed a single band at about 65 kDa by SDS-polyacrylamide gel electrophoresis after being treated with endo-beta-N-acetylglycosaminidase H, and this agrees with the predicted size based on the nucleotide sequence. About 75% of the original activity remained after heat treatment at 60 degrees C for 3 h. The optimal pH and temperature of the purified amylase were 7.0 and 60 degrees C, respectively. The purified amylase exhibited a high level of activity with raw sago starch. After 48-h treatment, the DPw of raw sago starch obviously decreased from 830,945 to 378,732. The surface of starch granules was rough, and some granules displayed deep cavities.