Cloning, heterologous expression, and characterization of Thielavia terrestris glucoamylase.

Thielavia terrestris is a soil-borne thermophilic fungus whose molecular/ cellular biology is poorly understood. Only a few genes have been cloned from the Thielavia genus. We detected an extracellular glucoamylase in culture filtrates of T. terrestris and cloned the corresponding glaA gene. The coding region contains five introns. Based on the amino acid sequence, the glucoamylase was 65% identical to Neurospora crassa glucoamylase. Sequence comparisons suggested that the enzyme belongs to the glycosyl hydrolase family 15. The T. terrestris glaA gene was expressed in Aspergillus oryzae under the control of an A. oryzae alpha-amylase promoter and an Aspergillus niger glucoamylase terminator. The 75-kDa recombinant glucoamylase showed a specific activity of 2.8 micromol/(min x mg) with maltose as substrate. With maltotriose as a substrate, the enzyme had an optimum pH of 4.0 and an optimum temperature of 60 degrees C. The enzyme was stable at 60 degrees C for 30 min. The Km and kcat of the enzyme for maltotriose were determined at various pHs and temperatures. At 20 degrees C and pH 4.0, the enzyme had a Km of 0.33 +/- 0.07 mM and a kcat of (5.5 +/- 0.5) x 103 min(-1) for maltotriose. The temperature dependence of kcat/Km indicated an activation free energy of 2.8 kJ/mol across the range of 20-70 degrees C. Overall, the enzyme derived from the thermophilic fungus exhibited properties comparable with that of its homolog derived from mesophilic fungi.

Cloning, heterologous expression, and enzymatic characterization of a thermostable glucoamylase from Talaromyces emersonii.

The gene encoding a thermostable glucoamylase from Talaromyces emersonii was cloned and, subsequently, heterologously expressed in Aspergillus niger. This glucoamylase gene encodes a 618 amino acid long protein with a calculated molecular weight of 62,827Da. T. emersonii glucoamylase fall into glucoside hydrolase family 15, showing approximately 60% sequence similarity to glucoamylase from A. niger. The expressed enzyme shows high specific activity towards maltose, isomaltose, and maltoheptaose, having 3-6-fold elevated k(cat) compared to A. niger glucoamylase. T. emersonii glucoamylase showed significantly improved thermostability with a half life of 48h at 65 degrees C in 30% (w/v) glucose, compared to 10h for glucoamylase from A. niger. The ability of the glucoamylase to hydrolyse amylopectin at 65 degrees C is improved compared to A. niger glucoamylase, giving a significant higher final glucose yield at elevated temperatures. The increased thermal stability is thus reflected in the industrial performance, allowing T. emersonii glucoamylase to operate at a temperature higher than the A. niger enzyme.