Purification and characterization of two thermostable proteases from the thermophilic fungus Chaetomium thermophilum.

Thermostable protease is very effective to improve the industrial processes in many fields. Two thermostable extracellular proteases from the culture supernatant of the thermophilic fungus Chaetomium thermophilum were purified to homogeneity by fractional ammonium sulfate precipitation, ion-exchange chromatography on DEAE-Sepharose, and PhenylSepharose hydrophobic interaction chromatography. By SDS-PAGE, the molecular mass of the two purified enzymes was estimated to be 33 kDa and 63 kDa, respectively. The two proteases were found to be inhibited by PMSF, but not by iodoacetamide and EDTA. The 33 kDa protease (PRO33) exhibited maximal activity at pH 10.0 and the 63 kDa protease (PRO63) at pH 5.0. The optimum temperature for the two proteases was 65 degrees C. The PRO33 had a K(m) value of 6.6 mM and a V(max) value of 10.31 micromol/l/min, and PRO63 17.6 mM and 9.08 micromol/l/min, with casein as substrate. They were thermostable at 60 degrees C. The protease activity of PRO33 and PRO63 remained at 67.2% and 17.31%, respectively, after incubation at 70 degrees C for 1 h. The thermal stability of the two enzymes was significantly enhanced by Ca2+. The residual activity of PRO33 and PRO63 at 70 degrees C after 60 min was approximately 88.59% and 39.2%, respectively, when kept in the buffer containing Ca2+. These properties make them applicable for many biotechnological purposes.