Comparison of Biochemical Properties of Wild Type Glucoamylases and Those of High Glucoamylase Mutant Strains of Aspergillus oryzae

Abstract

  The properties of mutant glucoamylases（GlaBs） from Aspergillus oryzae H-1 and G-4 strains with a high level of GlaB activity were compared with those of native GlaBs from parental strains. Amino acid compositions of both mutant GlaBs were different from those of native GlaBs, with a particular decrease in Ser residue of mutant GlaBs. Moreover, N-linked suger chains of native and mutant GlaBs were considered a complex or hybrid type, and not a high mannose type. Because N-linked sugar chain of each GlaB mainly consisted of galactose and N-acetylglucosamine, there was little mannose. Since a little more galactose and N-acetylglucosamine combined per mannose were found in the N-linked suger chains of mutant GlaBs than of native GlaBs, this suggested that each N-linked sugar chain length of mutant GlaBs was possibly shorter. Native GlaBs were not deglycosylated with Endoglycosidase H, while both mutant GlaBs were deglycosylated. This result suggests N-linked suger chain structures of both mutant GlaBs changed from a complex type in native GlaBs to a hybrid type. In circular dichroism spectra analysis, Tm values of GlaBs from H-1 and G-4 strains were approximately 53℃ both, and thus similar to Tm of native GlaBs. Therefore, the thermostability of mutant GlaBs was shown as not being affected by mutation.