A thermostable α-galactosidase from Lenzites elegans (Spreng.) ex Pat. MB445947: purification and properties

An alpha-galactosidase was isolated from a culture filtrate of Lenzites elegans (Spreng.) ex Pat. MB445947 grown on citric pectin as carbon source. It was purified to electrophoretic homogeneity by ammonium sulfate precipitation, gel filtration chromatography and anion-exchange chromatography. The relative molecular mass of the native purified enzyme was 158 kDa determined by gel filtration and it is a homodimer (Mr subunits = 61 kDa). The optimal temperature for enzyme activity was in the range 60-80 A degrees C. This alpha-galactosidase showed a high thermostability, retaining 94 % of its activity after preincubation at 60 A degrees C for 2 h. The optimal pH for the enzyme was 4.5 and it was stable from pH 3 to 7.5 when the preincubation took place at 60 A degrees C for 2 h. It was active against several alpha-galactosides such as p-nitrophenyl-alpha-d-galactopyranoside, alpha-d-melibiose, raffinose and stachyose. The alpha-galactosidase is a glycoprotein with 26 % of structural sugars. Galactose was a non-competitive inhibitor with a Ki = 22 mM versus p-nitrophenyl-alpha-d-galactoside and 12 mM versus alpha-d-melibiose as substrates. Glucose was a simple competitive inhibitor with a Ki = 10 mM. Cations such as Hg2+ and p-chloromercuribenzoate were also inhibitors of this activity, suggesting the presence of -SH groups in the active site of the enzyme. On the basis of the sequence of the N-terminus (SPDTIVLDGTNFALN) the studied alpha-galactosidase would be a member of glycosyl hydrolase family 36 (GH 36). Given the high optimum temperature and heat stability of L. elegans alpha-galactosidase, this fungus may become a useful source of alpha-galactosidase production for multiple applications.