Multiple α-galactosidases from Aspergillus niger: purification, characterization and substrate specificities

Enzymes with α-galactosidase activity are produced by many organisms, often in multiple forms. Here we compare the biochemical and hydrolytic properties of four major α-galactosidase forms (α-gal I-IV) that were purified from the culture filtrate of Aspergillus niger. α-Gal II, III and IV appear to be isoforms of the same enzyme, and N-terminal amino acid sequence data suggest that they are closely related or identical to A. niger AglB in family 27 of the glycosyl hydrolases. α-Gal I is a completely different enzyme that belongs to family 36. α-Gal I had an isoelectric point of 4.15 and appears to be a tetramer composed of four 94-kDa subunits. α-Gal II, III and IV were dimers with monomeric molecular masses of 64 kDa and isoelectric points of 4.5, 4.7 and 4.8, respectively. α-Gal II-IV were stable when incubated for 17 h at 50°C and pH 2–5, whereas α-gal I was most stable at pH 5–6. All enzymes had maximal catalytic activity at pH 4.5 and 60°C, and hydrolyzed melibiose, raffinose and stachyose. α-Gal II-IV also degraded galactomanno-oligosaccharides and released 66% of the galactose side groups from polymeric locust bean gum galactomannan. α-Gal I released galactose from locust bean gum only in combination with A. niger β-mannosidase. Kinetic experiments showed that α-gal I hydrolyzed p-nitrophenyl-α-d-galactopyranoside and melibiose more efficiently than α-gal II-IV. The distinct hydrolytic and biochemical properties of α-gal I and α-gal II-IV further signifies the difference between α-galactosidases of family 27 and 36.