Synthesis of galactosyl glycerol from guar gum by transglycosylation of α-galactosidase from Aspergillus sp. MK14

A guar gum-hydrolyzing strain, Aspergillus sp. MK14, secreted α-galactosidase selectively in liquid culture. Its α-galactosidase activity (0.820 U/ml) was much higher than its β-mannosidase and β-mannanase activities (0.027 and 0.050 U/ml, respectively). The molecular weight was estimated to be 59,000 Da by SDS–PAGE. The optimal pH was 5 and it was active from pH 2.2 to 6.2. The optimal temperature was 60 °C and the activity was stable below 50 °C. Enzyme activity toward melibiose was much lower than that with pNP-α-d-galactopyranoside. The activities toward 61-α-d-galactosyl-mannobiose and 63,64-α-d-galactosyl-mannopentaose were relatively high (86.2% and 48.4% relative to pNP-α-d-galactopyranoside, respectively). MK14 crude enzyme released only the monosaccharides, galactose and mannose (Gal/Man: 0.64) from guar gum. When glycerol was added to the reaction mixture, the transglycosylation proceeded efficiently, and the synthesis of galactosyl glycerol was 76.6 mg/g of guar gum. MK14 α-galactosidase could use guar gum as a good substrate (donor) in the transglycosylation.