Purification of β-galactosidase from Erythrina indica: Involvement of tryptophan in active site

β-Galactosidase (EC: 3.2.1.23), one of the glycosidases detected in Erythrina indica seeds, was purified to 135 fold. Amongst the four major glycosidases detected β-galactosidase was found to be least glycosylated, and was not retained by Con-A CL Seralose affinity matrix. A homogenous preparation of the enzyme was obtained by ion-exchange chromatography, followed by gel filtration. The enzyme was found to be a dimmer with a molecular weight of 74 kDa and 78 kDa, by gel filtration and SDS-PAGE, respectively. The optimum pH and optimum temperature for enzyme activity were 4.4 and 50 °C, respectively. The enzyme showed a Km value of 2.6 mM and Vmax of 3.86 U/mg for p-nitrophenyl-β-D-galactopyranoside as substrate and was inhibited by Zn2+ and Hg2+. The enzyme activity was regulated by feed back inhibition as it was found to be inhibited by β-D-galactose. Chemical modification studies revealed involvement of tryptophan and histidine for enzyme activity. Involvement of tryptophan was also supported by fluorescence studies and one tryptophan was found to be present in the active site of β-galactosidase. Circular dichroism studies revealed 37% α helix, 27% β sheet and 38% random coil in the secondary structure of the purified enzyme.