Purification and characterization of a thermostable β-Galactosidase from kidney beans (Phaseolus vulgaris L.) cv. PDR14

Using five different steps, β-Galactosidase has been purified from kidney beans to apparent electrophoretic homogeniety with approximately 90-fold purificationwith a specific activity of 281 units mg−1 protein. A single bandwas observed in native PAGE. Activity staining of the native gel with 5-bromo4-chloro 3-indoxyl β-D-galactopyranoside (X-Gal) at pH 4.0 also produceda single band. Analytical gel filtration in Superdex G-75 revealed the molecularmass of the native protein to be approximately 75 kD. 10 percnt; SDS-PAGE under reducingconditions showed two subunits of molecular masses, 45 and 30 kD, respectively.Hence, β-galactosidase from kidney beans is a heterodimer. A typical proteinprofile with λmax at 280 nm was observed and A280/A260ratio was 1.52. The N-terminal sequence of the 45 kD band showed 86 percnt; sequencehomology with an Arabidopsis thaliana and 85 percnt; with Lycopersiconesculentum putative β-galactosidase sequences. The Electrospray MassSpectrometric analysis of this band also revealed a peptide fragment that had90 percnt; sequence homology with an Arabidopsis thaliana putative β-galactosidasesequence. The N-terminal sequencing of the 30 kD band as well as mass spectrometricanalysis both by MALDI-TOF and ES MS revealed certain sequences that matchedwith phytohemagglutinin of kidney beans. The optimum pH of the enzyme was 4.0and it hydrolysed o- and p-nitrophenyl β-D galactopyranosidewith a Km value of 0.63 mmol/L and 0.74 mmol/L, respectively.The energy of activation calculated from the Arrhenius equation was 14.8 kcal/molenzyme site. The enzyme was found to be comparatively thermostable showing maximumactivity at 67 °C. Thermal denaturation of the enzyme at 65 °C obeyssingle exponential decay with first order-rate constant 0.105 min−1.Galactose, a hydrolytic product of this enzyme was a competitive inhibitor witha Ki of 2.7 mmol/L.