Kinetic studies on exploring lactose hydrolysis potential of β galactosidase extracted from Lactobacillus plantarum HF571129

Abstract

A novel intracellular β-galactosidases produced by Lactobacillus plantarum HF571129, isolated from an Indian traditional fermented milk product curd was purified and characterized. The β-galactosidases is a hetrodimer with a molecular weight of 60 kDa (larger subunit) and 42 kDa (smaller subunit), as estimated by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE). The enzyme was purified 7.23 fold by ultrasonication, ultrafiltration and gel filtration chromatography with an overall recovery of 30.41 %. The optimum temperature for hydrolysis of its preferred substrates, o-nitrophenyl- β-D-galactopyranoside (ONPG) and lactose, are 50 °C (both), and optimum pH for these reactions is 6.5 and 7.5, respectively. The β-galactosidases showed higher affinity for ONPG (K_m, 6.644 mM) as compared to lactose (K_m, 23.28 mM). Galactose, the end product of lactose hydrolysis was found to be inhibited (47 %). The enzyme activity was drastically altered by the metal ion chelators EDTA, representing that this enzyme is a metalloenzyme. The enzyme was activated to a larger extent by Mg²⁺ (73 % at 1 mM), while inhibited at higher concentrations of Na⁺ (54 % at 100 mM), K⁺ (16 % at 100 mM) and urea (16 % at 100 mM). The thermal stability study indicated an inactivation energy of E_d = 171.37 kJ mol⁻¹. Thermodynamic parameters such as ∆H, ∆S and ∆G, were determined as a function of temperature. About 88 % of lactose was hydrolyzed at room temperature within 1 h. The study suggested that this enzyme showed its obvious superiority in the industrial lactose conversion process.