Towards efficient enzymatic conversion of d-galactose to d-tagatose: purification and characterization of l-arabinose isomerase from Lactobacillus brevis

l-arabinose isomerase (l-AI) (EC 5. 3. 1. 4. l-AI) that mediates the isomerization of d-galactose to d-tagatose was isolated from Lactobacillus brevis (MF 465792), and was further purified and characterized. Pure enzyme with molecular weight of 60.1 kDa was successfully obtained after the purification using Native-PAGE gel extraction method, which was a monomer in solution. The l-AI was found to be stable at 45–75 °C, and at pH 7.0–9.0. Its optimum temperature and pH was determined as 65 °C and 7.0, respectively. Besides, we found that Ca2+, Cu2+, and Ba2+ ions inhibited the enzyme activity, whereas the enzyme activity was significantly enhanced in the presence of Mg2+, Mn2+, or Co2+ ions. The optimum concentration of Mn2+ and Co2+ was determined to be 1 mM. Furthermore, we characterized the kinetic parameters for l-AI and determined the Km (129 mM) and the Vmax (0.045 mM min− 1) values. Notably, L. brevisl-AI exhibited a high bioconversion yield of 43% from d-galactose to d-tagatose under the optimal condition, and appeared to be a more efficient catalyst compared with other l-AIs from various organisms.