Characterization of an L-arabinose isomerase from the Lactobacillus plantarum NC8 strain showing pronounced stability at acidic pH

Abstract

Gene araA encoding the l-arabinose isomerase (l-AI) from Lactobacillus plantarum NC8 was cloned and expressed in Escherichia coli. It encodes a polypeptide of 474 residues having 55% identities with l-AIs from Bacillus stearothermophilus US100 and Thermus sp. IM6501. The active form of the purified recombinant l-AI NC8 enzyme is a hexamer composed of six identical 55-kDa subunits. The purified enzyme was optimally active at 60°C and pH 7.5. It required divalent cations such as Co2+ and Mn2+ for maximal activity and thermostability. The l-AI NC8 was exceptionally active and stable at acidic pH. Indeed, it exhibited 68% of its maximal activity at pH 5.5 and retained 89% of activity after a 24-h incubation at pH 5. The apparent Km values of the enzyme for l-arabinose and d-galactose were 43.4 and 69.7 mM, respectively, and its catalytic efficiency was c. 10-fold higher for the physiological substrate l-arabinose (15.5 mM-1 min-1) than d-galactose (1.6 mM-1 min-1). The bioconversion yield of d-galactose to d-tagatose by the purified l-AI NC8 after 6 h at 60°C was 30%. © 2007 Federation of European Microbiological Societies.