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

Gene araA encoding thel-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 withl-AIs from Bacillus stearothermophilus US100 and Thermus sp. IM6501. The active form of the purified recombinantl-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. Thel-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 K_m values of the enzyme forl-arabinose andd-galactose were 43.4 and 69.7 mM, respectively, and its catalytic efficiency was c. 10-fold higher for the physiological substratel-arabinose (15.5 mM⁻¹ min⁻¹) thand-galactose (1.6 mM⁻¹ min⁻¹). The bioconversion yield ofd-galactose tod-tagatose by the purifiedl-AI NC8 after 6 h at 60 °C was 30%. [ABSTRACT FROM AUTHOR]