Thermostable L-arabinose isomerase from Bacillus stearothermophilus IAM 11001 for D-tagatose production: gene cloning, purification and characterisation.

BACKGROUND: D-Tagatose, as one of the rare sugars, has been found to be a natural and safe low-calorie sweetener in food products and is classified as a GRAS substance. L-Arabinose isomerase (L-Al, EC 5.3.1.4), catalysing the isomerisations of L-arabinose and D-galactose to L-ribulose and D-tagatose respectively, is considered to be the most promising enzyme for the production of D-tagatose. RESULTS:The araA gene encoding an L-Al from Bacillus stearothermophilus IAM 11001 was cloned, sequenced and overexpressed in Escherichia coli. The gene is composed of 1491 bp nucleotides and codes for a protein of 496 amino acid residues. The recombinant L-Al was purified to electrophoretical homogeneity by affinity chromatography. The purified enzyme was optimally active at 65 degrees C and pH 7.5 and had an absolute requirement for the divalent metal ion Mn2+ for both catalytic activity and thermostability. The enzyme was relatively active and stable at acidic pH of 6. The bioconversion yield of D-galactose to D-tagatose by the purified L-Al after 12 h at 65 degrees C reached 36%. CONCLUSION: The purified L-Al from B. stearothermophilus IAM 11001 was characterised and shown to be a good candidate for potential application in D-tagatose production. (C) 2010 Society of Chemical Industry