An L-arabinose isomerase from Acidothermus cellulolytics ATCC 43068: cloning, expression, purification, and characterization

The araA gene encoding an L-arabinose isomerase (L-AI) from the acido-thermophilic bacterium Acidothermus cellulolytics ATCC 43068 was cloned and overexpressed in Escherichia coli . The open reading frame of the L-AI consisted of 1,503 nucleotides encoding 501 amino acid residues. The recombinant L-AI was purified to homogeneity by heat treatment, ion-exchange chromatography, and gel filtration. The molecular mass of the enzyme was estimated to be approximately 55 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The purified enzyme was optimally active at 75°C and pH 7.5. It required divalent metal ions, either Mn 2+ or Co 2+ , for both enzymatic activity and thermostability improvement at higher temperatures. The enzyme showed relatively high activity and stability at acidic pH. It exhibited over 90% of its maximal activity at pH 6.0 and retained 80% of activity after 12 h incubation at pH 6.0. Catalytic property study showed that the enzyme had an interesting catalytic efficiency. Its apparent K m , V max , and catalytic efficiency ( k cat / K m ) for D-galactose was 28.9 mM, 4.9 U/mg, and 9.3 mM −1 min −1 , respectively. The enzyme carried out the isomerization of D-galactose to D-tagatose with a conversion yield over 50% after 12 h under optimal conditions, suggesting its potential in D-tagatose production.