Heterologous expression and characterization of Bacillus coagulans l-arabinose isomerase
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Bacillus coagulans has been of great commercial interest over the past decade owing to its strong ability of producing optical pure l-lactic acid from both hexose and pentose sugars including l-arabinose with high yield, titer and productivity under thermophilic conditions. The l-arabinose isomerase (L-AI) from Bacillus coagulans was heterologously over-expressed in Escherichia coli. The open reading frame of the L-AI has 1,422 nucleotides encoding a protein with 474 amino acid residues. The recombinant L-AI was purified to homogeneity by one-step His-tag affinity chromatography. The molecular mass of the enzyme was estimated to be 56 kDa by SDS-PAGE. The enzyme was most active at 70°C and pH 7.0. The metal ion Mn2+ was shown to be the best activator for enzymatic activity and thermostability. The enzyme showed higher activity at acidic pH than at alkaline pH. The kinetic studies showed that the K m, V max and k cat/K m for the conversion of l-arabinose were 106 mM, 84 U/mg and 34.5 mM−1min−1, respectively. The equilibrium ratio of l-arabinose to l-ribulose was 78:22 under optimal conditions. l-ribulose (97 g/L) was obtained from 500 g/l of l-arabinose catalyzed by the enzyme (8.3 U/mL) under the optimal conditions within 1.5 h, giving at a substrate conversion of 19.4% and a production rate of 65 g L−1 h−1.
Keywordsl-arabinose l-arabinose isomerase Bacillus coagulans l-ribulose Bioconversion
This research was supported by the Science and Engineering Research Council (SERC) of the Agency for Science, Technology and Research (A*STAR) of Singapore (SERC grant no 0921590133). We are grateful to Dr. Keith Carpenter for critical reading of this manuscript.
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