Characterization of an L-arabinose isomerase from Bacillus subtilis

An isolated gene from Bacillus subtilis str. 168 encoding a putative isomerase was proposed as an L-arabinose isomerase (L-AI), cloned into Escherichia coli, and its nucleotide sequence was determined. DNA sequence analysis revealed an open reading frame of 1,491 bp, capable of encoding a polypeptide of 496 amino acid residues. The gene was overexpressed in E. coli and the protein was purified using nickel-nitrilotriacetic acid chromatography. The purified enzyme showed the highest catalytic efficiency ever reported, with a kcat of 14,504 min−1 and a kcat/Km of 121 min−1 mM−1 for L-arabinose. A homology model of B. subtilis L-AI was constructed based on the X-ray crystal structure of E. coliL-AI. Molecular dynamics simulation studies of the enzyme with the natural substrate, L-arabinose, and an analogue, D-galactose, shed light on the unique substrate specificity displayed by B. subtilisL-AI only towards L-arabinose. Although L-AIs have been characterized from several other sources, B. subtilisL-AI is distinguished from other L-AIs by its high substrate specificity and catalytic efficiency for L-arabinose.

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