Expression, purification and characterization of a recombinant levan fructotransferase

A 1.6 kb DNA fragment including the IftM gene, encoding a levan fructotransferase (LFTase) of Microbacterium sp. AL-210, was subcloned into a high-expression vector, pET-29b, and the recombinant enzyme was overexpressed in Escherichia coli. Most of the LFTase activity was detected in the cytoplasmic fraction after induction with isopropyl beta-D-thiogalactoside. The recombinant enzyme with a tag of six histidine residues at the C-terminus was purified 132-fold by affinity and gel-filtration chromatography. Analysis of the N-terminal amino acid sequence revealed that the first 42 amino acids were post-translationally cleaved off. The molecular mass of the purified LftM was approx. 54 kDa as determined by SDS/PAGE, which corresponded well with a predicted size from the nucleotide sequence of the IftM gene lacking 42 amino acids. The enzyme converted levan into difructose anhydride IV (DFA IV) with a K-m of 2 mg/ml and a V-max of 40.6 mumol/min at pH 7.0 and 40 degreesC. The pH-dependence study of the enzyme for DFA IV production showed that LftM had a broad pH optimum (5.0-8.0) and the pK(a) values obtained were 4.5 and 8.9 at 40 degreesC. These results suggest that the acidic residues at the active site may play important roles for the catalytic mechanism of the LFTase.