Action of levan fructotransferase of Arthrobacter ureafaciens on levanoligosaccharides

Levan fructotransferase of the bacterium Arthrobacter ureafaciens, which produces di-D-fructose 2,6':6,2' dianhydride (difructose anhydride IV) from levan by an intramolecular transfructosylation reaction, was purified to give a single protein band of pI 4.5-4.7 on isoelectric focusing. It had a molecular weight of 128,000 on gel-filtration on Sephadex G-200 and 60,000 on SDS-polyacrylamide disc gel-electrophoresis, suggesting that the enzyme is composed of two identical subunits. The shortest levanoligosaccharide chain required for the difructose anhydride IV formation was determined to be tetraose. TLC of the enzymic digest of a modified levanhexaose derived from levanhexaose by the reduction of the reducing end to an alditol residue with sodium borohydride gave the difructose anhydride IV spot, suggesting that the enzyme attacks the modified levanhexaose molecule from the direction of the non-reducing fructose end. The enzymic digests of levantetraose, -pentaose, and -hexaose as the substrate gave, in addition to the difructose anhydride IV spot, spots of oligofructans of lower mobility than the original substrate on TLC. From the digest of levantetraose, a hexaoligofructan and a smaller amount of a pentaoligofructan but no fructose were separated, indicating enzymic intermolecular levanbiosyl and fructosyl transfer reactions.