Structural and Functional Insights into Intramolecular Fructosyl Transfer by Inulin Fructotransferase

Inulin fructotransferase (IFTase), a member of glycoside hydrolase family 91, catalyzes depolymerization of β-2,1-fructans inulin by successively removing the terminal difructosaccharide units as cyclic anhydrides via intramolecular fructosyl transfer. The crystal structures of IFTase and its substrate-bound complex reveal that IFTase is a trimeric enzyme, and each monomer folds into a right-handed parallel β-helix. Despite variation in the number and conformation of its β-strands, the IFTase β-helix has a structure that is largely reminiscent of other β-helix structures but is unprecedented in that trimerization is a prerequisite for catalytic activity, and the active site is located at the monomer-monomer interface. Results from crystallographic studies and site-directed mutagenesis provide a structural basis for the exolytic-type activity of IFTase and a functional resemblance to inverting-type glycosyltransferases.