FIGURE 8.

Proposed mechanism of XdINV transfructosylating activity. A, view of the XdINV-neoerlose complex, showing the ligand in cyan and the main interactions of its Glu-α(1–4)Glu moiety located at subsites +1 and +2. The position of this portion represents a potential binding mode of acceptor maltose for the formation of a β(2–6) linkage of glucose to fructose. The position of a putative sucrose acceptor molecule (white) has been inferred by superposition of its glucose to the glucose found at subsite +1 of neoerlose and adjusting manually the fructose unit by small torsion of its glycosidic bond, to best packing to Trp-105 side chain. B, scheme of the proposed mechanism. The donor-substrate sucrose is hydrolyzed by nucleophilic attack forming the fructosyl-XdINV intermediate; a subsequent sucrose molecule (the acceptor substrate) can enter the active site pocket with binding mode 1 generating 1-kestose or, alternatively, with binding mode 2 and generating neokestose.