Crystal structure of a lactosucrose-producing enzyme, Arthrobacter sp. K-1 β-fructofuranosidase.

Arthrobacter sp. K-1 β-fructofuranosidase (ArFFase), a glycoside hydrolase family 68 enzyme, catalyzes the hydrolysis and transfructosylation of sucrose. ArFFase is useful for producing a sweetener, lactosucrose (4(G)-β-D-galactosylsucrose). The primary structure of ArFFase is homologous to those of levansucrases, although ArFFase catalyzes mostly hydrolysis when incubated with sucrose alone, even at high concentration. Here, we determined the crystal structure of ArFFase in unliganded form and complexed with fructose. ArFFase consisted of a five-bladed β-propeller fold as observed in levansucrases. The structure of ArFFase was most similar to that of Gluconacetobacter diazotrophicus levansucrase (GdLev). The structure of the catalytic cleft of ArFFase was also highly homologous to that of GdLev. However, two amino acid residues, Tyr232 and Pro442 in ArFFase, were not conserved between them. A tunnel observed at the bottom of the catalytic cleft of ArFFase may serve as a water drain or its reservoir.