Identification and characterization of a novel inulin binding module (IBM) from the CFTase of Bacillus macerans CFC1

Production of a single cyclic type of fructooligosaccharide structure by inulin-degrading Paenibacillus sp. LX16 newly isolated from Jerusalem artichoke root

A novel inulin‐degrading bacterium was isolated from a soil sample collected on Jerusalem artichoke roots. It is a Gram‐positive, aerobic, motile and central endospore‐forming straight rod, and exhibits phenotypic properties being consistent with its classification in the genus Paenibacillus. The predominant cellular fatty acids were anteiso‐C15:0, C16:0 and anteiso‐C17:0. This strain represents a novel species of the genus Paenibacillus on the basis of phenotypic data together with phylogenetic analysis, and it is here designated as LX16 and deposited in China centre for type collection, China (= CCTCC 2015256). Strain LX16 could produce a cyclofructooligosaccharide fructanotransferase catalysing the formation of one type of fructooligosaccharide (FOS) from inulin. The FOS was identified as a cyclofructooligosaccharide with a degree of polymerization of 6. Such homology in inulin degradation products may be beneficial for the functional FOS production.

Structural and kinetic insights reveal that the amino acid pair Gln-228/Asn-254 modulates the transfructosylating specificity of Schwanniomyces occidentalis β-fructofuranosidase, an enzyme that produces prebiotics

Schwanniomyces occidentalis β-fructofuranosidase (Ffase) is a GH32 dimeric enzyme that releases fructose from the nonreducing end of various oligosaccharides and essential storage fructans such as inulin. It also catalyzes the transfer of a fructosyl unit to an acceptor producing 6-kestose and 1-kestose, prebiotics that stimulate the growth of bacteria beneficial for human health. We report here the crystal structure of inactivated Ffase complexed with fructosylnystose and inulin, which shows the intricate net of interactions keeping the substrate tightly bound at the active site. Up to five subsites were observed, the sugar unit located at subsite +3 being recognized by interaction with the β-sandwich domain of the adjacent subunit within the dimer. This explains the high activity observed against long substrates, giving the first experimental evidence of the direct role of a GH32 β-sandwich domain in substrate binding. Crucial residues were mutated and their hydrolase/transferase (H/T) activities were fully characterized, showing the involvement of the Gln-228/Asn-254 pair in modulating the H/T ratio and the type β(2-1)/β(2-6) linkage formation. We generated Ffase mutants with new transferase activity; among them, Q228V gives almost specifically 6-kestose, whereas N254T produces a broader spectrum product including also neokestose. A model for the mechanism of the Ffase transfructosylation reaction is proposed. The results contribute to an understanding of the molecular basis regulating specificity among GH-J clan members, which represent an interesting target for rational design of enzymes, showing redesigned activities to produce tailor-made fructooligosaccharides.