The levansucrase and inulosucrase enzymes of Lactobacillus reuteri 121 catalyse processive and non-processive transglycosylation reactions

Microbiology (Reading). 2006 Apr;152(Pt 4):1187-96. doi: 10.1099/mic.0.28484-0.

Abstract

Bacterial fructosyltransferase (FTF) enzymes synthesize fructan polymers from sucrose. FTFs catalyse two different reactions, depending on the nature of the acceptor, resulting in: (i) transglycosylation, when the growing fructan chain (polymerization), or mono- and oligosaccharides (oligosaccharide synthesis), are used as the acceptor substrate; (ii) hydrolysis, when water is used as the acceptor. Lactobacillus reuteri 121 levansucrase (Lev) and inulosucrase (Inu) enzymes are closely related at the amino acid sequence level (86 % similarity). Also, the eight amino acid residues known to be involved in catalysis and/or sucrose binding are completely conserved. Nevertheless, these enzymes differ markedly in their reaction and product specificities, i.e. in beta(2-->6)- versus beta(2-->1)-glycosidic-bond specificity (resulting in levan and inulin synthesis, respectively), and in the ratio of hydrolysis versus transglycosylation activities [resulting in glucose and fructooligosaccharides (FOSs)/polymer synthesis, respectively]. The authors report a detailed characterization of the transglycosylation reaction products synthesized by the Lb. reuteri 121 Lev and Inu enzymes from sucrose and related oligosaccharide substrates. Lev mainly converted sucrose into a large levan polymer (processive reaction), whereas Inu synthesized mainly a broad range of FOSs of the inulin type (non-processive reaction). Interestingly, the two FTF enzymes were also able to utilize various inulin-type FOSs (1-kestose, 1,1-nystose and 1,1,1-kestopentaose) as substrates, catalysing a disproportionation reaction; to the best of our knowledge, this has not been reported for bacterial FTF enzymes. Based on these data, a model is proposed for the organization of the sugar-binding subsites in the two Lb. reuteri 121 FTF enzymes. This model also explains the catalytic mechanism of the enzymes, and differences in their product specificities.

MeSH terms

  • Binding Sites
  • Chromatography, Ion Exchange
  • Chromatography, Thin Layer
  • Fructans / metabolism
  • Glycosylation
  • Hexosyltransferases / isolation & purification
  • Hexosyltransferases / metabolism*
  • Lactobacillus reuteri / enzymology*
  • Oligosaccharides / metabolism
  • Substrate Specificity
  • Sucrose / metabolism

Substances

  • Fructans
  • Oligosaccharides
  • fructooligosaccharide
  • Sucrose
  • levan
  • Hexosyltransferases
  • levansucrase
  • inulosucrase