Branching and molecular weight in levan: A detailed analysis of structural variability and enzymatic hydrolysis susceptibility

Levan, a β(2 → 6) linked D-fructofuranosyl polymer, is gaining significant attention in basic and applied research. It has been demonstrated that most properties are related to levan molecular weight but also its β(2 → 1) branching degree. In this paper the relationship between levan branching degree, particle size, and molecular weight is reviewed, exploring also how these structural parameters influence levan susceptibility to exo- and endolevanase hydrolysis for levans produced by three recombinants bacterial levansucrases. We found almost no association between molecular weight and neither particle size nor branching degree in levans described in the literature including those evaluated in this work. We also found that all evaluated levans form spherical nanoparticles. Interestingly, in enzyme assays with the synthesized levans, increasing branching and decreasing particle size are inversely associated with lower exolevanase (Bs-SacB) and endolevanase (Bl-LevB) hydrolysis rates. After a limited but exclusive β(2 → 6) exolevanase hydrolysis a limit-levan structure may be obtained. In the case of endolevanase hydrolysis, branching not only decreases endolevanase activity but also affects the type of oligosaccharides obtained, probably due to limited access to the enzyme to linear regions of the polymer.