A close look at the structural features and reaction conditions that modulate the synthesis of low and high molecular weight fructans by levansucrases

•Fructans are versatile homopolymers with an enormous potential in a wide variety of industrial sectors.•Levansucrases produce fructans of diverse molecular weight, polydispersity and branching ratio.•Traditional and novel strategies to control levan molecular weight are reviewed.•Protein interactions, chemical modifications and protein engineering effects on levan biosynthesis are discussed.•An in-depth analysis of the distinctive structural hallmarks of bacterial levansucrases is provided. The physicochemical properties and biological activity of levan, a generic term given to oligo- and polysaccharides consisting of fructose units linked predominantly by β(2-6) bonds, are attributable to both its size and structural complexity. Branching in β(2-1) contributes to diversify levan structures and properties. There is a broad spectrum of applications for levan and accordingly it has been the subject of several comprehensive reviews. A thorough analysis focused on the product specificity of enzymes from the Glycoside-Hydrolase family 68 that synthesize levan is however missing. We analyze here traditional and novel strategies to manipulate bacterial levansucrases in favor of the generation of low- or high-molecular weight levan, including site directed mutagenesis and chemical engineering. A comparison of highly variable structural elements of levansucrases is presented in the context of their capacity to synthesize saccharides of different sizes, employing the levansucrases from Bacillus subtilis and Bacillus megaterium as references.