Fructosyltransferase Enzymes for Microbial Fructan Production

Fructans are fructose-based oligo- and polysaccharides synthesized using sucrose as substrate. Depending on the glycosidic bonds in their structure, they are classified as inulin and levan types or a mixture of these, namely graminans and agavins. Fructans constitute one of the most widespread functional biomolecules in nature and they occur in microbes and plants and to a lesser extent in some fungi and certain algal species. The escalating number of evidence on their health-promoting effects made fructans an important class of platform chemicals. In fact, they have the largest market share among the natural functional additives in the food sector. Plants are the main resources of inulin-, graminan-, and agavin-type fructans but levan type of fructans are commercially produced by microorganisms. In microbes, levan and inulin are synthesized by extracellular fructosyltransferase (FT) enzymes named levansucrase (EC 2.4.1.10) and inulosucrase (EC 2.4.1.9), respectively. Although microbial levan producers are widespread in nature, microbial inulin production is only limited in few Gram-positive bacteria. This chapter first introduces fructans and its microbial and enzymatic production processes followed by the discussion on different classes and structure-functional features of FT enzymes.