fructosyltransferases

Fructosyltransferases are enzymes that catalyze the transfer of a fructosyl moiety from a donor, typically sucrose, to an acceptor molecule. They are part of the glycoside hydrolase family 32 (GH32) and are widely studied for their role in fructan biosynthesis in plants and in certain microbes.

Reaction and mechanism: In transfructosylation reactions, a fructosyl unit is transferred to a growing fructan chain or to another sugar, forming fructans such as kestose, inulins, or levan-like polymers. Under different conditions, hydrolysis can predominate, releasing fructose. Most FTs operate via a covalent fructosyl-enzyme intermediate and use two catalytic aspartate residues; product distribution depends on substrate availability and enzyme type.

Occurrence and function: In higher plants, FTs contribute to storage carbohydrates known as fructans, which function as carbon and osmotic reserves and may aid stress tolerance. In bacteria and fungi, related enzymes can synthesize fructosylated compounds for cell walls or extracellular polysaccharides.

Classification and examples: Plant fructosyltransferases include initiators and chain-elongating enzymes such as those that initiate fructan synthesis from sucrose and those that elongate chains or alter linkage patterns to yield different fructan types. Levan-sucrase or levansucrase enzymes are also GH32 members capable of producing levan. Some enzymes display both transferase and hydrolase activities depending on substrate concentration.

Industrial relevance: Fructosyltransferases are exploited to produce fructooligosaccharides (FOS) and inulin-type fructans used as prebiotics and functional food ingredients. Plant breeding and biotechnology aim to modify fructan content to improve stress tolerance or storage properties.