ABSTRACT
Levansucrase is a kind of transferase which catalyzes a fructosyl transfer from sucrose to various acceptor molecules. The product, levan, consists of D-fructofuranosyl residues linked predominantly by -(2,6) linkage as a main chain with some -(2,1) branching points. Both linkage types are formed by the single enzyme levansucrase. The enzyme catalyzes the following reactions:
1. Polymerization reaction
ðSucroseÞn!ðGlucoseÞn þ Levan or Oligosaccharides
2. Hydrolysis reaction
SucroseþH2O! FructoseþGlucose ðLevanÞn þH2O! ðLevanÞn1 þ Fructose 3. Acceptor reaction
SucroseþAcceptor Molecules! Fructosyl-AcceptorþGlucose
4. Exchange reaction
Sucroseþ ½14CGlucose! Fructose-½14CGlucoseþGlucose
5. Disproportionation reaction
½levanm þ ½levann! ½levanm1 þ ½levannþ1
The enzyme catalyzes hydrolysis and polymerization reactions concomitantly (Reaction 1), resulting in fructose homopolymer (levan) and free glucose. This reaction occurs when sucrose exists as the sole fructosyl donor and acceptor, and involves three steps: initiation, propagation, and termination (1). Chains of levan grow in a stepwise fashion by repeated transfer of a hexosyl group from the donor to growing acceptor molecules. The enzyme primarily catalyzes a coupled reaction by a Ping-Pong mechanism, i.e., sucrose hydrolysis followed by transfructosylation involving a fructosyl-enzyme intermediate (2).
