Sucrose:Fructan 6-Fructosyltransferase,a Key Enzyme for Diverting Carbon from Sucrose to Fructan in Barley Leaves

Sucrose:sucrose 6-fructosyltransferase, an enzyme activity recently identified in fructan-accumulating barley (Hordeum vulgare) leaves, was further characterized. The purified enzyme catalyzed the transfer of a fructosyl group from sucrose to various acceptors. It displayed some beta]-fructosidase (invertase) activity, indicating that water could act as fructosyl acceptor. Moreover, it transferred the fructosyl residue of unlabeled sucrose to U-14C]Glc, producing U-14C]sucrose and unlabeled glucose. Most significantly for fructan synthesis, the enzyme used as acceptors but not as donors a variety of oligofructans containing beta](2->1)- and beta](2->6)-linked fructosyl moieties. Thus, it acted as a general sucrose:fructan fructosyltransferase. The products formed by the enzyme from sucrose and various purified, structurally characterized oligofructans were analyzed by liquid chromatography and identified by comparison with structurally characterized standards. The results showed that the enzyme formed exclusively beta](2->6) fructosyl-fructose linkages, either initiating or elongating a fructan chain of the phlein type. We propose, therefore, to rename the purified enzyme sucrose:fructan 6-fructosyltransferase.