A multifunctional galactinol synthase catalyzes the synthesis of fagopyritol A1 and fagopyritol B1 in buckwheat seed

Abstract

Fagopyritols are galactosyl cyclitols in buckwheat (Fagopyrum esculentum Moench, Polygonaceae) seeds with structural similarities to a putative insulin mediator deficient in non-insulin dependent diabetes mellitus and polycystic ovary syndrome. Based on the assumption that the multifunctional enzyme galactinol synthase (GolS, UDP-galactose:myo-inositol galactosyltransferase, EC 2.4.1.123) may have homology to the enzyme responsible for the synthesis of fagopyritols, we isolated two full-length and one partial cDNA clones encoding GolS homologues from buckwheat seeds. We designated the genes corresponding to these three cDNAs as Fagopyrum esculentum GolS (FeGolS)-1, FeGolS-2 and FeGolS-3. The full-length FeGolS-1 and FeGolS-2 cDNAs are 1269 bp and 1326 bp in length and encode polypeptides of 38.3 kDa and 40.7 kDa, respectively. According to the deduced amino acid sequences, FeGolS-1 and FeGolS-2 share a high level of sequence similarity with GolSs in other species. FeGolS-2 and FeGolS-3 contain an insertion of 17 or 18 amino acid residues near the carboxyl terminus, respectively, which is absent in FeGolS-1 and other GolSs. Both bacterially expressed recombinant FeGolS-1 and FeGolS-2 proteins exhibited GolS activities when assayed in the presence of UDP-galactose and myo-inositol. In the presence of UDP-galactose and d-chiro-inositol, FeGolS-1 catalyzed the synthesis of fagopyritol B1 whereas FeGolS-2 catalyzed the synthesis of both fagopyritol A1 and fagopyritol B1 in a 1:4 mole ratio. These results demonstrated that multifunctional GolS homologues in buckwheat seeds confer fagopyritol synthase activities and that the specificity for fagopyritol A1 synthesis may be mediated by a unique class of GolS homologues. © 2004 Elsevier Ireland Ltd. All rights reserved.