Acceptor specificity of 4-α-glucanotransferases of mammalian glycogen debranching enzymes

Glycogen debranching enzyme (GDE) has two distinct active sites for its 4-alpha-glucano. transferase and amylo-alpha-1,6-glucosidase activities. The GDE 4-alpha-glucanotransferases of mammals show stringent donor specificity; only alpha-glucans with an alpha-1,6-linked maltotetraosyl or maltotriosyl branch function as donors of a maltotriosyl or maltosyl residue. In this study, we investigated the acceptor specificity of the 4-a-glucanotransferases using methyl alpha-maltooligosides, p-nitrophenyl alpha-maltooligosides, and pyridylaminated maltooligosaccharides of various sizes as the acceptor substrates, and phosphorylase limit dextrin as the donor substrate. High-performance liquid chromatography analysis of the transfer products indicated that maltotriosyl and maltosyl residues were specifically transferred from phosphorylase limit dextrin to acceptors with a maltopentaosyl residue comprising a nonreducing-end. These results suggest that the acceptor binding sites in the active sites of mammalian GDE 4-alpha-glucanotransferases are composed of tandem subsites that are geometrically complementary to five glucose residues.