Crystal structures of glycogen-debranching enzyme mutants in complex with oligosaccharides

Debranching is a critical step in the mobilization of the important energy store glycogen. In eukaryotes, including fungi and animals, the highly conserved glycogen-debranching enzyme (GDE) debranches glycogen by a glucanotransferase (GT) reaction followed by a glucosidase (GC) reaction. Previous work indicated that these reactions are catalyzed by two active sites located more than 50 Å apart and provided insights into their catalytic mechanisms and substrate recognition. Here, five crystal structures of GDE in complex with oligosaccharides with 4–9 glucose residues are presented. The data suggest that the glycogen main chain plays a critical role in binding to the GT and GC active sites of GDE and that a minimum of five main-chain residues are required for optimal binding.