Mechanism of action of enzymes involved in rare glycogen storage diseases

Glycogen is the major glucose reserve in eukaryotes, acting as an energy source and maintaining glucose homeostasis. Dysregulation of glycogen metabolism leads to diseases, such as glycogen storage diseases, including neurodegeneration and lysosomal dysfunction. Glycogen synthesis is carried out by glycogenin (GN), glycogen synthase (GS) and glycogen branching enzyme (GBE). GN initiates glycogen synthesis through autoglucosylation to produce a primer glucose chain of 8-12 units. GS interacts with GN to elongate the primer chain and GBE introduces branches, producing globular glycogen particles. GS is activated by glucose-6-phosphate (G6P) binding and inhibited by phosphorylation.

The work described in this thesis aims to understand how GS and GN coordinate to synthesise glycogen and further investigate the complex regulation of GS. Currently, there is no structural information of this complex and no structure of human GS. In addition, the mechanism of inactivation by phosphorylation remains elusive.

Structural and biochemical techniques were used to investigate the GS-GN complex. Optimisation of the expression and purification of the full-length human GS-GN complex allowed the first structural analysis of this complex by electron microscopy. Both low- and high-resolution 3D reconstructions attained reveals the stoichiometry of the complex and elucidates the mechanism of GS inactivation by phosphorylation. Overall, the work in this thesis provides new insights into glycogen synthesis regulation and facilitates studies of glycogen-related diseases.