Glycogen serves as a crucial storage molecule for glucose, characterized by its highly branched structure, which optimizes space within cells. The linear chains of glycogen are formed through α(1→4) glycosidic linkages between glucose units, while branch points are created via α(1→6) glycosidic bonds. Typically, these sugar chains consist of 12 to 14 glucose subunits.
At the core of glycogen lies a protein called glycogenin, which initiates glycogen synthesis by attaching the first glucose units to its tyrosine residues. Glycogen is synthesized by the enzyme glycogen synthase, which utilizes uridine diphosphate glucose (UDP-glucose) instead of ATP. During this process, UDP is released, and the elongation of sugar chains occurs at the non-reducing end, maintaining the typical length of 12 to 14 subunits.
The regulation of glycogen synthase is intricately linked to insulin signaling. Glycogen synthase is normally inactivated by glycogen synthase kinase 3 (GSK3), but insulin inhibits GSK3, thereby activating glycogen synthase. Additionally, protein phosphatase 1 (PP1) plays a vital role in this activation by dephosphorylating glycogen synthase, and its activity is stimulated by insulin and glucose-6-phosphate. Conversely, glucagon and epinephrine inhibit glycogen synthase, as these hormones are released in response to low blood sugar levels, promoting the mobilization of glucose rather than its storage.
In the context of glycolysis, glycogen synthase operates alongside other enzymes, such as hexokinase, to convert glucose into glycogen through a series of steps. While glycogen synthase is responsible for forming the straight chains, the branching enzyme facilitates the creation of branches by transferring segments of 6 to 10 glucose subunits from the existing chain to the sixth position of a glucose unit, forming α(1→6) bonds. This branching process is essential for the compact structure of glycogen, allowing for efficient storage and mobilization of glucose when needed.