4.5: Glycogen synthesis

Glycogen synthesis is the process of storing glucose and occurs primarily in the liver and the skeletal muscle. The metabolic pathways in these tissues are similar, but the utility of glycogen stores is different. Briefly, liver glycogen is catabolized primarily in response to elevated glucagon, and the glucose 6-phosphate generated is dephosphorylated and released into circulation. In contrast, muscle glycogen is only used by the muscle itself; muscle lacks glucose 6-phosphatase and glucose 6-phosphate released from muscle glycogen is oxidized in glycolysis. Although discussed here as a point of comparison, glycogenolysis is a fasted state pathway and occurs in response to glucagon and epinephrine. This will be discussed in section 5.1.

Glycogen synthesis

Initially glucose 6-phosphate, is isomerized to glucose 1-phosphate. UDP-glucose pyrophosphorylase synthesizes UDP-glucose from glucose 1-phosphate and UTP, and this is the source of all the glycosyl residues added to the growing glycogen chain (figure 4.18). Glycogen synthase is the regulatory enzyme for the pathway and is responsible for linking glycosyl residues in a 1,4 linkage. The reaction typically occurs on existing glycogen stores; however, in the absence of any stored glycogen the reaction can occur on the protein primer, glycogenin.

Figure 4.18: Glycogen synthesis.

Regulation of glycogen synthesis

Glycogen synthesis is regulated by a single enzyme, glycogen synthase. This enzyme is primarily regulated through covalent modification. It is active when dephosphorylated and inactive when phosphorylated. The phosphorylation/dephosphorylation is facilitated by glucagon and insulin levels, respectively (table 4.4).

Summary of pathway regulation

Metabolic pathway	Major regulatory enzyme(s)	Allosteric effectors	Hormonal effects
Glycogenesis	Glycogen synthase	Glucose 6-P (+)	Insulin ­\(\uparrow\) Glucagon \(\downarrow\) (liver) Epi \(\downarrow\) (muscle)
Glycogenolysis (see section 5.1)	Glycogen phosphorylase	AMP (+) muscle \(\ce{Ca}^{2+}\) (+) in muscle	Glucagon \(\uparrow\)­ (liver) Epi \(\uparrow\)­ (muscle)

Table 4.4: Summary of pathway regulation.

References and resources

Text

Ferrier, D. R., ed. Lippincott Illustrated Reviews: Biochemistry, 7th ed. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins, 2017, Chapter 6: Bioenergetics and Oxidative Phosphorylation: Section V, VI, Chapter 8: Introduction to Metabolism and Glycolysis, Chapter 9: TCA Cycle and Pyruvate Dehydrogenase Complex: Section IIA, IIB, Chapter 11: Glycogen Metabolism: Section V, VI, Chapter 16: Fatty Acid Ketone Body and TAG Metabolism: Section II, IV, V, Chapter 23: Metabolic Effect of Insulin and Glucagon, Chapter 25: Diabetes Mellitus.

Le, T., and V. Bhushan. First Aid for the USMLE Step 1, 29th ed. New York: McGraw Hill Education, 2018, 72–78, 85–89.

Lieberman, M., and A. Peet, eds. Marks' Basic Medical Biochemistry: A Clinical Approach, 5th ed. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins, 2018, Chapter 2: The Fed or Absorptive State, Chapter 19: Basic Concepts of Regulation: Section IV.A.1.2, Chapter 20: Cellular Bioenergetics, Chapter 22: Generation of ATP from Glucose: Section I.A.B.C, III, Chapter 24: Oxidative Phosphorylation and the ETC: Section I.E, II, III, Chapter 31: Synthesis of Fatty Acids: Section I.A.B, IV, V.

Figures

Grey, Kindred, Figure 4.18 Glycogen synthesis. 2021. https://archive.org/details/4.18_20210924. CC BY 4.0.