# Glycogenesis
**Glycogenesis** is the biosynthetic pathway by which glucose is converted into glycogen, the branched glucose polymer that serves as the principal short-term carbohydrate store in animals and fungi. The reverse process, the breakdown of glycogen to release glucose units, is called glycogenolysis.[1](https://www.ncbi.nlm.nih.gov/books/NBK549820/?report=printable) Glycogenesis proceeds through an activated sugar donor, [UDP-glucose](https://www.edgepedia.org/udp-glucose), and requires three specialized enzymes beyond basic glucose metabolism: glycogenin, glycogen synthase, and the glycogen branching enzyme.[2](https://www.ncbi.nlm.nih.gov/sites/books/NBK539802/)
| Key fact | Detail |
|---|---|
| Definition | Formation of glycogen from glucose; the opposing degradative pathway is glycogenolysis[1](https://www.ncbi.nlm.nih.gov/books/NBK549820/?report=printable) |
| Activated donor | UDP-glucose, formed from glucose-1-phosphate and uridine triphosphate by UDP-glucose pyrophosphorylase[2](https://www.ncbi.nlm.nih.gov/sites/books/NBK539802/) |
| Chain elongation | Glycogen synthase forms α(1→4) glycosidic bonds at the nonreducing end of a preexisting chain and cannot initiate synthesis de novo[2](https://www.ncbi.nlm.nih.gov/sites/books/NBK539802/) |
| Primer | Glycogenin, a self-glucosylating protein, builds a nascent chain of roughly 8 to 12 (commonly up to 10–20) glucose residues[2](https://www.ncbi.nlm.nih.gov/sites/books/NBK539802/) |
| Branching | The branching enzyme transfers a terminal segment of about 6 to 8 residues to a C6 hydroxyl, forming an α(1→6) linkage once a chain reaches at least 11 residues[2](https://www.ncbi.nlm.nih.gov/sites/books/NBK539802/) |
| Human stores | About 500 g of glycogen in skeletal muscle and about 100 g in the liver[3](https://www.frontiersin.org/journals/physiology/articles/10.3389/fphys.2011.00112/full) |
| Hormonal control | Insulin promotes glycogenesis; glucagon, cortisol, and epinephrine favor breakdown[1](https://www.ncbi.nlm.nih.gov/books/NBK549820/?report=printable) • [2](https://www.ncbi.nlm.nih.gov/sites/books/NBK539802/) |
## The pathway
Glycogenesis begins with the phosphorylation of glucose to glucose-6-phosphate (G6P), catalyzed by hexokinase in most tissues or by the liver-specific isoform glucokinase. This step traps glucose inside the cell and commits it to intracellular metabolism. Phosphoglucomutase then converts G6P to glucose-1-phosphate (G1P).[2](https://www.ncbi.nlm.nih.gov/sites/books/NBK539802/)
The next step creates the activated donor. G1P reacts with uridine triphosphate in a reaction catalyzed by UDP-glucose pyrophosphorylase, producing UDP-glucose and pyrophosphate. Rapid hydrolysis of the pyrophosphate by inorganic pyrophosphatase makes the reaction effectively irreversible and drives it forward; UDP-glucose then serves as the glucose donor for all subsequent synthesis.[2](https://www.ncbi.nlm.nih.gov/sites/books/NBK539802/)
**Elongation.** [Glycogen synthase](https://www.edgepedia.org/glycogen-synthase), the rate-limiting enzyme of the pathway, transfers the glucose moiety of UDP-glucose to the nonreducing end of a preexisting glycogen chain, forming a new α(1→4) glycosidic linkage.[2](https://www.ncbi.nlm.nih.gov/sites/books/NBK539802/) • [4](https://pmc.ncbi.nlm.nih.gov/articles/PMC4892876/) The enzyme cannot start a chain from nothing: it requires a primer. That primer is made by glycogenin, a self-glucosylating protein that covalently attaches glucose from UDP-glucose to the hydroxyl group of a conserved tyrosine residue within its own structure. Successive rounds of autoglycosylation extend this nascent chain to approximately 8 to 12 glucose residues (commonly up to 10–20), producing the short α(1→4)-linked oligosaccharide on which glycogen synthase then acts.[2](https://www.ncbi.nlm.nih.gov/sites/books/NBK539802/) A review of glycogen storage diseases reports that up to seven glucose units can be added to glycogenin, a number treated as critical because glycogen synthase can only extend an existing chain.[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC6050127/)
**Branching.** The glycogen branching enzyme (amylo-α(1→4)→α(1→6) transglycosylase) creates the branch points that give glycogen its structure. Once a growing chain reaches sufficient length, typically at least 11 residues, the enzyme transfers a terminal segment of approximately 6 to 8 glucose residues from the linear chain to an internal C6 hydroxyl group, forming an α(1→6) linkage. Branching increases the number of nonreducing ends, which enhances solubility and permits more rapid glucose mobilization during glycogenolysis.[2](https://www.ncbi.nlm.nih.gov/sites/books/NBK539802/)
Glycogen synthesis differs outside mammals: in yeast, glycogenin initiates synthesis and glycogen synthase elongates chains using UDP-glucose as the glucose donor, while branching enzyme rearranges existing glucan segments to form branches; in most bacteria, GlgC produces ADP-glucose, which glycogen synthase uses to initiate and elongate chains without requiring glycogenin, and branching enzyme transfers existing glucan segments rather than using ADP-glucose directly.[6](https://pmc.ncbi.nlm.nih.gov/articles/PMC2927715/) • [14](https://pmc.ncbi.nlm.nih.gov/articles/PMC6339891/)
## Regulation
Glycogenesis runs when the body is well fed and slows during fasting. The primary hormones that regulate the cycle of glycogenesis and glycogenolysis are insulin, glucagon, and cortisol.[1](https://www.ncbi.nlm.nih.gov/books/NBK549820/?report=printable) Insulin, the fed-state signal, promotes synthesis through coordinated dephosphorylation: it activates protein phosphatase 1 and the kinase Akt, and Akt-mediated inactivation of glycogen synthase kinase-3 (GSK-3) activates glycogen synthase, increasing hepatic glycogen stores.[2](https://www.ncbi.nlm.nih.gov/sites/books/NBK539802/) • [4](https://pmc.ncbi.nlm.nih.gov/articles/PMC4892876/) Glucagon, the fasting signal, inhibits glycogen synthesis in the liver while stimulating glycogenolysis.[7](https://doi.org/10.1152/ajpendo.00492.2002)
**Allosteric control** adds a second layer. Glycogen synthase activity is stimulated by glucose 6-phosphate and inhibited by a high glycogen concentration, so the product and the pathway's own substrate feed back on the key enzyme.[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC4802397/) In muscle, glycogen synthase is inactivated by phosphorylation on multiple serine residues by kinases including casein kinase-1, protein kinase A, and GSK-3.[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC4892876/) Control of synthesis in muscle is shared between glucose transport into the cell and the step catalyzed by glycogen synthase, and the liver and muscle use different tissue-specific isoforms of the key enzymes.[9](https://pubmed.ncbi.nlm.nih.gov/11949930/)
## Physiological role of the two depots
In humans the majority of glycogen is stored in skeletal muscle, about 500 g, with about 100 g in the liver.[3](https://www.frontiersin.org/journals/physiology/articles/10.3389/fphys.2011.00112/full) The two depots serve different purposes. Liver glycogen delivers glucose to the bloodstream during fasting and exercise, while muscle glycogen, in a tissue that lacks glucose-6-phosphatase, is used locally to fuel contraction.[2](https://www.ncbi.nlm.nih.gov/sites/books/NBK539802/) After an overnight fast, the liver produces glucose roughly equally from two sources: glycogen breakdown and glucose formation using lactate, amino acids, glycerol, and pyruvate as substrates.[10](https://www.annualreviews.org/doi/10.1146/annurev.nutr.27.061406.093806) The depots are physiologically linked: lactate produced by contracting muscle is converted to glucose in the liver and contributes to hepatic glycogen replenishment.[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC4802397/)
## History
The pathway's discovery is tied to Claude Bernard, the French physiologist working in Paris in the mid-19th century. In 1848 Bernard reported on the glucogenic function of the liver in a paper titled "On the origin of sugar in the animal body," a finding that changed prevailing views of animal metabolism, which until then, following Jean Baptiste Dumas's 1841 lectures, held that animals only degraded the energy substrates synthesized by plants.[11](https://pmc.ncbi.nlm.nih.gov/articles/PMC12948771/)
Serendipity moved the work forward in 1855. While perfusing livers with cold water to remove all sugar, Bernard was unable one day to make his usual duplicate sugar measurement; the next day much sugar had reappeared in the washed liver. He went on to study this reappearance methodically, and on 24 September 1855 he presented a paper to the Academy of Science demonstrating that the liver held a substance which the body turned into glucose; he then set out to identify this precursor.[11](https://pmc.ncbi.nlm.nih.gov/articles/PMC12948771/) In March 1857 he published the paper describing the isolation of glycogen.[11](https://pmc.ncbi.nlm.nih.gov/articles/PMC12948771/) Bernard nonetheless believed a "vital force" was required for glycogen synthesis, writing in 1857 that the glucogenic matter "must be created by the vital activity of the organ."[11](https://pmc.ncbi.nlm.nih.gov/articles/PMC12948771/) Later historians, including Grmek and Holmes, showed that Bernard's notebooks reveal a gradual, complicated development of these discoveries that his later lectures simplified.[11](https://pmc.ncbi.nlm.nih.gov/articles/PMC12948771/)
The enzymology followed a century later. In 1936 Carl and [Gerty Cori](https://www.edgepedia.org/gerty-cori) showed that glucose-1-phosphate is the product of glycogen breakdown by an enzyme present in mammalian muscle, which they named phosphorylase; they later demonstrated that phosphorylase acting on glucose-1-phosphate in vitro could produce a glycogen-like polysaccharide.[12](https://www.bmj.com/content/bmj/1/5033/1431.full.pdf)
## Clinical significance
Because glycogenesis depends on a defined set of enzymes, defects in them cause disease. Glycogen storage disease type 0 results from a deficiency of glycogen synthase itself, caused by mutations in GYS1, which encodes the muscle isoform, or GYS2, which encodes the liver isoform.[2](https://www.ncbi.nlm.nih.gov/sites/books/NBK539802/) More broadly, pathogenic variants in genes encoding enzymes of glycogen synthesis, degradation, or regulation produce the glycogen storage diseases, a heterogeneous group of inherited disorders presenting with features such as fasting hypoglycemia, hepatomegaly, cardiomyopathy, and exercise intolerance.[2](https://www.ncbi.nlm.nih.gov/sites/books/NBK539802/) Insufficient insulin-stimulated activation of muscle glycogen synthesis is also described as one of the major components of type 2 diabetes mellitus.[13](https://onlinelibrary.wiley.com/doi/10.1111/febs.12059)
## References
1. [Paredes-Flores MA, Rahimi N, Mohiuddin SS. Biochemistry, Glycogenolysis. StatPearls, NCBI Bookshelf](https://www.ncbi.nlm.nih.gov/books/NBK549820/?report=printable)
2. [Daghlas SA, Rahimi N. Biochemistry, Glycogen. StatPearls, NCBI Bookshelf](https://www.ncbi.nlm.nih.gov/sites/books/NBK539802/)
3. [Jensen J, Rustad PI, Kolnes AJ, Lai YC. The Role of Skeletal Muscle Glycogen Breakdown for Regulation of Insulin Sensitivity by Exercise. Frontiers in Physiology, 2011](https://www.frontiersin.org/journals/physiology/articles/10.3389/fphys.2011.00112/full)
4. [Wang X et al. Regulation of glucose metabolism from a liver-centric perspective. Experimental and Molecular Medicine, 2016](https://pmc.ncbi.nlm.nih.gov/articles/PMC4892876/)
5. [Ellingwood SS, Cheng A. Biochemical and Clinical Aspects of Glycogen Storage Diseases. Journal of Endocrinology, 2018](https://pmc.ncbi.nlm.nih.gov/articles/PMC6050127/)
6. [Wilson WA, Roach PJ et al. Regulation of glycogen metabolism in yeast and bacteria. FEMS Microbiology Reviews, 2010](https://pmc.ncbi.nlm.nih.gov/articles/PMC2927715/)
7. [Jiang G, Zhang BB. Glucagon and regulation of glucose metabolism. American Journal of Physiology-Endocrinology and Metabolism, 2003](https://doi.org/10.1152/ajpendo.00492.2002)
8. [Adeva-Andany MM et al. Glycogen metabolism in humans. BBA Clinical, 2016](https://pmc.ncbi.nlm.nih.gov/articles/PMC4802397/)
9. [Roach PJ. Glycogen and its metabolism. Current Molecular Medicine, 2002](https://pubmed.ncbi.nlm.nih.gov/11949930/)
10. [Cherrington AD et al. Splanchnic Regulation of Glucose Production. Annual Review of Nutrition, 2007](https://www.annualreviews.org/doi/10.1146/annurev.nutr.27.061406.093806)
11. [Claude Bernard's route to the isolation of glycogen. PMC, 2024/2025](https://pmc.ncbi.nlm.nih.gov/articles/PMC12948771/)
12. [Claude Bernard and the Discovery of Glycogen. British Medical Journal, 1957](https://www.bmj.com/content/bmj/1/5033/1431.full.pdf)
13. [Palm DC, Rohwer JM, Hofmeyr J-HS. Regulation of glycogen synthase from mammalian skeletal muscle. The FEBS Journal, 2013](https://onlinelibrary.wiley.com/doi/10.1111/febs.12059)
14. [pmc.ncbi.nlm.nih.gov](https://pmc.ncbi.nlm.nih.gov/articles/PMC6339891/)
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*Topic: Encyclopedia › Life and health › Biological foundations › Biochemistry and metabolism › Metabolism and metabolic pathways › Carbohydrate and energy metabolism › Gluconeogenesis and glycogen metabolism*
*Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 21, 2026 · Edited: Sep 30, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license