Glucagon
Template:Short description Template:About Template:Infobox gene Glucagon is a peptide hormone, produced by alpha cells of the pancreas. It raises the concentration of glucose and fatty acids in the bloodstream and is considered to be the main catabolic hormone of the body.<ref>Template:Cite book</ref> It is also used as a medication to treat a number of health conditions. Its effect is opposite to that of insulin, which lowers extracellular glucose.<ref name="Campbell">Template:Cite book</ref> It is produced from proglucagon, encoded by the GCG gene.
The pancreas releases glucagon when the amount of glucose in the bloodstream is too low. Glucagon causes the liver to engage in glycogenolysis: converting stored glycogen into glucose, which is released into the bloodstream.<ref>Template:Cite book</ref> High blood-glucose levels, on the other hand, stimulate the release of insulin. Insulin allows glucose to be taken up and used by insulin-dependent tissues. Thus, glucagon and insulin are part of a feedback system that keeps blood glucose levels stable. Glucagon increases energy expenditure and is elevated under conditions of stress.<ref>Template:Cite journal</ref> Glucagon belongs to the secretin family of hormones.
Structure
Glucagon is a 29-amino acid polypeptide. Its primary structure in humans is: NH2-His-Ser-Gln-Gly-Thr-Phe-Thr-Ser-Asp-Tyr-Ser-Lys-Tyr-Leu-Asp-Ser-Arg-Arg-Ala-Gln-Asp-Phe-Val-Gln-Trp-Leu-Met-Asn-Thr-COOH (HSQGTFTSDYSKYLDSRRAQDFVQWLMNT).
The polypeptide has a molecular mass of 3485 daltons.<ref>Template:Cite journal</ref> Glucagon is a peptide (nonsteroid) hormone.
Physiology
Production
The hormone is synthesized and secreted from alpha cells (α-cells) of the islets of Langerhans, which are located in the endocrine portion of the pancreas. Glucagon is produced from the preproglucagon gene Gcg. Preproglucagon first has its signal peptide removed by signal peptidase, forming the 160-amino acid protein proglucagon.<ref name=Muller2017/> Proglucagon is then cleaved by proprotein convertase 2 to glucagon (amino acids 33-61) in pancreatic islet α cells. In intestinal L cells, proglucagon is cleaved to the alternate products glicentin (1–69), glicentin-related pancreatic polypeptide (1–30), oxyntomodulin (33–69), glucagon-like peptide 1 (72–107 or 108), and glucagon-like peptide 2 (126–158).<ref name=Muller2017/>
In rodents, the alpha cells are located in the outer rim of the islet. Human islet structure is much less segregated, and alpha cells are distributed throughout the islet in close proximity to beta cells. Glucagon is also produced by alpha cells in the stomach.<ref name="Unger_2012">Template:Cite journal</ref>
Recent research has demonstrated that glucagon production may also take place outside the pancreas, with the gut being the most likely site of extrapancreatic glucagon synthesis.<ref>Template:Cite journal</ref>
Regulation
Production, which is otherwise freerunning, is suppressed/regulated by amylin, a peptide hormone co-secreted with insulin from the pancreatic β cells.<ref name = "Zhang_2018">Template:Cite journal</ref> As plasma glucose levels recede, the subsequent reduction in amylin secretion alleviates its suppression of the α cells, allowing for glucagon secretion.
Secretion of glucagon is stimulated by:
- Hypoglycemia
- Epinephrine (via β2, α2,<ref name="Layden_2010">Template:Cite journal</ref> and α1<ref name=alpha1and2>Template:Cite journal</ref> adrenergic receptors)
- Arginine
- Alanine (often from muscle-derived pyruvate/glutamate transamination (see alanine transaminase reaction).
- Acetylcholine<ref name="HoneyWEIRf1980">Template:Cite journal</ref>
- Cholecystokinin
- Gastric inhibitory polypeptide
- Gastrin<ref>Template:Cite journal</ref>
Secretion of glucagon is inhibited by:
- Somatostatin
- Amylin<ref name = "Zhang_2018" />
- Insulin (via GABA)<ref name="pmid16399504">Template:Cite journal</ref>
- PPARγ/retinoid X receptor heterodimer.<ref name="pmid17962386">Template:Cite journal</ref>
- Increased free fatty acids and keto acids into the blood.<ref name="Johnson2003">Template:Cite book</ref>
- Increased urea production
- Glucagon-like peptide-1
Function
Glucagon generally elevates the concentration of glucose in the blood by promoting gluconeogenesis and glycogenolysis.<ref>Template:Cite book</ref> Glucagon also decreases fatty acid synthesis in adipose tissue and the liver, as well as promoting lipolysis in these tissues, which causes them to release fatty acids into circulation where they can be catabolised to generate energy in tissues such as skeletal muscle when required.<ref>Template:Cite journal</ref>
Glucose is stored in the liver in the form of the polysaccharide glycogen, which is a glucan (a polymer made up of glucose molecules). Liver cells (hepatocytes) have glucagon receptors. When glucagon binds to the glucagon receptors, the liver cells convert the glycogen into individual glucose molecules and release them into the bloodstream, in a process known as glycogenolysis. As these stores become depleted, glucagon then encourages the liver and kidney to synthesize additional glucose by gluconeogenesis. Glucagon turns off glycolysis in the liver, causing glycolytic intermediates to be shuttled to gluconeogenesis.
Glucagon also regulates the rate of glucose production through lipolysis. Glucagon induces lipolysis in humans under conditions of insulin suppression (such as diabetes mellitus type 1).<ref>Template:Cite journal</ref>
Glucagon production appears to be dependent on the central nervous system through pathways yet to be defined. In invertebrate animals, eyestalk removal has been reported to affect glucagon production. Excising the eyestalk in young crayfish produces glucagon-induced hyperglycemia.<ref name="Leinen_1983">Template:Cite journal</ref>
Mechanism of action
Glucagon binds to the glucagon receptor, a G protein-coupled receptor, located in the plasma membrane of the cell. The conformation change in the receptor activates a G protein, a heterotrimeric protein with αs, β, and γ subunits. When the G protein interacts with the receptor, it undergoes a conformational change that results in the replacement of the GDP molecule that was bound to the α subunit with a GTP molecule.<ref>Template:Cite web</ref> This substitution results in the releasing of the α subunit from the β and γ subunits. The alpha subunit specifically activates the next enzyme in the cascade, adenylate cyclase.
Adenylate cyclase manufactures cyclic adenosine monophosphate (cyclic AMP or cAMP), which activates protein kinase A (cAMP-dependent protein kinase). This enzyme, in turn, activates phosphorylase kinase, which then phosphorylates glycogen phosphorylase b (PYG b), converting it into the active form called phosphorylase a (PYG a). Phosphorylase a is the enzyme responsible for the release of glucose 1-phosphate from glycogen polymers. An example of the pathway would be when glucagon binds to a transmembrane protein. The transmembrane proteins interacts with Gɑβ𝛾. Gαs separates from Gβ𝛾 and interacts with the transmembrane protein adenylyl cyclase. Adenylyl cyclase catalyzes the conversion of ATP to cAMP. cAMP binds to protein kinase A, and the complex phosphorylates glycogen phosphorylase kinase.<ref>Template:Cite journal</ref> Phosphorylated glycogen phosphorylase kinase phosphorylates glycogen phosphorylase. Phosphorylated glycogen phosphorylase clips glucose units from glycogen as glucose 1-phosphate.
Additionally, the coordinated control of glycolysis and gluconeogenesis in the liver is adjusted by the phosphorylation state of the enzymes that catalyze the formation of a potent activator of glycolysis called fructose 2,6-bisphosphate.<ref name="Hue L & Rider MH_1987">Template:Cite journal</ref> The enzyme protein kinase A (PKA) that was stimulated by the cascade initiated by glucagon will also phosphorylate a single serine residue of the bifunctional polypeptide chain containing both the enzymes fructose 2,6-bisphosphatase and phosphofructokinase-2. This covalent phosphorylation initiated by glucagon activates the former and inhibits the latter. This regulates the reaction catalyzing fructose 2,6-bisphosphate (a potent activator of phosphofructokinase-1, the enzyme that is the primary regulatory step of glycolysis)<ref name="Claus, TH et al_1984">Template:Cite book</ref> by slowing the rate of its formation, thereby inhibiting the flux of the glycolysis pathway and allowing gluconeogenesis to predominate. This process is reversible in the absence of glucagon (and thus, the presence of insulin).
Glucagon stimulation of PKA inactivates the glycolytic enzyme pyruvate kinase,<ref name="Feliu JE, Hue L & Hers HG_1976">Template:Cite journal</ref> inactivates glycogen synthase,<ref name="Jiang G, Zhang BB_2003">Template:Cite journal</ref> and activates hormone-sensitive lipase,<ref name="Hayashi Y_2021">Template:Cite journal</ref> which catabolizes glycerides into glycerol and free fatty acid(s), in hepatocytes.
Glucagon also inactivates acetyl-CoA carboxylase, which creates malonyl-CoA from acetyl-CoA, through cAMP-dependent and/or cAMP-independent kinases.<ref name="Swenson TL & Porter JW_1985">Template:Cite journal</ref>
Malonyl-CoA is a product formed by ACC during denovo synthesis and an allosteric inhibitor of Carnitine palmitoyltransferase I (CPT1), a mitochondrial enzyme important for bringing fatty acids into the intermembrane space of the mitochondria for β-oxidation.<ref name="Wang Y, Yu W, Li S, Guo D, He J, Wang Y_2022">Template:Cite journal</ref> Glucagon decreases malonyl-CoA through inhibition of acetyl-CoA carboxylase and through reduced glycolysis through its aforementioned reduction in Fructose 2,6-bisphosphate. Thus, reduction in malonyl-CoA is a common regulator for the increased fatty acid metabolism effects of glucagon.
Pathology
Abnormally elevated levels of glucagon may be caused by pancreatic tumors, such as glucagonoma, symptoms of which include necrolytic migratory erythema,<ref name="pmid27422767">Template:Cite journal</ref> reduced amino acids, and hyperglycemia. It may occur alone or in the context of multiple endocrine neoplasia type 1.<ref name="pmid21167379">Template:Cite journal</ref>
Elevated glucagon is the main contributor to hyperglycemic ketoacidosis in undiagnosed or poorly treated type 1 diabetes. As the beta cells cease to function, insulin and pancreatic GABA are no longer present to suppress the freerunning output of glucagon. As a result, glucagon is released from the alpha cells at a maximum, causing a rapid breakdown of glycogen to glucose and fast ketogenesis .<ref name="pmid18939392">Template:Cite journal</ref> It was found that a subset of adults with type 1 diabetes took 4 times longer on average to approach ketoacidosis when given somatostatin (inhibits glucagon production) with no insulin.Template:Citation needed Inhibiting glucagon has been a popular idea of diabetes treatment, however, some have warned that doing so will give rise to brittle diabetes in patients with adequately stable blood glucose.Template:Citation needed
The absence of alpha cells (and hence glucagon) is thought to be one of the main influences in the extreme volatility of blood glucose in the setting of a total pancreatectomy.
History
In the early 1920s, several groups noted that pancreatic extracts injected into diabetic animals would result in a brief increase in blood sugar prior to the insulin-driven decrease in blood sugar.<ref name=Muller2017>Template:Cite journal</ref> In 1922, C. Kimball and John R. Murlin identified a component of pancreatic extracts responsible for this blood sugar increase, terming it "glucagon", a portmanteau of "glucose agonist".<ref name=Muller2017/><ref name="Kimball_1923">Template:Cite journal</ref>Template:Failed verification In the 1950s, scientists at Eli Lilly isolated pure glucagon, crystallized it, and determined its amino acid sequence.<ref name=Muller2017/><ref>Template:Cite journal</ref><ref name="Bromer_1957">Template:Cite journal</ref> This led to the development of the first radioimmunoassay for detecting glucagon, described by Roger Unger's group in 1959.<ref name=Muller2017/>
A more complete understanding of its role in physiology and disease was not established until the 1970s, when a specific radioimmunoassay was developed.<ref>Template:Cite journal</ref>
In 1979, while working in Joel Habener's laboratory at Massachusetts General Hospital, Richard Goodman collected islet cells from Brockman bodies of American anglerfish in order to investigate somatostatin.<ref name="Molteni">Template:Cite news</ref> By splicing DNA from anglerfish islet cells into bacteria, Goodman was able to identify the gene which codes for somatostatin.<ref name="Molteni" /> P. Kay Lund joined the Habener lab and used Goodman's bacteria to search for the gene for glucagon.<ref name="Molteni" /> In 1982, Lund and Goodman published their discovery that the proglucagon gene codes for three distinct peptides: glucagon and two novel peptides.<ref name="Molteni" /> Graeme Bell at Chiron Corporation led a team which isolated the two latter peptides, which are now known as glucagon-like peptide-1 and glucagon-like peptide-2.<ref name="Molteni" /> This opened the door to the discovery of the glucagon-like peptide-1 receptor and then drugs which target that receptor, known as GLP-1 receptor agonists.<ref name="Molteni" />
See also
- Cortisol
- Diabetes mellitus
- Glucagon-like peptide-1
- Glucagon-like peptide-2
- Insulin
- Islets of Langerhans
- Pancreas
- Proglucagon
- Tyrosine kinase
References
External links
- PDBe-KB provides an overview of all the structure information available in the PDB for Human Glucagon