Glucagon: Chemistry and Functions | Hormones

In this article we will discuss about the Chemistry and Functions of Glucagon.

Chemistry of Glucagon:

1. Glucagon is a polypeptide with a molecu­lar weight of 3,485.

2. It consists of 29 amino acid having 15 dif­ferent amino acids. The amino acids are arranged in a straight chain.

3. It contains no cystine, proline or isoleucine but contains sufficient amounts of me­thionine and tryptophan.

4. It can be crystallized in the absence of zinc or other metals.

5. A pro-glucagon precursor of about 9,000 Daltons is also identified.

6. It originates primarily in the alpha-cells of the pancreas, although a considerable amount comes from extra-hepatic alpha-cells in the stomach and other portions of the gastrointestinal tract.

7. A glucagon-like immunoreactive factor (GLI) is also identified in gastric and duo­denal mucosa but less active than pancre­atic glucagon.

Factors influencing secretions of glucagon:

1. Low blood glucose increases secretion of pancreatic glucagon.

2. Most amino acids, particularly arginine, cause a rapid secretion of glucagon from the pancreas.

3. Fatty acids inhibit glucagon release.

4. Exercise stimulates the secretion of glu­cagon.

5. During mixed meals, both insulin and glu­cagon are secreted, but the carbohydrate meal causes insulin release. High protein meal favours glucagon secretion.

6. In stress, insulin secretion is inhibited but glucagon secretion is stimulated.

Functions of Glucagon:

1. Glucagon is sensitive to the adenylate cy­clase receptor sites in the liver and in­creases cAMP level. cAMP activates the enzyme protein kinase which further acti­vates phosphorylase. Phosphorylase causes glycogenolysis making available of glucose in blood. cAMP suppresses gly­cogen synthetase. Thus, glucagon in­creases glycogen breakdown and inhibits synthesis of glycogen.

2. Glucagon can also activate various phospho-enzymes by activating protein kinase and inhibit de-phospho-enzymes. Thus, glucagon can directly stimulate glu­coneogenesis by activation of pyruvate carboxylase and fructose-1, 6-di-phosphatase.

3. It also inhibits glucose oxidation by in­hibiting pyruvate kinase and pyruvate dehydrogenase.

4. Elevated glucagon concentration also in­creases ketogenesis.

5. It increases potassium release from the liver.

6. In adipose tissue as well as in liver, it in­creases the breakdown of lipids to fatty acids and glycerol.

7. Crystalline glucagon polypeptide is used for the treatment of hypoglycemic persons. Glucagon is also used as a diagnostic test for glycogen storage disease.

Normal level of glucagon (fasting) in serum: 20-100 Pg/ml.