Questions and Answers
What is the normal insulin level in blood?
What is the role of adrenaline during stressful conditions or exercise?
What is the half-life of insulin in the plasma?
What are the two polypeptide chains of insulin?
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How are the two chains of insulin joined together?
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What is involved in the degradation of insulin?
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What is the result of insulin binding to its receptor?
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What is the first step in the biosynthesis of insulin?
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What is the trigger for insulin secretion?
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What is induced by insulin at the transcriptional level?
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What is the effect of insulin on Glucose-6-phosphatase?
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What is the role of GLUT2 in insulin secretion?
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What is the result of the influx of Ca2+ in insulin secretion?
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What is the role of insulin in DNA synthesis?
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What is the primary function of insulin in relation to glucose metabolism?
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Which of the following hormones also stimulates insulin secretion?
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What is the primary glucose transporter in skeletal muscle and adipose tissue?
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What is the effect of insulin on glycolysis?
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What is the result of insulin's anti-lipolytic effect?
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What is the normal glucagon level in blood?
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What is the result of a decrease in blood glucose level on glucagon secretion?
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What is the effect of free fatty acids and ketone bodies on glucagon secretion?
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What binds to GTP in the mechanism of glucagon action?
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What is the result of insulin's effect on gluconeogenesis and glycogenolysis?
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What is the result of GTP-G protein activating adenylate cyclase?
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What is the primary target of glucagon for its glycogenolytic effect?
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Which enzyme is inhibited by glucagon?
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What is the effect of glucagon on plasma FFA level?
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What is the role of cAMP in the glucagon signaling pathway?
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Which enzyme is induced by glucagon to favour gluconeogenesis?
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Study Notes
Insulin
- Insulin is a protein hormone synthesized and secreted by the β-cells of the islets of Langerhans in the pancreas.
- Responsible for regulating blood glucose levels.
- Normal insulin level in blood: 5-15 mIU/mL.
- Insulin is composed of two polypeptide chains (glycine and phenylalanine chains) with 21 and 30 amino acids, respectively.
- The two chains are joined together by two interchain disulphide bonds and an intrachain disulphide bond in the α-chain.
Biosynthesis of Insulin
- Insulin is synthesized as preproinsulin.
- Preproinsulin is rapidly converted to proinsulin in the endoplasmic reticulum.
- Proinsulin is transported to the Golgi apparatus, where it is cleaved by a protease to form insulin.
Secretion of Insulin
- Insulin secretion is triggered by an elevation of glucose levels.
- GLUT2 allows the entry of glucose into the β-cell.
- Glucose is metabolized, producing ATP, which closes potassium channels and activates calcium channels.
- The influx of Ca2+ causes the release of insulin into the blood.
Factors Affecting Insulin Secretion
- Glucose
- Gastrointestinal hormones (e.g., secretin, cholecystokinin, and gastrin)
- Incretin hormones (e.g., glucagon-like peptide-1 and glucose-dependent insulinotropic polypeptide)
- Proteins and amino acids (e.g., leucine and arginine)
- Epinephrine
Degradation of Insulin
- Insulin is rapidly degraded by the liver.
- Plasma half-life is less than 5 minutes.
- Insulin-specific protease (insulinase) is involved in the degradation of insulin.
Mechanism of Insulin Action
- Insulin receptors and signal transduction:
- Insulin binds to the α subunits of the receptor.
- Oligomerization of α units triggers the tyrosine kinase activity of the β subunit.
- The phosphorylated sites act as binding sites for insulin receptor substrates (IRS).
- Gene transcription (new enzyme synthesis):
- Insulin induces the synthesis of enzymes such as glucokinase, pyruvate kinase, phosphofructokinase, and acetyl CoA carboxylase.
- Insulin represses the synthesis of enzymes such as glucose-6-phosphatase, phosphoenolpyruvate carboxykinase, and fructose-1,6-bisphosphatase.
- Activation of enzymes:
- Insulin activates existing enzymes by covalent modification (phosphorylation or dephosphorylation).
- Insulin activates protein phosphatase I, which dephosphorylates enzyme proteins.
- DNA synthesis:
- Insulin increases DNA synthesis, cell growth, and anabolism through the IRS pathway.
Physiological Actions of Insulin
- Regulation of carbohydrate metabolism:
- Insulin facilitates the uptake of glucose by tissues.
- Glycolysis is stimulated by insulin.
- Insulin promotes glucose storage and utilization.
- Regulation of lipid metabolism:
- Lipogenesis is favored by insulin.
- Insulin inhibits lipolysis in adipose tissue.
Glucagon
- Glucagon is a protein hormone synthesized and secreted by the α-cells of the islets of Langerhans in the pancreas.
- Glucagon has the opposite effect of insulin.
- Normal glucagon level in blood: 50-100 pg/mL.
- Glucagon is a polypeptide hormone with 29 amino acids.
- Plasma half-life is about 5 minutes.
Biosynthesis of Glucagon
- Glucagon is synthesized initially as proglucagon (160 amino acids).
- Proglucagon undergoes cleavage to form glucagon.
Factors Affecting Glucagon Secretion
- Decrease in blood glucose level
- Amino acids (e.g., arginine, alanine) derived from a protein-containing meal
- Epinephrine and norepinephrine
- Free fatty acids and ketone bodies suppress secretion of glucagon
- Insulin and somatostatin inhibit glucagon secretion
Mechanism of Glucagon Action
- Glucagon combines with a membrane-bound receptor.
- This activates the GDP-bound G-protein, converting it into GTP.
- The α subunits of the G protein dissociate from the β and γ subunits.
- The α subunits bind to GTP, activating adenylate cyclase to convert ATP to cAMP.
- cAMP combines with the regulatory subunit of the protein kinase.
Physiological Actions of Glucagon
- Glycogenolysis:
- Glucagon stimulates glycogenolysis in the liver.
- The active form of glycogen phosphorylase is formed under the influence of glucagon.
- Gluconeogenesis:
- Glucagon favors gluconeogenesis by inducing enzymes such as PEPCK, glucose-6-phosphatase, and fructose-1,6-bisphosphatase.
- Increases plasma FFA level:
- In adipose tissue, glucagon favors beta-oxidation, as it activates carnitine acyltransferase.
- Inhibits glycogen synthesis:
- Glucagon inhibits glycogen synthesis by inactivating glycogen synthase.
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Description
This quiz covers the structure, function, and biosynthesis of insulin, a protein hormone that regulates blood glucose levels. Learn about its composition, properties, and normal levels in the blood.
