Biology: Insulin Structure and Function

Podcast

Play an AI-generated podcast conversation about this lesson

Questions and Answers

What is the normal insulin level in blood?

1-5 mIU/mL

15-25 mIU/mL

25-50 mIU/mL

5-15 mIU/mL (correct)

What is the role of adrenaline during stressful conditions or exercise?

Suppresses insulin release and mobilizes glucose from liver for energy purpose (correct)

Stimulates insulin release

Activates protein phosphatase I

Increases DNA synthesis

What is the half-life of insulin in the plasma?

Several hours

About 1 hour

Less than 5 minutes (correct)

More than 30 minutes

What are the two polypeptide chains of insulin?

Glycine and phenylalanine chains Signup and view all the answers

How are the two chains of insulin joined together?

By 2 interchain disulphide bonds Signup and view all the answers

What is involved in the degradation of insulin?

Insulinase Signup and view all the answers

What is the result of insulin binding to its receptor?

Oligomerization of α units and tyrosine kinase activity Signup and view all the answers

What is the first step in the biosynthesis of insulin?

The insulin is synthesized as preproinsulin Signup and view all the answers

What is the trigger for insulin secretion?

An elevation of glucose level Signup and view all the answers

What is induced by insulin at the transcriptional level?

Phosphofructokinase Signup and view all the answers

What is the effect of insulin on Glucose-6-phosphatase?

Represses its synthesis Signup and view all the answers

What is the role of GLUT2 in insulin secretion?

It allows the entry of glucose into the β-cell Signup and view all the answers

What is the result of the influx of Ca2+ in insulin secretion?

The release of insulin into the blood Signup and view all the answers

What is the role of insulin in DNA synthesis?

Increases DNA synthesis Signup and view all the answers

What is the primary function of insulin in relation to glucose metabolism?

Uptake of glucose by tissues Signup and view all the answers

Which of the following hormones also stimulates insulin secretion?

All of the above Signup and view all the answers

What is the primary glucose transporter in skeletal muscle and adipose tissue?

GluT4 Signup and view all the answers

What is the effect of insulin on glycolysis?

Insulin stimulates glycolysis Signup and view all the answers

What is the result of insulin's anti-lipolytic effect?

Decreased lipolysis in adipose tissue Signup and view all the answers

What is the normal glucagon level in blood?

50-100 pg/mL Signup and view all the answers

What is the result of a decrease in blood glucose level on glucagon secretion?

Increase in glucagon secretion Signup and view all the answers

What is the effect of free fatty acids and ketone bodies on glucagon secretion?

Suppress glucagon secretion Signup and view all the answers

What binds to GTP in the mechanism of glucagon action?

The α subunits of the G protein Signup and view all the answers

What is the result of insulin's effect on gluconeogenesis and glycogenolysis?

Insulin inhibits gluconeogenesis and glycogenolysis Signup and view all the answers

What is the result of GTP-G protein activating adenylate cyclase?

Conversion of ATP to cAMP Signup and view all the answers

What is the primary target of glucagon for its glycogenolytic effect?

Liver Signup and view all the answers

Which enzyme is inhibited by glucagon?

Glycogen synthase Signup and view all the answers

What is the effect of glucagon on plasma FFA level?

Increases plasma FFA level Signup and view all the answers

What is the role of cAMP in the glucagon signaling pathway?

Activates protein kinase Signup and view all the answers

Which enzyme is induced by glucagon to favour gluconeogenesis?

PEPCK Signup and view all the answers

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.

Studying That Suits You

Use AI to generate personalized quizzes and flashcards to suit your learning preferences.

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.