Insulin Structure & Physiology

Sections

Full-Length Text

INSULIN

• Peptide hormone secreted by pancreatic beta cells
• Promotes anabolic (synthetic pathways): energy requiring
• Binds receptor tyrosine kinases (liver, muscle, adipose tissue, etc.)

PANCREAS

Islets of Langerhans (1-2% of pancreatic beta cells)

• Beta cell: secretes insulin
• Alpha cell: secretes glucagon (opposes insulin)

INSULIN SYNTHESIS AND STRUCTURE

Components of insulin

• N-terminal signal peptide: targets preproinsulin to endoplasmic reticulum
• B chain: portion of final hormone
• C peptide: marker of endogenously synthesized insulin
• A chain: portion of final hormone
• Two disulfide bonds: 1 within A chain, another between B and A chains

Synthesis

• Signal peptide cleaved from preproinsulin in ER of beta cells to form proinsulin
• C peptide cleaved from proinsulin in Golgi apparatus
• Two products: C peptide and insulin

C peptide

• Longer half-life than insulin: marker of insulin synthesis and secretion

Insulin

• Half-life ~ 6 min.

INSULIN SECRETION

Pancreatic beta cell surface

• Voltage-dependent Ca2+ channel (closed)
• ATP-sensitive K+ channel (open): K+ flows down concentration gradient out of cell
• GLUT2: tissue-specific glucose transporter (beta cells and liver cells)

Steps of secretion

1. Eat carbohydrate rich meal: plasma glucose is high
2. Glucose enters beta cell via GLUT2
3. Glucokinase phosphorylates/sequesters glucose in cell (glucose --> glucose 6P)

• Glucokinase: tissue specific (beta cells and liver cells), high Km and high Vmax

4. Glycolysis: Glucose 6P --> ATP
5. ATP binds K+ channel and closes it: depolarizes membrane & activates Ca2+ channel
6. Ca2+ influx promotes exocytosis and release of insulin secretory granules

Pancreatic beta cells: most important glucose-sensing cells

• GLUT2: high Km, only bind glucose when plasma glucose is high
• Glucokinase: High Km, high Vmax & no product inhibition (can continue trapping glucose even when intracellular glucose concen. rise)

INSULIN MECHANISM OF ACTION

Key insulin-sensitive tissues: liver, muscle & adipose tissue

1. Insulin binds receptor tyrosine kinase: activates intracellular RTK beta subunits
2. Autophosphorylation: activated RTK phosphorylates other intracellular proteins
3. Initiates signaling cascade
4. Muscle & adipose: signaling cascade mobilizes GLUT4 transporters from intracellular storage to cell surface

• Increases glucose absorption
• Seconds after insulin binding
• Does not occur in hepatocytes

5. Activates anabolic enzymes: glycogen, protein and lipid synthesis (~minutes/hours)

• Promotes glucose storage when glucose is abundant

6. Inhibits catabolic enzymes: glycogen & lipid breakdown(~minutes/hours)
7. Inhibits gluconeogenesis(~minutes/hours)
8. Long-term response: transcriptional control (~hours/days)

Full-Length Text

• Here we will learn about insulin, a key regulatory hormone in metabolism.

• To begin, start a table to learn some key features of insulin.

• Denote that it is a peptide hormone secreted by pancreatic beta cells.
  • It promotes anabolic (synthetic) pathways in the body, which require energy.
  • It binds receptor tyrosine kinases in specific tissues, which include the liver, muscle and adipose tissue.

We will elaborate on each of these points in this tutorial.

• To begin, draw a pancreas.

• Label a cluster of cells in the pancreas "Islets of Langerhans."
  • They comprise around 1% of pancreatic cells.

We will focus on one specific cell in this cluster: a beta cell.

• Write that it secretes the hormone insulin.
  • The alpha cells of the islets of Langerhans secrete glucagon, which opposes insulin.

To start, let's learn insulin's structure.

• Illustrate its two precursor molecules.

• We'll draw preproinsulin in the shape of a paperclip in four specific segments:
  • First, the N-terminal signal peptide; write that it targets preproinsulin to the endoplasmic reticulum after it is translated in the cytosol.
  • B chain, a part of the final hormone.
  • C-peptide, a marker of endogenously synthesized insulin.
  • A chain, also a part of the final hormone.

• Next, indicate that the signal peptide is cleaved in the endoplasmic reticulum of beta cells.

• Show that this results in proinsulin, which comprises the:
  • B chain
  • C-peptide
  • A chain

• Show that a pair of disulfide bonds hold the B and A chains together.

• Draw another disulfide bond between two amino acids in the A chain.

• Next, show that in the Golgi apparatus of beta cells, the C-peptide is cleaved, which results in two products:
  • Insulin, which is the B chain and A chain.
  • C-peptide.

• Write that the half-life of insulin is about 6 minutes.

• Indicate that C-peptide has a longer half-life than insulin and is a marker of insulin synthesis and secretion.

Now, let's transition to insulin secretion.

• Draw a pancreatic beta cell as a circular outline.

• Within it show some secretory granules.
  • It is these granules that, upon stimulation, will exocytose and release insulin.

Let's see how.

• On the surface of the beta cell, draw:
  • A voltage-dependent calcium channel that is closed.
  • An ATP-sensitive potassium channel.
  • GLUT2: a glucose transporter.

• Indicate that K+ flows down its concentration gradient out of the cell through the potassium channel. - This gradient is maintained by a sodium potassium pump, which we do not include here.

Now, imagine that you eat a carbohydrate rich meal. Your plasma glucose levels are high!

• Show that glucose enters a pancreatic beta cell via GLUT2.
  • From here, glycolysis occurs, in which the cell breaks down glucose to synthesize ATP.

• Draw the first step of glycolysis: the enzyme glucokinase phosphorylates glucose to glucose 6-phosphate and traps it in the cell.

• Show that glucose-6-phosphate breaks down to ATP (we will not draw the carbohydrate intermediates).

• Show that ATP binds the potassium channel.
  • Indicate that this binding causes it to close.
  • This depolarizes the membrane, which activates the voltage-dependent calcium channel.

• Show that the calcium influx promotes exocytosis and the release of insulin we mentioned at the beginning of this section.

• Thus, high plasma glucose stimulates the beta cell to secrete insulin, and it relies on GLUT2 and glucokinase to sense increases in plasma glucose levels.

• Indicate that GLUT2 transporters are tissue-specific: they are only present in pancreatic beta cells and liver cells.

• More importantly, write that they have a high Km; thus, they only bind glucose when plasma glucose is high.

• As for glucokinase, indicate that it is also specific to the liver and pancreas.

• And indicate that:
  • It has a high Km (like GLUT2) and it also has a high Vmax.
  • It is not inhibited by its product, glucose-6-phosphate. It can continue trapping glucose in the cell even when intracellular concentrations rise.

• Thus, write that pancreatic beta cells are the most important glucose-sensing cells in the body, and they rely on GLUT2 and glucokinase!

Finally, let's learn the function that insulin plays in metabolism.

• Draw a cell membrane with a large protein embedded in it.
  • This is our insulin receptor.

• Indicate that it is a receptor tyrosine kinase (RTK).

• Indicate that this cell membrane represents that of three key insulin-sensitive tissues: liver, muscle, and adipose tissue. Insulin predominantly acts upon these three tissues.

Now back to our RTK.

• Label the alpha and beta subunits.

• Step 1: insulin binds the RTK, which activates the intracellular beta subunits.

• Step 2: autophosphorylation allows the RTK to phosphorylate other intracellular proteins.

• Step 3: this initiates a signaling cascade.

What does this signaling cascade do? We'll outline three key responses, starting with the most immediate.

• Show that in muscle and adipose cells, this signaling cascade mobilizes GLUT4 glucose-transporters from intracellular storage to the cell surface.

• Indicate that this increases glucose absorption, and that it occurs seconds after insulin binding.

• Note that this does not occur in hepatocytes, which have GLUT2 transporters not GLUT4.

• Next, show that it activates enzymes involved in anabolic processes: glycogen, protein and lipid synthesis.
  • It promotes storage when glucose is abundant.

• Indicate that it inhibits enzymes in catabolic processes such as glycogen and lipid breakdown, preventing the mobilization of stored carbohydrates, and in gluconeogenesis, preventing the production of new glucose.

• Indicate that this occurs in a matter of minutes to hours.

• Write that the long-term response is at the transcriptional level: it produces an increase in the number of anabolic enzymes.
  • Show that this occurs over a course of hours to days.