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Role of Insulin and Glucagon in the regulation of metabolism
Wk 11 LOs
1. Outline the synthesis of insulin and its release into the circulation
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Insulin is a small molecule consisting of two amino acid chains linked by disulfide linkages
It is synthesized in the beta cells of the Islets of Langerhans in the pancreas
Ribosomes on the surface of the endoplasmic reticulum (ER) translate the insulin mRNA to
form an insulin preprohormone (11.5 kDa).
This is then cleaved in the ER to form proinsulin (~9 kDa).
Most of this is then further cleaved in the Golgi apparatus to form mature insulin and a
cleavage peptide, C peptide.
Both mature insulin and the C peptide are then packaged together into secretory granules
and secreted into the blood stream together after physiologic stimulation.
Secretion of Insulin
 glucose enters the beta cells of the pancreas via GLUT2 transporters
 glucose is then phosphorylated by glucokinase, which traps it inside the cell, and is further
metabolized to create ATP
 ↑ ATP causes ATP-gated K channel to close
 ↑ K+ ions inside the cell depolarizes the membrane and cause voltage gated Calcium
channels to open
 the brisk increase in intracellular [calcium] causes the insulin storage granules to move to
the surface of the cell and for insulin to be released via exocytosis
 the Islets of the pancreas are highly vascular and ensures prompt diffusion of insulin into
the nearby blood vessels
 insulin secretion is biphasic; there is the initial release of insulin stores and then a second
prolonged phase where new insulin has to be synthesized and packaged before release
2. List the signals that trigger the release of (a) insulin and (b) glucagon from the islets
(a) insulin
 high blood glucose levels (>5mmol/L)
 some GI hormones
 ↑ certain amino acids (leucine and arginine) after a protein rich meal
 glucagon
 parasympathetic nervous system and β-receptors of sympathetic nervous system
(b) glucagon
 low blood glucose levels (<4mmol/L)
 some GI hormones
 ↑ blood concentration of certain amino acids (promotes rapid conversion of
aa→glucose
 parasympathetic nervous system and α-receptors of sympathetic nervous system
 exercise/fasting
3. Draw a diagram showing how insulin induces responses in a cell after binding to its plasma
membrane receptor, including the tyrosine kinase activity of the insulin receptor and the
interaction of insulin with the MAP kinase signaling pathway
Insulin receptor (IR)
 membrane-bound tyrosine kinase receptor with 2 α subunits and 2 β subunits;
 the β subunits in the cytoplasmic domain have tyrosine kinase
activity
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the tyrosine kinase activity of the IR phosphorylates the Insulin Receptor Substrates
(IRS) as well as other cytoplasmic proteins such as SHC
IRS are docking proteins to which other proteins bind and become activated
Insulin signaling Pathways
1. The Ras/MAP kinase pathway
 the SH2 domain of the Grb2 protein (growth factor receptor binding protein)
recognizes and binds the phosphorylated tyrosine residues on SHC
 SOS (a guanine nucleotide exchange factor – GEF) bound to GRB, then promotes
the exchange of inactive GDP bound to Ras to GTP, which leads to active Ras
 this leads to a cascade of protein kinase activation, but the major downstream
target of Ras-GTP is MAPK (mitogen activated protein kinase)
 the pathway increases gene expression and cell growth
 transcription – slow response
2. The PI3K (phosphatidyl inositol 3-kinase) pathway
 IRS-1 activates PI3K
 PI3K phosphorylates PIP2→PIP3 (phosphatidylinositol
biphosphate→phosphatidylinsoitol triphosphate)
 ↑PIP3 activates protein kinase B (PKB)
 PKB phosphorylates existing target proteins:
o movement of GLUT4 to membrane
o phosphorylation of glycogen synthase kinase which inactivates it, thereby
allowing glycogen synthase to remain active
o synthesis of lipids, proteins and glucagon
 Phosphorylation – rapid response
4. Draw a diagram to show how glucagon induces responses in a cell after binding to
its receptor by activating cAMP-dependent protein kinase
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Glucogon is a 29aa single polypeptide chain that is synthesized in the α cells of the Islets
of Langerhans (also has prepro and pro precursors)
it binds to a G-protein coupled receptor (GPCR) on the surface of cells
activation of the GPCR allows Gs (in the cytoplasm) to dissociate and become activated
Gs activation leads to the activation of adenylate cyclase (AC)
AC catalyses production of cAMP which activates protein kinase A (PKA)
PKA phosphorylates (and therefore modifies the activities of):
o glycogen synthase
o acetyl Co-A carboxylase
o phosphorylase
PKA also activates transcription factors such as CREB leading to transcription of certain
genes
5. Outline how the insulin:glucagon ratio regulates blood glucose concentrations in
feeding and fasting
Insulin and glucagon function as important feedback control systems for maintaining a normal
blood glucose concentration. The normal plasma insulin:glucagon ratio is around 2
Fed State:
 When the blood glucose level is too high (e.g. after a meal), insulin is secreted and causes
the blood glucose concentration to decrease toward normal.
 Insulin:glucagon ratio is high and may reach 10:1 (due to ↑insulin)
 ↑ glucose uptake through GLUT4
 ↑ glycogen synthesis (liver and muscle)
 ↓ lipolysis and ↑ fat storage
 ↓ proteolysis
Fasting/exercising state:
 Decrease in blood glucose stimulates glucagon secretion. Glucagon then functions in the
opposite direction to increase blood glucose concentration to normal.
 Insulin:glucagon ratio is low and when <0.5 (due to ↑glucagon and ↓insulin)
 ↑ glycogenolysis
 ↑ aa metabolism
 ↑ gluconeogenesis
 ↑ lipolysis
Under most normal conditions, the insulin feedback mechanism is much more important than the
glucagon mechanism, but in instances of starvation, or excessive utilization of glucose during
exercise or other stressful situations, the glucagon mechanism also becomes important.
6. Describe how insulin promotes glycogen synthesis, glycolysis, fatty acid synthesis
and the uptake of glucose into muscle and adipose tissue
Glycogen Synthesis
Insulin promotes liver uptake and storage of glucose by:
 inactivating liver phosphorylase, the principal enzyme that causes liver glycogen to
be split into glucose
 ↑ transcription of glucokinase, one of the enzymes that causes the initial
phosphorylation of glucose to glucose-6-P, trapping it inside the cell
 phosphorylates glycogen synthase kinase which activates glycogen synthase
Glycolysis
 insulin ↑ glycolysis by activating the following enzymes:
o glucokinase
o phosphofructokinase
o pyruvate kinase
Fatty acid synthesis
Insulin promotes the conversion of excess glucose into fatty acids:
 insulin increases the transport of glucose into liver cells
 after the liver glucose concentration reaches 5-6%, glycogen synthesis is inhibited and all
additional glucose entering the liver is available to form fat
 ↑ pyruvate dehydrogenase - pyruvate → Acetyl-CoA
 ↑ acetyl-CoA carboxylase which carboxylates acetyl-CoA to form malonyl-CoA, the first
stage of fatty acid synthesis
 ↑ activity of fatty acid synthase
 ↑ activity of lipoprotein lipase
Uptake of glucose into muscle and adipose tissue
 ↑ recruitment of GLUT4 glucose transporters to the plasma membrane of muscle and fat
cells
7. Describe how glucagon stimulates glycogen breakdown, gluconeogenesis and fatty acid
oxidation
Glycogenolysis
 inhibits glycogen synthase (inhibits glycogen synthesis) and glucokinase
 ↑ activity of glycogen phosphorylase
 ↑ activity of glucose-6-phosphorylase
Gluconeogenesis
 ↑ activity of phosphoenolpyruvate carboxykinase
 ↑ activity of fructose – 1,6 – bisphosphatase
 ↑ activity of glucose-6-phosphorylase
 ↓ activity of glucokinase, phosphofructokinase and pyruvate kinase
Fatty Acid Oxidation
 ↓ activity of Acetyl-CoA carboxylase which ↓ malonyl CoA which ↑ activity of CAT
(carnitine acylcarnitine transferase – helps break down fat)
 ↑ activity of hormone sensitive lipase