block 4- enzymes and ligand binding Flashcards
(14 cards)
how do we get a rise in glucose in the body?
-when we eat a meal rich in carbohydrates = Our body breaks down the carbohydrate (starch) into glucose, using an enzyme called amylase, which
results in increase glucose concentration in the blood
-A rise in blood glucose can be toxic, if the levels are too high for a long time → excess glucose can create
reactive oxygen species (ROS) that lead to chronic oxidative stress
how does the body respond to an increase in glucose levels?
-The body responds by stimulating the beta cells of the Islets of Langerhans in the pancreas to release
insulin molecules into the blood, which initiates the signal transduction pathway
-The pathway stimulates the absorption of the glucose by the cells and the subsequent transformation of
glucose into glycogen form. → How does insulin induce the signal transduction pathway
structure of insulin
-insulin is expressed as preproinsulin= 110 amino-acid precursor
-Preproinsulin has its signal peptide removed → proinsulin (86 amino-acids).
-During folding, proinsulin then forms 3 disulphide bonds
between the A-chain (C-terminus of proinsulin) and the B-
chain (N-terminus).
-The proinsulin is then cleaved by 2 serine proteases (one
of which is related to subtilisin), and carboxypeptidase,
resulting in the mature, active insulin.
Active insulin is a 51 amino-acid protein, consisting of two
chains (which are then stored in beta cells ready for use).
where are insulin receptors found?
- they are expressed both on the surface of hepatocytes (liver cells).where it promotes glycogenesis and inhibits gluconeogenesis; and
skeletal muscle and fat tissue, where it facilitates glucose uptake by
activating the glucose transporter GLUT4.
what happens when there is dsyregulation of insulin receptors
-linked to many human diseases e.g. cancer and diabetes
glycogenesis
conversion of glucose to
glycogen
gluconeogenesis
biochemical synthesis of
glucose
what happens when insulin binds to the insulin receptor
- Binding of insulin to the extracellular part of the insulin
receptor… - stimulates the interaction between insulin receptor
substrate (IRS) and the intracellular part of the insulin
receptor - which activates PI3K…
- which activates Protein kinase B (AKT)…
- which stimulates the translocation of glucose
transporter 4 (GLUT4) to the cell surface. - which allows the facilitated diffusion of glucose across
the plasma membrane, reducing blood sugar levels - furthermore, AKT inhibits GSK-3 which allows for
glycogen synthase (GS) to covert glucose to glycogen
insulin receptor domain structure?
-insulin receptors are derived from a single
polypeptide containing over 1300 amino
acids (> 150,000 Da).
Multi-domain protein, which is post-
translationally cleaved into two subunits: ⍺-
subunit and β-subunit, which are cross-linked
with disulphide bonds to form a homodimer.
→ Important for maintaining the 3D-structure.
→ Lots of domains give the protein flexibility.
componenets of the insulin receptor domain structure?
- Extracellular Domain (ECD):
Located outside the cell
Contains 6 individual domains
Functions:
Provides mobility and flexibility
Responsible for recognition and binding of insulin
🔹 2. Transmembrane Domain (TM):
A short 23-amino acid α-helical structure
Anchors the receptor in the cell membrane
🔹 3. Intracellular (Cytoplasmic) Domain:
Inside the cell
Contains tyrosine kinase activity
Triggers downstream insulin signaling when insulin binds
activatiom of insulin receptor following insulin binding
- Conformational Change:
Insulin binding causes a major structural change in the receptor.
It shifts from an inverted U-shape to a T-shaped conformation.
This movement brings the intracellular kinase domains closer together, enabling activation.
🔹 2. Insulin Binding Sites:
Each insulin receptor is made of two monomers (protomers).
Each monomer has two insulin-binding sites:
Site 1:
Formed by the L1 and L2 domains of one monomer and the CR domain of the other.
Site 2:
Involves the FnIII-3 domains (membrane-proximal).
In the inactive Λ-shape, these domains are far apart.
🔹 3. Activation:
Insulin binding brings the FnIII-3 domains close together.
This triggers autophosphorylation of the intracellular tyrosine kinase domains, initiating the insulin signaling cascade.
What happens when the insulin receptor tyrosine kinase domains are brought together?
Insulin binding induces a conformational change in the receptor from an inverted “U” to a “T” shape.
This brings the intracellular tyrosine kinase domains into close proximity, triggering autophosphorylation on specific tyrosine residues.
Each kinase domain includes:
N-lobe (binds ATP)
C-lobe (binds substrate)
Activation loop (blocks active site when inactive)
The hinge region allows the kinase to open, enabling ATP and substrate binding.
Key tyrosines (e.g., Tyr1162, Tyr1163) in the activation loop are phosphorylated.
In the inactive state, Tyr1162 acts like a pseudosubstrate, blocking the active site.
Tyr1163 helps stabilize this inhibition through a hydrogen bond.
Phosphorylation relieves this autoinhibition, fully activating the kinase.
how do kinase enzymes work?
-Kinases are enzymes that regulate cellular processes by catalysing the phosphorylation reaction on
protein substrates, using ATP as an enzyme cofactor
-tyrosine kinases, serine-threonine kinases and histidine kinases – dictates what residue type is
phosphorylated by that particular kinase
What is the function of the activated insulin receptor tyrosine kinase?
Upon activation, the receptor’s active site becomes exposed, allowing it to phosphorylate target proteins.
One major target is Insulin Receptor Substrate (IRS).
IRS does not initiate signaling itself but serves as a docking platform.
Phosphorylated IRS recruits downstream effectors like phosphoinositide 3-kinase (PI3K).
This triggers the PI3K–Akt signaling pathway, leading to:
Glucose uptake
Glycogen synthesis
Inhibition of gluconeogenesis
Cell growth and survival
