block 4- enzymes and ligand binding Flashcards
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 preprpinsulin= 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?
he extracellular component has 6 individual
domains. These individual domains enable
mobility and flexibility of the ECD, and are involved
with the recognition and binding of insulin.
The transmembrane domain (TM) consists of a
short 23 amino acid helical structure.
The intracellular component has 3 individual
domains, including the enzymatic kinase domain
(TK), and a juxtamembrane (JM) region for binding
with signalling proteins.
activatiom of insulin receptor following insulin binding
- when insulin binds = conformational restructuring on receptor
- forms a T-shape conformation from a inverted u shape
-each monomer has 2 insulin binding sites. binding site one composed mainly of L1 and L2 domains of one protomer and the CR domain of another.
-insulin binding site 2 is primarilyn the inactive Λ-shape conformation the membrane proximal FnIII-3
domains are far apart
Following insulin binding, these domains are brought together
what hapeens when you bring the tyresine kinases domains together?
-has a Nlobe and c-lobe and activation loop
-Hinge region allows the separate lobes
to open to accommodate both ATP and
substrate to bind to the kinase
-ATP binds to the n-lobe
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
phosphorylation of tyrosine
-missed explanation
-see lecture
why the phosphorylation of tyrosine?
-the crystal structure of an inactive insulin
receptor tyrosine kinase revealed a novel
autoinhibition mechanism.
The ‘activation loop’ behaves like a
pseudosubstrate and blocks the active site of the kinase, using Tyr1162
-Tyr1163 stabilises this conformation using
a hydrogen-bond to glutamic acid residue
(negatively charged
what is the action of activated tyrosine kinase?
the activation of the tyrosine kinase,
following the conformational change of
the activation loop ‘opens up’ the active
site.
→ Allows for the binding and
phosphorylation of substrate proteins,
such as Insulin Receptor Substrate
(IRS).
-IRS acts as adapter proteins as it doesn’t itself activate anything, instead the phosphorylated IRS
acts as an attachment point for phosphoinositide 3-kinase (PI3K) → downstream signalling.
