Lecture 5 + 6 Cell signals 1 + 2

Question 1

What is signal transduction?

Answer 1

we need receptors to respond to extracellular signals like hormones
rectors can be intracellular or on the cell surface, as most signalling molecules do not cross the membrane
the initial ligand binding is then transduced throughout the cell via other intracellular signalling components to generate a response.

Question 2

what are the 3 super families of cell surface receptor?

Answer 2

g protein coupled receptors (7TM) called GPCR’s - GPCR’s are used in a massive range of drug treatments (40%)
ligand gated ion channels
receptors with intrinsic enzymic activity - tyrosine kinanses - insulin receptor

Question 3

what is a ligand, what is its relationship to receptors?

Answer 3

ligands bind to receptors, they are incredibly specific, so they only interact with that receptor.

Question 4

what is an agonist ? give examples with clinical use.

Answer 4

they bind to the receptor and activate a response
B2 adrenoceptor agonsits like salbutamol
these are anti - asthma drugs that cause bronchodilation
u- opiod rector agonists like morphine
for pain relief

Question 5

what is an antagonists, give examples with clinical use.

Answer 5

antagonists bind to the recptor and do not activate it - they block effects of agonists
B2 adrenoceptor antagonists like propanolol
this helps cardiovascular hypertension
the D2 dopmine receptor antagonists like haloperidol
anti schizophrenic drug

Question 6

why are GPCR’s useful clinically?

Answer 6

they make up 40 % of prescription drugs - and are used to treat many different issues from heart problems to asthma

GPCR's can respond to a variety of different stimuli -
sense light, odours, taste
ions
neurotransmitters
hormones

Question 7

what is the structure of a GPCR and how does this relate to its function?

Answer 7

they have a common 7 transmembrane structure
this 7TM is always the same size, yet GPCR’s vary in size, this is due to variations in size in the-
extracellular N terminus
intracellular C terminus
GPCRs can be 300-1200 amino acids in length

it binds in two ways
forming a high affinity transmembrane binding pocket
the N terminal region can form the binding domain

when the ligand binds the GPCR will undergo a confirmation shape change to activate it
the activated GPCR can now find and activate intracellular G proteins

Question 8

how do G proteins signal responses within the cell ?

Draw this if you can.

Answer 8

a g protein is made up of alpha-a beta-b and gamma-y sub units
the deactivated G protein will have a and by sections
the alpha section will be binded to GDP
when it binds to the activated GPCR the GDP is exchanged with GTP
this switches ‘on’ the g protein, which due to affinty changes now dissociates into a and by sections
these can now both interact with effector proteins - to produce the desired effect

Question 9

how is G protein signalling terminated ?

how can this be varied?

Answer 9

GTPase on the alpha section will hydrolyse GTP back to GDP.
the a unit will now reform the inactive aby g protein
switching off the signal

we have proteins that can regulate GTPase activity, this can allow for a variable timer function in the pathway
in/decreasing GTPase activity will affect levels of signal amplification.

Question 10

GPCR’s specifically interacts with specific G proteins.

what does this mean and why does this happen?

Answer 10

the alpha sub unit is what determines the specific interaction. can be Gas or Gai or Gaq for example
these specific sub units then interact with specific effector proteins
this means extracellular signalling via GPCR’s will activate a single or small amount of G proteins and effectors to give a SPECIFIC cellular response.

Question 11

Give examples of these specific g proteins and there affects.

Answer 11

the B-adrenoceptor - preferentially binds to Gas to stimulate adenylyl cyclase activity
the a2-adrenoceptor - preferentially binds to Gai to inhibt adenylyl cyclase activity

Question 12

how do the toxins cholera (CTx) and pertussis (PTx) interfere with G protein function?

Answer 12

PTx stops the loss of GDP for GTP, to bind on the a sub unit
this stops the pathway and signal transduction effects

CTx blocks the actions of GTPase converting GDP to GTP. so the reaction is stuck permanently ‘ON’ - prevents termination
as recombination cannot happen

Question 13

what is the pathway from signal to respone?

Answer 13

ligand binds to GPCR - there is a singal transudction pathway - reaches and effector - activation of a cellular response.

Question 14

What is a benefit of the bilayer in cell signal transudction

Answer 14

it is a 2D surface in effect.

this allows all components to be in a localised ordered domain, allowing for quicker responses

Question 15

effectors can be enzymes or ion channels

give examples of both.

Answer 15

adenylyl cyclase - converts ATP to cyclic AMP
phospholipase C - PIP2 to IP3 and DAG

ion channels
Voltage gated ion channels - VOCC’s
GIRKs - K+ channels

Question 16

Give examples of agonist stimulated regulation of adenylyl cyclase.

Answer 16

adenylyl cyclase can be stimulated by Gs coupled receptors such as B adrenoceptors
once the Gs Proteins is activated it can activate Adnenylyl cyclase (AC) to convert ATP into cyclic AMP
which can then siganl many thinds such as CAMP dependent protein kinase (PKA)

AC can be inhibited by Gi coupled receptors such as a2- adrenoceptors.
the activated Gi will inhibit AC stopping the production of Cyclic AMP, so no more signalling effects.
it can inhibit the effects of Gs proteins

Question 17

How does Cyclic AMP exert actions through cyclic AMP dependent Protein kinase receptors (PKA’s)

Answer 17

PKA is a molecule made of 2 regulatory and 2 catalytic sub units
cAMP can bind twice to each regulatory receptor section
this releases the catalytic subunits, than can now cause effects.

protein kinase will phosphorylate proteins

Question 18

How does agonist stimulated reception of phospholipase C work ?

Answer 18

IP3 activation can increase cytoplasmic calcium ten times in seconds - quick response

a Gq protein couples to a receptor such as a1- adrenoceptor. it is activated and can then activate the enzyme phospholipase C this will hydrolyse PIP2 to IP3 and DAG
these are second messengers
IP3 acts at a IP3 receptor at the endoplasmic rectiulum
opens a calcium channel, calcium enters the cytoplasm rapidly
the DAG binds to a PKC (protein kinase) which can then
also be activated by calcium from ER.
the PKC can then go onto phosphorylate proteins

Question 19

what is the concept of signal amplification?

Answer 19

a few molecules of eg adrenaline bind to cell surface B adrenoceptors to cause a massive cellular response
as it activates a Gs protein which stimulates adenylyl cyclase, which activates even more PKA molecules which each interact with multiple target proteins.
this is a cascade reaction with the signal amplifying at each step.

Question 20

clinical example - Intropy

Answer 20

intropy (more forceful heartbeat)
adrenaline and sympathetically released noradrenaline intercatct with ventricular B1-adrenoceptors to increase contraction force.
Gs stimulates AC to make cAMP, signals PKA, which phosphorylates a VOCC to allow more Ca into the cell, more calcium means a stronger contraction
PKA can also allows Ca relase from the ER

Question 21

clinical example - Smooth Muscle Contraction

Answer 21

sympathetically released noradrenaline can interact with a1-adrenorecptors on vascular smooth muscle to cause vasoconstriction of arterioles

utilises the Gq - phospholipase C - IP3 - Ca2+, DAG, protein kinase C pathway

the calcium initiates contraction
the Protein kinase C phosphorylates proteins to provide sustained contraction

parasymathetically relased acetylcholine can interact with bronchiolar smooth muscle M3 muscarininc receptors to cause bronchoconstriction - also uses this pathway.

Question 22

Clinical Example - modulation of neurotransmitter release

Answer 22

in the CNS and Peripheral-NS neurotransmitter release is often modulated by presynaptic G protein coupled recptors.

a Gi U opioid receptor is triggered by morphine, to relase the Gi alpha sub unit and the Beta gamma subunit
the by sub unit can bind to a VOCC and vlock Calcium from entering the cell, by inhibiting the VOCC.

this reduces Calcium infulx and neurotransmitter release

Question 23

summarise the benefits of signal transduction in biological membranes

Answer 23

1) diversity - so many different GPCR’s that there is a diverse range of stimuli and effects possible
2) Specificity - specific ligand - recpetor interactions, specific G protein a subunits, coupled to particular effector pathways allows for an incredibly specific response
3) Amplification - a small change extracellulary cab through multiple cascading pathways provide significant cellular changes. making this system very responsive