Lecture 7 - Signal Transduction in Biological Membranes

Question 1

describe the structure of a g protein

Answer 1

heterotrimeric - made of three distinct subunits - alpha, beta, gamma
beta and gamma subunits tightly bound - function as a single unit
alpha subunit binds GTP and hydrolyses it to GDP

Question 2

how does a g protein work?

Answer 2

1. agonist binds to receptor
2. protein interaction releases GDP, binds GTP
3. alpha-GTP and beta-gamma are released and interact with effectors
4. GTP hydrolysed to GDP
5. alpha-GDP and beta-gamma reform heterotrimer

Question 3

what are the three intermediate g proteins?

Answer 3

gs - stimulatory - stimulate adenylyl cyclase to produce camp
gi - inhibitory - inhibit adenylyl cyclase reducing camp
gt - transducing - activates a phosphodiesterase enzyme to hydrolyse cyclic gmp to 5’-gmp

Question 4

what is the g protein pathway for adrenaline and noradrenaline?

Answer 4

receptor: B-adrenoceptor
g protein: gs
effector: stimulates adenylyl cyclase
physiological response: glycogenolysis, lipolysis

Question 5

what are the two g protein pathways for acetylcholine?

Answer 5

1. receptor: M3
   g protein: gq
   effector: stimulates phospholipase c
   physiological response: smooth muscle contraction
2. receptor: M2
   g protein: gi
   effector: inhibits adenylyl cyclase and stimulates k channel
   physiological response: slowing of cardiac pacemaker

Question 6

what is the g protein pathway for light?

Answer 6

receptor: rhodopsin
g protein: gt
effector: stimulates cyclic gmp phosphodiesterase
physiological response: visual excitation

Question 7

which g protein alpha-subunits do adrenergic receptors utilise?

Answer 7

a1 - gq (stimulates phospholipase c)
a2 - gi (inhibits adenylyl cyclase)
B1 - gs (stimulates adenylyl cyclase)
B2 - gs (stimulates adenylyl cyclase)
(QISS QIQ)

Question 8

which g protein alpha-subunits do cholinergic receptors utilise?

Answer 8

M1 - gq (stimulates phospholipase c)
M2 - gi (inhibits adenylyl cyclase)
M3 - gq (stimulates phospholipase c)

Question 9

how many possible g-alpha-beta-gamma combinations are there?

Answer 9

over 1000

Question 10

what does cholera toxin do?

Answer 10

eliminates gtpase activity of gs-alpha so it is irreversibly activated

Question 11

what does pertussus toxin do?

Answer 11

interferes with gdp/gtp exchange on gi-alpha so it is irreversibly inactivated

Question 12

what diseases are caused by mutations to gpcrs?

Answer 12

retinitis pigmentosa
nephrogenic diabetes insipidus
familial male precocious puberty

Question 13

what causes retinitis pigmentosa?

Answer 13

loss of function mutation to rhodopsin

Question 14

what causes nephrogenic diabetes insipidus?

Answer 14

loss of function mutation to v2 vasopressin receptor

Question 15

what causes familial male precocious puberty?

Answer 15

gain of function mutation to lh receptor

Question 16

what is adenylyl cyclase?

Answer 16

integral plasma membrane protein
enzyme that hydrolyses cellular atp to generate cyclic amp
camp interacts with pka to phosphorylate other proteins

Question 17

what is phospholipase c?

Answer 17

enzyme that hydrolysis PIP2 to IP3
IP3 interacts with specific intracellular receptors on the er
allows Ca to leave the er lumen and enter the cytoplasm

Question 18

what is cyclic gmp phosphodiesterase?

Answer 18

specialised mechanism in photoreceptive cells of retina

regulates breakdown of cyclic gmp phosphodiesterase by gt

Question 19

explain the deactivation of gpcr pathways

Answer 19

1. once a receptor has productively interacted with a g protein, the agonist binding is weakened so agonist-receptor dissociation can happen
2. while activated, the receptor is susceptible to protein kinases that phosphorylate the receptor and prevent it from activating further g proteins
3. active lifetime of alpha-GTP may be limited by cellular factors that stimulate intrinsic GTPase activity of the subunit
4. enzymatic activities in the cell mean that the basal state is favoured so cells contain high activity enzymes that metabolise second messengers, returning levels to basal
5. enzymatic cascades activated downstream of second messenger/protein kinase activation to oppose their effect

Question 20

how is chonotropy in the heart regulated?

Answer 20

rate at which san fires an ap can be affected by ACh release by parasympathetic nerves
predominant receptor type is M2 cholinoceptors
activation increases open probability of K channels via gi
increased permeability to K causes hyperpolarisation, slowing intrinsic firing rate, causing negative chonotropic effect

Question 21

how is inotrophy regulated in the heart?

Answer 21

sympathetic innervation of cardiac ventricles can influence force of contraction (inotropy)
activation of B adrenoceptors increases open probability of VOCC via gs
gs both interact directly with VOCC and indirectly via camp –> pka –> phosphorylation and activation of VOCC
influx of Ca brings about positive inotropic effect

Question 22

how is arteriolar vasoconstriction brought about?

Answer 22

sympathetic release of noradrenaline acts on a1 adrenoceptors to stimulate phospholipase c and ip3 production via gq
ip3 releases er Ca and initiates a contractile response

Question 23

how is neurotramsitter release modulated?

Answer 23

presynaptic gpcr can influence neurotransmitter release
pre synaptic mu opiod receptors stimulated by endogenous opiods or analgesics to couple to g-a1 proteins
g-beta-gamma subunits released from heterotrimer interact with VOCC to reduce Ca entry, reducing neurotransmitter release