MR S7 - Signal Transduction in Biological Membranes

Question 0

Describe the structure of G proteins

Answer 0

Heterotrimeric (made up of three different or distinct subunits)
The b and g subunits bind tightly and act as a single unit
The a subunit has a guanine nucleotide binding site where GTP binds and is slowly hydrolysed to GDP

Question 1

How do GPCRs act?

Answer 1

By altering the activity of effectors via the activation of one or more guanine nucleotide binding proteins (G proteins)

Question 2

Describe GPCR structure when receptor is not activated

Answer 2

G protein is usually on the inner face of the membrane

Subunit a has a GDP bound

Question 3

Describe the sequence of events after an agonist binds to a GPCR

Answer 3

Receptor has high affinity for GDP-bound a subunit
Protein-protein interaction occurs
Results in the release of GDP and the binding of GTP
Affinity for a and b-g subunits is reduced so are released
Subunits interact with effectors

Question 4

How does interaction of G protein subunits stop?

Answer 4

Intrinsic GTPase activity of the a subunit hydrolyses GTP to GDP
The affinity of the a and b-g subunits increases
ABG heterotrimer reforms and awaits deactivation by agonist-bound receptor

Question 5

Describe the action of Gs

Answer 5

β receptor activated by adrenaline agonist
Gs is the a subunit of the G protein
Gs stimulates adenylyl cyclase to increase cAMP production

Question 6

Describe the action of Gi

Answer 6

α2 receptor activated by binding of adrenaline agonist or M2 receptor activated by binding of acetylcholine agonist
Gi is the a subunit of the G protein involved
Gi inhibits adenylyl cyclase to reduce cAMP production

Question 7

Describe the action of Gq

Answer 7

α1 receptor activated by binding of adrenaline agonist or M1 or M3 receptor activated by binding of acetylcholine agonist
Gq

Question 8

Describe the action of Gt

Answer 8

In retinal rod cells, the protein rhodopsin detects light and activates a G protein
Gt is the a subunit of the G protein involved
Gt activates phosphodiesterase which hydrolyses cyclic GMP to 5’-GMP

Question 9

Discuss G protein diversity

Answer 9

Human genome codes for 20a, 5b and 12+g subunits
Therefore 1000+ ABG combinations
800+ receptor types to interact with different Ga subunits
Can activate 10+ enzymes/ion channels

Question 10

Discuss the specificity of G protein action

Answer 10

An extracellular signal can activate 1 or many G proteins and effectors to bring about a specific cellular response

Question 11

Describe the action of the cholera toxin

Answer 11

Capable of ADP-ribosylating specific G proteins

CTx eliminates the GTPase activity of the Gs a subunit, which leads to it becoming permanently activated

Question 12

Describe the action of the Pertussis toxin

Answer 12

Capable of ADP-ribosylating specific G proteins

PTx interferes with the GDP/GTP exchange on the Gi a subunit, which leads to it’s permanent inactivation

Question 13

Name some diseases caused by mutations to GPCRs

Answer 13

Retinis pigmentosa
Nephrogenic Diabetes Isipitus
Familial Male Precocious Puberty

Question 14

Describe Retinis pigmentosa

Answer 14

Can be caused by a loss-of-function mutation in the gene coding for Rhodopsin

Question 15

Describe Nephrogenic diabetes insipidus

Answer 15

Can be caused by a loss-of-function mutation to the V2 vasopressin receptor

Question 16

Describe familial male precocious puberty

Answer 16

Caused by a gain-of-function mutation to the gene coding for the lute nosing hormone receptor
Causes receptor to be active without ligand binding

Question 17

Describe adenylyl cyclase

Answer 17

Integral plasma membrane protein
Can be activated (via Gs) or inhibited (via Gi) by activation of various receptors
Hydrolyses cellular ATP to cAMP

Question 18

Describe the action of cAMP

Answer 18

Interacts with a specific protein kinase: cyclic AMP dependent protein kinase (aka PKA)

Question 19

Describe the action of PKA

Answer 19

Phosphorylates a variety of proteins within the cell to affect activity

Question 20

Describe phospholipase C

Answer 20

An enzyme which hydrolyses the membrane phospholipid PIP2 to IP3
Activated by Gq

Question 21

Describe the action of IP3

Answer 21

Interacts with specific intracellular receptors on the endoplasmic reticulum to allow Ca to leave the lumen of the ER and enter the cytoplasm

Question 22

Describe cyclic GMP phosphodiesterase

Answer 22

Specialised mechanism found in the photoreceptive cells of the retina
Regulates the breakdown of cyclic GMP phosphodiesterase by Gt

Question 23

How is a GPCR deactivated?

Answer 23

After receptor interaction with the G protein, the agonist binding is weakened and receptor-agonist dissociation is likely
Whilst activated, the receptor-agonist is vulnerable to a variety of protein kinases which prevent it activating more G proteins
Cellular factors can influence the GTPase activity of the a subunit
In the cell, basal rates are favoured for enzymes so there are many highly active enzymes which metabolise second messengers
Enzyme cascades initiated by second messengers often act to oppose the second messengers

Question 24

Describe regulation of chronotropy in the heart

Answer 24

The rate of SAN impulses can be affected by ACh release from parasympathetic nerves
Receptor type is M2, which increases the open probability of K+ channels via the Gi subunit
This causes hyperpolarisation, slowing the intrinsic firing rate and resulting in a negative chronotropic effect

Question 25

Describe regulation of intropy of the heart

Answer 25

Sympathetic stimulation of the cardiac ventricles (and circulating adrenaline) can influence intropy
Activation of

Question 26

What is intropy?

Answer 26

The force of contraction of the cardiac ventricles

Question 27

Describe arteriolar vasoconstriction

Answer 27

Sympathetic release of noradrenaline acts on α1 adrenoceptors to stimulate phospholipase c -> IP3 production via Gq
The IP3 causes Ca release into cell from ER and initiates contractile response

Question 28

Describe modulation of neurotransmitter release

Answer 28

Presynaptic GPCRs can influence neurotransmitter release
Presynaptic m-opioid receptors can be stimulated by indigenous opioids and by analgesics eg morphine to couple to Gi a subunits
The Gbg subunits are liberated from the heterotrimer and interact with VOCCs to reduce Ca entry into cell, thus reducing neurotransmitter release