MR S7 - Signal Transduction in Biological Membranes Flashcards
Describe the structure of G proteins
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
How do GPCRs act?
By altering the activity of effectors via the activation of one or more guanine nucleotide binding proteins (G proteins)
Describe GPCR structure when receptor is not activated
G protein is usually on the inner face of the membrane
Subunit a has a GDP bound
Describe the sequence of events after an agonist binds to a GPCR
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
How does interaction of G protein subunits stop?
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
Describe the action of Gs
β receptor activated by adrenaline agonist
Gs is the a subunit of the G protein
Gs stimulates adenylyl cyclase to increase cAMP production
Describe the action of Gi
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
Describe the action of Gq
α1 receptor activated by binding of adrenaline agonist or M1 or M3 receptor activated by binding of acetylcholine agonist
Gq
Describe the action of Gt
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
Discuss G protein diversity
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
Discuss the specificity of G protein action
An extracellular signal can activate 1 or many G proteins and effectors to bring about a specific cellular response
Describe the action of the cholera toxin
Capable of ADP-ribosylating specific G proteins
CTx eliminates the GTPase activity of the Gs a subunit, which leads to it becoming permanently activated
Describe the action of the Pertussis toxin
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
Name some diseases caused by mutations to GPCRs
Retinis pigmentosa
Nephrogenic Diabetes Isipitus
Familial Male Precocious Puberty
Describe Retinis pigmentosa
Can be caused by a loss-of-function mutation in the gene coding for Rhodopsin
Describe Nephrogenic diabetes insipidus
Can be caused by a loss-of-function mutation to the V2 vasopressin receptor
Describe familial male precocious puberty
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
Describe adenylyl cyclase
Integral plasma membrane protein
Can be activated (via Gs) or inhibited (via Gi) by activation of various receptors
Hydrolyses cellular ATP to cAMP
Describe the action of cAMP
Interacts with a specific protein kinase: cyclic AMP dependent protein kinase (aka PKA)
Describe the action of PKA
Phosphorylates a variety of proteins within the cell to affect activity
Describe phospholipase C
An enzyme which hydrolyses the membrane phospholipid PIP2 to IP3
Activated by Gq
Describe the action of IP3
Interacts with specific intracellular receptors on the endoplasmic reticulum to allow Ca to leave the lumen of the ER and enter the cytoplasm
Describe cyclic GMP phosphodiesterase
Specialised mechanism found in the photoreceptive cells of the retina
Regulates the breakdown of cyclic GMP phosphodiesterase by Gt
How is a GPCR deactivated?
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
Describe regulation of chronotropy in the heart
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
Describe regulation of intropy of the heart
Sympathetic stimulation of the cardiac ventricles (and circulating adrenaline) can influence intropy
Activation of
What is intropy?
The force of contraction of the cardiac ventricles
Describe arteriolar vasoconstriction
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
Describe modulation of neurotransmitter release
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
