Signal Transduction Flashcards
List some functions that G proteins are responsible for
Muscle contraction Stimulus-secretion coupling Catabolic/anabolic processes Light Smell Taste
What do G protein-coupled receptors (GPCRs) do? And how? (in general)
Alter the activity of effectors (enzymes or ion channels)
Do this by activating one or more types of guanine nucleotide binding proteins.
Structure of a G-protein
Heterotrimeric (3 distinct subunits stuck together)
3 distinct subunits α, β, γ
α has a guanine nucleotide binding site
Structure of a GPCR?
Single polypeptide chains
7 transmembrane spanning domains
Extracellular N terminal
Intracellular C terminal
What regions of the GPCR are responsible for binding?
2-3 of the transmembrane domains
Or
The N-terminal region
How does interaction of the G protein with the GPCR activate it?
Causes GTP to exchange for GDP on the G protein α subunit.
GDP -> GTP
What happens after the G protein has been activated?
α-GTP and βγ dissociate.
Each interact with effector proteins
What stops the two sub units interacting with the effector proteins? What happens next?
Until the α-GTPase activity hydrolysed GTP back to GDP.
α-GDP and βγ then reform an inactive heterotrimeric complex.
What does Gs stimulate? What kind of pathway is it?
Adenylyl cyclase to produce cAMP
Stimulatory
What do Gi do? Type of pathway?
Inhibits adenylyl cyclase so there is less cAMP
Inhibitory
What do Gq proteins do?
Interact with the membrane bound enzyme PIP2 to generate the 2nd messengers InsP3 and DAG
What does Gt (transducin) do?
Rhodopsin activates it which in turn activates the enzyme that hydrolyses cGMP ➡️ 5’-GMP
What does the pertussis toxin do to GPCRs?
Contains the enzyme ADP-ribosyl transferase which modifies and inactivates Gi-type proteins. This uncouples receptor-effector linkage.
What does the cholera toxin do?
Contains an enzyme which specifically modifies Gs-type proteins, leading to irreversible activation.
What is retinitis pigmentosa caused by?
A loss of function mutation to rhodopsin.
What is nephron epic diabetes insipidus caused by?
Loss of function mutation to V2 vasopressin receptor
What is familial male precocious puberty caused by?
A gain of function mutation (where the receptor is effective without the ligand) to the lute using hormone receptor.
What activates and what inhibits adenylyl cyclase?
Activated via Gs and inhibited by Gi
What does adenylyl cyclase do? What does this lead to?
Hydrolyses ATP to generate cAMP
cAMP interacts with a specific protein kinase (PKA)
PKA then phosphorylates a variety of other proteins within the cell to affect activity.
What effects does adenylyl cyclase have on the cell?
Increased glycogenolysis and gluconeogenesis in the liver
Increased lipolysis in adipose tissue
Relaxation of smooth muscle
Positive inotropic and chronotropic effects in the heart
What is phospholipase C
An enzyme that hydrolyses membrane phospholipid PIP2 ➡️ IP3
What activates phospholipase C?
Gq
What does IP3 do?
Interacts with specific intracellular receptors on the ER to allow Ca2+ to leave the ER lumen and enter the cytoplasm.
What is cyclic GMP phosphodiesterase and what does it do?
A specialised mechanism found in the photoreceptors of the retina.
Rhodopsin is excitated by a photon of light which causes Gt to regulate the breakdown of the second messenger cGMP
Causes channel closure and membrane hyperpolarisation.
What happens when cGMP levels are high in the dark?
Can open a second-messenger operated ion channel which allows calcium and sodium to enter the cytoplasm.
Alters the signal output to the CNS.
How is a GPCR activated?
An agonist binds to it
What is the receptor desensitisation phenomenon?
While a GPCR is activated, protein kinases can phosphorylate the receptor and prevent it from activating further G-proteins
How are G proteins involved in controlling heart inotropy?
β-adrenoreceptors (GPCRs) are activated by adrenaline.
They increase the open probability of voltage-opened calcium channels via Gs
Influx of Ca brings about a positive inotropic effect
How does Gs act indirectly with VOCCs (in the context of inotropy)
cAMP ➡️ PKA ➡️ phosphorylation and activation of VOCCs.
How are GPCRs involved in chronotropy of the heart?
M2 muscarinic choline receptors are activated.
This increases the open probability of V-G potassium channels via Gi
Increased plasma membrane permeability to K+ causes hyperpolarisation, slowing the intrinsic firing rate, resulting in a negative chronotropic effect.
How are GPCRs involved in arteriolar vasoconstriction/smooth muscle contraction
Sympathetically released noradrenaline acts on α1-adrenoreceptors to stimulate phospholipase C and IP3 production via Gq.
IP3 releases ER calcium and initiates a contractile response.
How does bronchoconstriction occur?
Parasympathetically released Ach interacts with M3 muscarinic receptors on bronchiolar smooth muscle
How does modulation of neurotransmitter release occur?
Can be influenced by pre-synaptic GPCRs.
Presynaptic μ opioid receptors can be stimulated by endogenous opioids or by analgesics such as morphine.
Couples to Gα1 proteins
The G-βγ liberated fro. The heterotrimer interact with VOCCs to reduce calcium entry, reducing neurotransmitter release.
