Lecture 6 - GPCRs II Flashcards
What are the effectors of trimeric G proteins and how are they regulated?
Effectors of trimeric G proteins include enzymes that create second messengers and ion channels whose gating is regulated either directly (βγ subunits) or indirectly by second messengers and their effectors. Activated G proteins regulate the activities of enzymes that control the levels of second messengers.
What are the key second messenger systems regulated by GPCRs and how are they regulated?
The key second messenger systems regulated by GPCRs include the cAMP and cGMP systems, as well as the hydrophobic lipid system and calcium. The regulation of second messengers is achieved through various mechanisms such as the activation or inhibition of enzymes that control their synthesis and breakdown.
What is the cAMP second messenger system, and what is its role in the fight or flight response?
The cAMP second messenger system is regulated by adenylyl cyclase, which is activated by Gs and inhibited by Gi.
Ligand binds to receptor activating G protein
α subunit moves and binds to adenylate cyclase in the membrane
This activated enzyme catalyses formation of a cAMP from ATP
The cAMP (2nd messenger) activates Protein kinase A
PKA phosphorylates/activates protein
Initiates a response within the cell
Removl by phosphodiesterase
What is the cGMP second messenger system, and how is it similar to the cAMP system?
The cGMP second messenger system, involves the conversion of guanosine triphosphate (GTP) to 3’ , 5’ -cyclic guanosine monophosphate (cGMP) through the enzyme guanylate cyclase.
The cGMP system differs in that the enzyme can be receptor-bound or free in the cytoplasm, and the message is encoded in the concentration and frequency of changes in concentration.
The cGMP system is also regulated by the removal of cGMP through phosphodiesterase.
What is the hydrophobic lipid second messenger system, and how is it regulated?
Involves the cleavage of membrane lipids by phospholipase Cβ (PLC), generating two kinds of second messengers: one water-soluble and diffuses through the cytoplasm (e.g., IP3) and the other hydrophobic and remains in the membrane (e.g., DAG).
GPCRs can activate PLC to generate IP3 and DAG, and DAG activates protein kinase C (PKC) and stays in the membrane.
Lipid kinases add phosphate groups to lipids eg. To DAG to make phosphatidic acid , or to PI to generate PIP, PIP2, PIP3
Numerous isoforms of PLC and PKC are found in cells
What is the role of calcium as a second messenger?
Calcium regulates diverse cell functions, such as synaptic transmission, hormone secretion and synthesis, fertilization, muscle contraction, and cytokinesis.
In resting cells, cytosolic calcium concentration is kept low by ATP-driven calcium pumps.
Receptors regulate the activity of calcium channels to produce transient rises in calcium concentration.
Calcium influx into the cytosol is regulated by channels in the extracellular membrane and ligand-gated channels on the endoplasmic reticulum (ER).
Store-operated channels, made up of ORAI and gated by STIM, are responsible for store refilling and maintaining ER calcium levels. Calcium also plays an important role in the activation of T-lymphocytes.
What does Gi do ?
Inhibits adenylyl cyclase, open K+ channeos and close Ca2+ channels
What does Gs do ?
Stimulates adenylyl cyclase to produce cAMP, receptor kinase
What does Gq do ?
Activates phospholipase CB to produce IP3, which releases Ca2+
Describe - β2 Adrenoceptor regulation of metabolism in liver and skeletal muscle
Binding of single Epinephrine molecule to a receptor sets off a signalling cascade resulting in the phosphorylation/activation of enzymes controlling glycogen metabolism.
Signalling is switched off by :
Agonist dissociating from receptor
GTPase activity of Gαs
cAMP breakdown by phosphodiesterase
Dephosphorylation of enzymes
What are PKCs activated by ?
PKCs (protein kinase C) are Ser/Thr kinases, activated by DAG (C1 domain), and Ca2+ (C2 domain)
DAG binding causes dissociation of intramolecular pseudosubstrate domain from active site, once activated PKCs can provide either positive or negative feedback in signaling pathway
Phosphorylation of PLCβ provides negative feedback for GPCR signaling, makes the signalling ‘transient’
Phosphorylation of receptors can contribute to desensitization
What does overstimulation of GCPRs cause ?
Tachyphylaxis - e.g. LSD or salbutamol
Disease e.g. uncontrolled growth in cancer
2 mechanisms - GRK (receptor kinases) and β-arrestin
Stops G-protein from binding
Internalises receptor - degraded or recycled
