G Protein Coupled Receptors

Question 1

What are the 4 types of GPCRs?

Answer 1

Gas, Gai, Gaq, Gat

Question 2

What is a GPCR

Answer 2

Transmembrane protein receptor that crosses the membrane 7 times and phosphorylates a G protein on the intracellular side of the membrane when activated

Question 3

What are the three components of GPCR Signalling?

Answer 3

1) Receptor receives signal
2) G protein (Heterotrimeric protein) activated by phosphorylation of GTP from GDP
3) Effector activated by alpha subunit of G protein

Question 4

What is the mechanism of action of Gas and Gai?

Answer 4

1) Activate/ Inhibit Adenylyl Cyclase
2) Converts ATP to cAMP (2nd messenger)
3) Activates Protein Kinase A and is broken down by Phosphodiesterase to 5’ AMP
4) PKA activates Enzymes, Ion Channels and cAMP-Response Element Binding-protein — Activates gene expression

Question 5

What is the mechanism of action of Gaq?

Answer 5

1) Activates phospholipase C beta
2) Converts Phosphatidylinositol 4,5-Biphosphate into Inositol 1,4,5-Triphosphate and Diacylglycerol
3a) IP3 stimulates endoplasmic reticulum to release Calcium ions that binds to Calmodulin and activates enzymes, ion channels and contractile proteins
3b) DAG stimulates Protein Kinase C which activates Phospholipase A2 that converts phospholipids into arachidonic acids that are converted into eicosanoids
4) PKC also activates enzymes, ion channels and contractile proteins and C-Fos, C-jun that activates gene expression

Question 6

What is the mechanism of action of Gat?

Answer 6

Rhodopsin
1) Photon binds to Rhodopsin
2) Activates Gat (Transducin)
3) Activates cGMP-specific PDE
4) PDE breaks down cGMP
5) Closes Na/Ca channels
6) Membrane undergoes hyperpolarisation
7) Signals the brain
8) Drop in Ca2+ activates cytosolic guanylyl cyclase
9) Synthesis of cGMP reopens Na/Ca channel
10) Repeat the cycle

Question 7

How is signalling amplified?

Answer 7

1) One GPCR can activate multiple G proteins
2) One alpha subunit binding to the effector can release multiple secondary messengers
3) One secondary messenger can release even more messengers or activate multiple proteins

Question 8

How is GPCR activity regulated?

Answer 8

1) Ligand removal (Reuptake/ Enzymatic degradation)
2) Desensitization
2a) Uncoupling (Uncouple G protein from receptor)
2b) Internalisation (Endocytosis of GPCR)
2c) Down regulation (Break down of GPCR)

Question 9

What is the mechanism of desensitisation of GPCRs?

Answer 9

1) Ligand binds to GPCR and activates
2) After G protein leaves, G protein-coupled receptor kinase phosphorylates the end of the receptor that binds to the G protein
3) Beta-Arrestin binds to the phosphorylated end (Uncoupling)
4) Membrane with receptor than undergoes endocytosis (Internalisation)
5) Inside the endosome there are phosphatases that dephosphorylates the receptor for it to be recycled
6) In other endosomes the receptor is degraded when bound with lysosomes (Down-regulation)

Question 10

What are the two types of desensitization?

Answer 10

1) Homologous desensitization (Only to that particular agonist -> Normal desensitisation process)
2) Heterologous desensitizatoin (To all agonists -> Activated protein kinases phosphorylate other GPCRs with no specificity)