Week 7 - Signal Transduction

Question 1

State which group of receptors are targeted whilst designing drugs for asthma and give two examples of drugs designed to treat asthma.

Answer 1

B2 adrenoceptors (agonists)
E.g. salbutamol, salmeterol

Question 2

State which receptors are targeted whilst designing drugs for analgesia/anaesthesia, and state two examples of these drugs.

Answer 2

U-opioid receptors (agonists)

E.g. morphine, fentanyl

Question 3

State the class of drugs used to treat cardiovascular problems (for example, hypertension) and give two examples of these.

Answer 3

Beta adrenoreceptor antagonists

E.g. propranolol, atenolol

Question 4

State the class of drugs neuroleptics (anti-schizophrenics) belong to and state 2 examples of these.

Answer 4

D2 Dopamine receptor antagonists

E.g. haloperidol, sulpiride

Question 5

How is retinitis pigmentosa caused?

Answer 5

Loss of function mutation to rhodopsin

Question 6

List the three main superfamilies of cell surface receptors.

Answer 6

1. Ligand-gated (receptor-operated) ion channels, e.g. nicotinic acetylcholine receptor
2. Receptors with intrinsic enzyme activity (e.g. receptor tyrosine kinases, insulin receptor)
3. G protein coupled (7TM) receptor, e.g. muscarinic acetylcholine receptor

Question 7

How is nephrogenic diabetes insipidus caused?

Answer 7

Loss of function mutation to the V2 vasopressin receptor

Question 8

How is familial male precocious puberty caused?

Answer 8

Gain of function mutation to the luteinizing hormone (LH) receptor

Question 9

Outline the types of stimuli that GPCRs respond to.

Answer 9

1. Sensory GPCRs respond to light (e.g. rhodopsin, odours and tastes)
2. Ions (H+, Ca2+)
3. Neurotransmitters (e.g. acetylcholine, glutamate)
4. Peptide and non-peptide hormones (e.g. glucagon, adrenaline)
5. Large glycoproteins (e.g. TSH)

Question 10

Name the common structural features that all G protein-coupled receptors share.

Answer 10

1. Single polypeptide chain (300-1200 amino acids)
2. 7 Transmembrane spanning regions
3. Extracellular N terminal
4. Intracellular C terminal

Question 11

Outline the regions of GPCRs responsible for ligand binding.

Answer 11

1. Some receptors, ligand binding site formed by (2-3 of) the transmembrane domains
2. In other cases, the N-terminal region (and other extracellular domains) form the ligand binding site

Question 12

Briefly outline how G protein coupled receptors cause a change in cellular activity.

Answer 12

-Conformational change caused in the receptor by agonist binding
-An activated GPCR must interact with a guanine nucleotide binding protein (G protein)
-G protein moves around on the membrane: parts of itself which interact with the plasma membrane
-G proteins are made up of three subunits: “heterotrimeric” - alpha, beta and gamma (functionally a dimer)
-The GPCR G protein interaction activates the G protein by causing GTP to exchange for GDP on the alpha subunit
-The alpha-beta gamma complex immediately dissociates into alpha-GTP and free beta-gamma subunits
-Each can then interact with effector proteins (e.g. second messenger generating enzymes or ion channels)
Termination:
-Alpha-GTP and/or beta-gamma interaction with effectors lasts until the alpha subunit GTPase hydrolyses GTP back to GDP
-GTPase is not an efficient enzyme: GTP is not instantly hydrolysed, GTP binds and 1-4 seconds lapses before GTPase activity kicks in and hydrolyses GTP
-Alpha-GDP and beta-gamma subunits then reform an inactive heterotrimeric complex

Question 13

How do GPCRs ensure specificity of cellular response (i.e. what governs Receptor G-protein selection)?

Answer 13

1. Activated GPCRs preferentially interact with specific types of G protein (G alpha subunit: primary determinant)
2. G alpha and G beta gamma subunits: interact with specific effector proteins
3. An extracellular signal, working via a specific GPCR, will activate a single or small sub-population of G proteins and effectors in the cell to bring about a specific cellular response

Question 14

State the respective GPCRs, G proteins and effectors for the following ligands:
1, 2, 3. Adrenaline/Noradrenaline
4. Light
5, 6. Acetylcholine

Answer 14

1. GPCR - B adrenoceptor
   G protein: Gsalpha (G beta gamma)
   Stimulates Adenylyl cyclase
2. GPCR: Alpha 2 adrenoceptor
   G protein: Gialpha (G beta gamma)
   Inhibits adenylyl cyclase
3. GPCR: Alpha 1 adrenoceptor
   G protein: Gqalpha (G beta gamma)
   Effector: Stimulates phospholipase C
4. GPCR: Rhodopsin
   G protein: Gtalpha (transducin)
   Effectors: Stimulates cyclic GMP and phosphodiesterase
5. GPCRs: M2/M4 muscarinic receptors
   G protein: Gialpha (G beta gamma)
   Effector: Inhibits Adenylyl cyclase
6. GPCRs: M1/M3 muscarinic receptors
   G protein: Gqalpha (G beta gamma)
   Effector: Stimulates phospholipase C