LG - Advanced Studies in GPCRs I

Question 1

Q: What are the main receptor types relevant to signal transduction? (4)

Answer 1

• GPCRs – signal via G-proteins, most common drug targets
• Enzyme-linked receptors – e.g. insulin receptor, activates phosphorylation cascades
• Ion channels – e.g. NMDA receptor, fast signalling via ion flow
• ACE2 receptor – enzyme receptor involved in angiotensin regulation and SARS-CoV-2 entry

Question 2

Q: What are key concepts about GPCRs and their function? (3)

Answer 2

• GPCRs activate intracellular signalling via G-protein coupling
• Enzyme-linked receptors signal via phosphorylation cascades
• Ion channels allow rapid signalling via ions; ACE2 has cardiovascular and viral entry roles

Question 3

Q: What is the importance of receptors in pharmacology? (3)

Answer 3

• Serve as drug targets for most therapeutics
• Understanding structure/function aids drug design
• Enables development of specific drugs with fewer side effects

Question 4

Q: Describe the GPCR activation process. (3)

Answer 4

• Ligand binds → conformational change in GPCR
• G-protein activation (GDP-GTP exchange)
• Gα dissociates and initiates downstream signalling

Question 5

Q: Why are GPCRs major drug targets? (3)

Answer 5

• 35–50% of marketed drugs act on GPCRs
• Activated by neurotransmitters, hormones, light, odors
• Control diverse physiological functions

Question 6

Q: What are the types of receptors based on structure? (4)

Answer 6

• Ion channels – open/close for ion flow
• Enzyme-linked – activate internal kinase cascades
• GPCRs – 7TM, G-protein signalling
• Intracellular receptors – bind hydrophobic ligands like steroids

Question 7

Q: What is the basic structure of GPCRs? (5)

Answer 7

• 7 transmembrane α-helices (TM1–TM7)
• Extracellular N-terminus and 3 ECLs – ligand recognition
• 3 ICLs – especially ICL2 and ICL3 for G-protein coupling
• Intracellular C-terminus – β-arrestin binding
• Ligand and G-protein binding sites are spatially distinct

Question 8

Q: What are two major GPCR classification systems? (2)

Answer 8

• Kolakowski (A–F): Rhodopsin (A), Secretin (B), Glutamate (C), etc.
• GRAFS: Glutamate, Rhodopsin, Adhesion, Frizzled, Secretin

Question 9

Q: What are the five main GPCR families in the GRAFS system? (5)

Answer 9

• Glutamate (G) – metabotropic glutamate and GABA receptors
• Rhodopsin (R) – largest family (light, odorant, hormones)
• Adhesion (A) – large ECDs, immune function
• Frizzled (F) – bind Wnt ligands
• Secretin (S) – peptide hormone receptors

Question 10

Q: What are the structural features and motifs of Rhodopsin (Class A) GPCRs? (4)

Answer 10

• 7 TM helices, short N-terminus, ligands bind within TM core
• NPxxY motif (TM7): conformational change, β-arrestin recruitment
• (E/DRY) motif (TM3): G-protein coupling, mutations cause constitutive activity
• Ligands are usually small molecules

Question 11

Q: What are key features of Secretin (Class B1) GPCRs? (4)

Answer 11

• Large N-terminal ECD (\~100–160 aa) with disulfide bridges
• Ligands: peptide hormones (e.g. glucagon, VIP, PACAP)
• 2-step activation: bind ECD, insert peptide C-terminus into TM core
• Activates Gs → cAMP; may also signal via Gq or β-arrestin

Question 12

Q: What are the roles of RAMPs (Receptor Activity-Modifying Proteins)? (4)

Answer 12

• Chaperone – aid GPCR trafficking to cell surface
• Pharmacology switch – modify ligand specificity
• Signalling switch – alter G-protein coupling
• Trafficking switch – regulate receptor fate (recycling/degradation)

Question 13

Q: What are the main features of Glutamate (Class C) GPCRs? (4)

Answer 13

• Recognise amino acid neurotransmitters (e.g. glutamate, GABA)
• Venus flytrap domain (VFT) – ligand binding causes closure
• May contain nine-cysteine domain (except GABAB)
• Function as obligate dimers, often via coiled-coil domains

Question 14

Q: What are key structural traits of Adhesion (Class B2) GPCRs? (4)

Answer 14

• Large ECD with GAIN domain enables autoproteolysis
• NTF: mediates cell adhesion; CTF (7TM): intracellular signalling
• GAIN and TM domains regulate activation and stability
• Functions in immune and developmental signalling

Question 15

Q: What are features of Frizzled (Class F) GPCRs? (4)

Answer 15

• Large cysteine-rich domain (CRD) binds Wnt ligands
• Ligand (e.g. Wnt) has “thumb” (palmitoylated Ser) and “index finger” (Cys loop)
• Couples to Dishevelled (DVL) and sometimes G-proteins
• Crucial in development and tissue regulation

Question 16

Q: Despite diversity, what unifies all GPCR families? (5)

Answer 16

• All possess 7-transmembrane (7TM) structure
• Use G-protein coupled signalling (some exceptions like FZD debated)
• Activated by extracellular ligands (e.g. peptides, Wnt, neurotransmitters)
• Mediate arrestin regulation, desensitisation, internalisation
• Together, they are targets for \~50% of marketed drugs

Question 17

Q: How do GPCRs amplify signals via G-proteins? (2)

Answer 17

• One GPCR can activate multiple G-proteins, amplifying response
• Allows strong, sustained signalling from a single stimulus

Question 18

Q: Describe the heterotrimeric G protein complex. (3)

Answer 18

• Composed of Gα, Gβ, Gγ subunits
• Inactive: Gα-GDP bound to Gβγ
• Active: GPCR binding promotes GDP→GTP exchange, Gα-GTP + Gβγ dissociate

Question 19

Q: Why are heterotrimeric G-proteins important in signalling? (3)

Answer 19

• Act as intermediaries between receptors and effectors
• Modular: 21 Gα, 5 Gβ, 12 Gγ → cell-type specific responses
• Dysregulation linked to cancer, CV disease, neuro disorders, metabolism

Question 20

Q: How is the G-protein cycle regulated? (4)

Answer 20

1. Inactive: Gα-GDP bound to Gβγ
2. Activation: GPCR triggers GDP release, GTP binds
3. Signal: Gα-GTP + Gβγ activate effectors
4. Termination: GTP hydrolysed → GDP (via intrinsic GTPase or RGS proteins)