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2.1 G Proteins — Test 1
Q1. G proteins are GTP-binding proteins that function to:✓ Couple hormone receptors to adjacent effector molecules
Q2. A heterotrimeric G protein is composed of the subunits:✓ alpha, beta and gamma
Q3. Which subunit of the G protein binds GDP or GTP and carries the stimulatory/inhibitory identity?✓ The alpha subunit
Q4. A G protein is in its ACTIVE state when its alpha subunit is bound to:✓ GTP
Q5. The intrinsic enzymatic activity that switches the G protein OFF is:✓ GTPase activity (hydrolysing GTP to GDP)
Q6. In the cAMP second-messenger system, the stimulatory G protein (Gs) acts on adenylate cyclase to:✓ Activate it
Q7. Cholera toxin causes profuse secretory diarrhoea by ADP-ribosylating the G protein so that:✓ Gs is locked active and adenylate cyclase stays on, raising cAMP
Q8. Pertussis toxin acts by ADP-ribosylating:✓ Gi (relieving its inhibition of adenylate cyclase)
Q9. Receptors that signal through G proteins are structurally:✓ Seven-transmembrane (G-protein-coupled) receptors
Q10. The TRH (thyrotropin-releasing hormone) receptor belongs to the:✓ G-protein-coupled receptor family
Q11. Water-soluble (peptide/catecholamine) hormones typically initiate their action by:✓ Activating G-protein-coupled receptors
Q12. Match each toxin with its molecular target and select the correct option.✓ A-ii, B-i
Q13. The correct sequence of G-protein activation after hormone binding is:✓ GDP released, GTP binds alpha, alpha activates the effector
Q14. Which of the following hormones does NOT use a G-protein-coupled receptor?✓ Insulin
Q15. Both the adenylate cyclase system and the IP3 system have in common that they:✓ Use G proteins to couple receptor to effector
Q16. The effector enzyme activated by Gs in the cAMP pathway is:✓ Adenylate cyclase
Q17. The condition in which one hormone, present at pathologically high levels, mimics another by binding its receptor is called:✓ Specificity spillover
Q18. After activation, the G-protein signal is terminated when:✓ Intrinsic GTPase hydrolyses GTP to GDP
Q19. Stimulatory and inhibitory activities of G proteins reside specifically in the:✓ alpha-s and alpha-i subunits
Q20. One receptor activating many G proteins (each making many cAMP molecules) illustrates the principle of:✓ Signal amplification