Multiple Choice
G protein-coupled receptors (GPCRs) are transmembrane proteins with ligand binding sites on the extracellular side of the membrane (near its N-terminal). What is the role of the G-protein in the GPCR signaling pathway?
12. Biosignaling
G protein-Coupled Receptors
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Gi is the inhibitory G protein of various effector enzymes. If a toxin released from a bacterial infection enters cells & covalently modifies the α-subunit of Gi so that it can no longer bind GTP, which of the following is a likely result?
A
Increase in the synthesis of the secondary messenger molecule.
B
Decrease in the synthesis of the secondary messenger molecule.
C
The inhibitory G protein α-subunit will not dissociate from the β-γ subunits.
D
Only A and C are true.
E
Only B and C are true.
Verified step by step guidance1
Understand the role of Gi protein: Gi is an inhibitory G protein that, when activated, inhibits the activity of certain effector enzymes, leading to a decrease in the production of secondary messenger molecules like cAMP.
Recognize the effect of GTP binding: Normally, the α-subunit of Gi binds GTP to become active. This active form can then dissociate from the β-γ subunits to exert its inhibitory effect on effector enzymes.
Analyze the impact of the toxin: The toxin modifies the α-subunit of Gi, preventing it from binding GTP. Without GTP binding, the α-subunit cannot become active and cannot dissociate from the β-γ subunits.
Predict the consequences on secondary messenger synthesis: Since the α-subunit cannot inhibit effector enzymes, there will be an increase in the synthesis of secondary messenger molecules, as the inhibition is lost.
Conclude the likely outcomes: The inability of the α-subunit to dissociate from the β-γ subunits (C) and the increase in secondary messenger synthesis (A) are both true, making the correct answer 'Only A and C are true.'
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