Q.49 Which one of the following statements regarding G proteins is INCORRECT?
(A) GDP is bound to G protein in the resting stage
(B) GTP bound α subunit cannot reassemble with βγ dimer
(C) All G proteins are trimeric
(D) Activation of G protein may result in activation or inhibition of the target enzymes
G Proteins INCORRECT Statement: All G Proteins Are Trimeric MCQ Solution
G proteins regulate cellular signaling through GDP/GTP cycling and α-βγ dissociation, but not all follow heterotrimeric structure. The incorrect statement identifies this diversity in G protein families.
Option Analysis
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(A) GDP is bound to G protein in the resting stage: CORRECT. Inactive heterotrimeric G proteins (αβγ) bind GDP on α-subunit at plasma membrane, associated with GPCR.
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(B) GTP bound α subunit cannot reassemble with βγ dimer: CORRECT. GTP binding induces conformational change causing α-GTP dissociation from βγ; intrinsic GTPase hydrolyzes GTP→GDP for reassembly.
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(C) All G proteins are trimeric: INCORRECT. Heterotrimeric G proteins (Gαβγ) couple GPCRs, but small G proteins (Ras, Rho, Rab, Arf, Ran) are monomeric GTPases regulating vesicle trafficking, cytoskeleton, nuclear transport.
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(D) Activation of G protein may result in activation or inhibition of the target enzymes: CORRECT. Gαs activates adenylyl cyclase (↑cAMP); Gαi inhibits it (↓cAMP); Gq activates PLC (↑IP3/DAG).
Correct Answer
(C). Only ~20 Gα, 5 Gβ, 12 Gγ genes encode heterotrimers; >150 monomeric small G proteins exist.
G proteins incorrect statement “All G proteins are trimeric” traps students confusing heterotrimeric GPCR effectors with ubiquitous small GTPases. Heterotrimeric G proteins (Gαs/i/o/q/12/13) comprise 36 subtypes regulating 800+ GPCRs (40% drug targets); monomeric Ras superfamily (>150 members) controls proliferation, trafficking, cytoskeleton via effector domains like Raf, PI3K.
G Protein Families Compared
| Type | Structure | Examples | Function | Localization |
|---|---|---|---|---|
| Heterotrimeric | Gαβγ | Gαs, Gαi, Gq | GPCR signaling | Plasma membrane |
| Small/Monomeric | α only | Ras, RhoA, Rab5 | Proliferation, cytoskeleton | Cytosol/membranes |
Cycle Mechanics (A,B,D Correct)
Resting: Gα-GDP-βγ–GPCR [A ✓]
Ligand: GPCR* → GDP↔GTP
Active: Gα-GTP + βγ [B ✓]
Effect: ↑AC (Gαs) / ↓AC (Gαi) [D ✓]
Inactive:Gα-GDP + βγ → reassemble
Exam Traps & Mnemonics
C traps: Visualize Ras (oncogene, 30% cancers) vs Gαs (McCune-Albright). Mnemonic: “Hetero-TRIMeric = Three (αβγ); Mono-meric = One (Ras)”. Drug targets: 34% pharmaceuticals hit GPCRs via heterotrimeric G proteins only.
Biotech relevance: Gα mutants (cholera toxin locks Gαs-GTP); Ras-G12V (pancreatic cancer); Rho kinase inhibitors (hypertension). Option C distinguishes signaling superfamilies critical for systems biology, cancer therapy.
~150 small G proteins vs 36 heterotrimeric subtypes; C definitively incorrect.
