Q.10 Choose the correct signal transduction pathway.
(A) Hormone → 7 TM receptor → G protein → cAMP → PKA
(B) Hormone → G protein → 7 TM receptor → cAMP → PKA
(C) Hormone → 7 TM receptor → G protein → PKA → cAMP
(D) Hormone → 7 TM receptor → cAMP → G protein → PKA

The correct G protein-coupled receptor (GPCR) signaling pathway follows hormone binding to 7TM receptor, activating G protein, then adenylyl cyclase producing cAMP to activate PKA, making (A) the correct answer. This cAMP pathway mediates responses to hormones like epinephrine and glucagon.​

Correct Answer

The correct option is (A) Hormone → 7 TM receptor → G protein → cAMP → PKA. Ligand (hormone) binds extracellular GPCR (7 transmembrane domains), inducing conformational change that activates heterotrimeric G protein by GDP-GTP exchange on Gα subunit. Active Gαs stimulates adenylyl cyclase to convert ATP → cAMP, which binds regulatory subunits of PKA, releasing catalytic subunits for phosphorylation cascades.​​

Option Explanations

Gs-coupled GPCR pathway amplification: single receptor activates multiple G proteins; cAMP diffuses to activate numerous PKAs.

• (A) Hormone → 7 TM receptor → G protein → cAMP → PKA: Correct; standard sequence where Gαs → adenylyl cyclase → cAMP → PKA activation.​

• (B) Hormone → G protein → 7 TM receptor → cAMP → PKA: Incorrect; G protein cannot be activated without prior receptor conformational change.​

• (C) Hormone → 7 TM receptor → G protein → PKA → cAMP: Incorrect; PKA activated by cAMP binding, not vice versa—cAMP produced first by adenylyl cyclase.​

• (D) Hormone → 7 TM receptor → cAMP → G protein → PKA: Incorrect; cAMP generated downstream of G protein; receptor does not directly produce cAMP.​

Mechanism Overview

Hormone binding → GPCR activation → Gαs-GTP dissociates → binds adenylyl cyclase → ATP → cAMP → PKA phosphorylates CREB → gene expression. Pathway terminates by phosphodiesterase (cAMP → AMP) and Gα GTPase. Key in glycogenolysis, hormone secretion.​