cAMP–PKA Pathway

Core Concept

The cAMP pathway is a canonical G-protein–coupled receptor (GPCR) signaling cascade. A hormone or neurotransmitter binds a seven-transmembrane receptor, which activates a heterotrimeric G protein. The G protein's alpha subunit (Gs, stimulatory) activates the membrane enzyme adenylyl cyclase, which converts ATP to the second messenger cyclic AMP (cAMP). cAMP binds the regulatory subunits of protein kinase A (PKA), releasing its catalytic subunits to phosphorylate serine/threonine residues on target proteins. The response is turned off by phosphodiesterases that hydrolyze cAMP and by phosphatases that remove the phosphates.

Key Components

• GPCR: Seven-transmembrane receptor (e.g., β-adrenergic receptor for epinephrine).
• Heterotrimeric G protein: α, β, γ subunits. Gsα stimulates adenylyl cyclase; Giα inhibits it.
• Adenylyl cyclase: Integral membrane enzyme converting ATP → cAMP + PPi.
• cAMP: Diffusible second messenger.
• Protein kinase A (PKA): Tetramer of two regulatory (R) and two catalytic (C) subunits.
• Phosphodiesterase (PDE): Hydrolyzes cAMP to 5′-AMP.
• CREB and other PKA targets: Mediate transcriptional and metabolic responses.

Mechanism / How It Works

1. Ligand binding induces a conformational change in the receptor, enabling it to act as a guanine nucleotide exchange factor (GEF) for the G protein.
2. G protein activation: Gsα exchanges GDP for GTP, dissociates from the βγ dimer, and becomes active.
3. Adenylyl cyclase activation: GTP-bound Gsα binds and stimulates adenylyl cyclase, sharply raising cytosolic cAMP.
4. PKA activation: Four cAMP molecules (two per regulatory subunit) bind the R subunits, causing a conformational change that releases the catalytic subunits, which are now active kinases.
5. Phosphorylation: Catalytic subunits phosphorylate target proteins on serine/threonine in the consensus motif Arg-Arg-X-Ser/Thr. In liver and muscle, PKA phosphorylates phosphorylase kinase and glycogen synthase to mobilize glucose; in other cells it phosphorylates transcription factors such as CREB.
6. CREB transcription: PKA translocates to the nucleus and phosphorylates CREB at Ser133, recruiting the coactivator CBP/p300 and switching on cAMP-response-element (CRE) genes.

Energy and Directionality

Each step is powered and directional. GTP hydrolysis by Gsα (intrinsic GTPase) is slow but finite, so Gsα self-inactivates. ATP is consumed twice: once by adenylyl cyclase to make cAMP (releasing PPi, hydrolyzed to 2 Pi, making the reaction essentially irreversible) and once per phosphorylation event by PKA. cAMP is short-lived because phosphodiesterases hydrolyze it to 5′-AMP. Thus the pathway is a driven cascade: signal amplification (one receptor → many G proteins → many cAMP → many PKA → many phosphorylated targets) with built-in timers for shutdown.

Experimental Evidence / Technique

• Sutherland's discovery: Earl Sutherland showed that epinephrine plus a liver membrane fraction produced a heat-stable factor (cAMP) that activated glycogen phosphorylase — the first second messenger (Nobel Prize, 1971).
• FRET biosensors (e.g., Epac-based or PKA-based sensors) report real-time cAMP dynamics in single cells.
• Cholera toxin ADP-ribosylates Gsα, locking it in the GTP-bound state; the resulting unregulated cAMP production opens CFTR chloride channels in intestinal epithelium, causing the massive secretory diarrhea of cholera.
• Pertussis toxin ADP-ribosylates Giα, preventing it from inhibiting adenylyl cyclase.

How it works

1. Ligand binding induces a conformational change in the receptor, enabling it to act as a guanine nucleotide exchange factor (GEF) for the G protein.
2. G protein activation: Gsα exchanges GDP for GTP, dissociates from the βγ dimer, and becomes active.
3. Adenylyl cyclase activation: GTP-bound Gsα binds and stimulates adenylyl cyclase, sharply raising cytosolic cAMP.
4. PKA activation: Four cAMP molecules (two per regulatory subunit) bind the R subunits, causing a conformational change that releases the catalytic subunits, which are now active kinases.
5. Phosphorylation: Catalytic subunits phosphorylate target proteins on serine/threonine in the consensus motif Arg-Arg-X-Ser/Thr. In liver and muscle, PKA phosphorylates phosphorylase kinase and glycogen synthase to mobilize glucose; in other cells it phosphorylates transcription factors such as CREB.
6. CREB transcription: PKA translocates to the nucleus and phosphorylates CREB at Ser133, recruiting the coactivator CBP/p300 and switching on cAMP-response-element (CRE) genes.

Common confusions

• cAMP vs. AMP: cAMP is a cyclic, second-messenger form; AMP is an ordinary nucleotide and a degradation product.
• PKA activation mechanism: cAMP does not phosphorylate PKA; it binds the regulatory subunits and causes them to release the catalytic subunits.
• Gs vs. Gi: Both regulate adenylyl cyclase but in opposite directions; Gq is an entirely different effector (phospholipase C).
• Cholera vs. pertussis: Cholera over-activates Gs; pertussis disables Gi. Both increase cAMP, but through different G proteins.

Quick review

• Ligand → GPCR → Gsα(GTP) → adenylyl cyclase → ATP → cAMP → PKA (R2C2 → free catalytic) → Ser/Thr phosphorylation → response.
• Off switches: GTP hydrolysis, phosphodiesterase, phosphatases, receptor desensitization.