Cyclic AMP (cAMP)

Overview of the cAMP Second-Messenger System

• First G-protein–linked second-messenger pathway discussed.
• Second messenger = cyclic adenosine monophosphate (cAMP).
• Coupled to the stimulatory G-protein $G<em>s$ ("s" for stimulatory). • Central membrane enzyme: adenylyl cyclase (AC) – inactive until bound by $G</em>s\alpha$–GTP.

Step-by-Step Signal Activation

1. Ligand–Receptor Binding
   – Extracellular ligand docks with its specific membrane receptor.

2. Receptor Conformational Change
   – Ligand binding alters receptor shape → exposes a cytosolic G-protein interaction site.

3. G-Protein Activation
   – GDP on $G<em>s\alpha$ exchanged for GTP → $G</em>s$ becomes active.
   – Reaction: $G<em>s\alpha\text{–GDP} + GTP \rightarrow G</em>s\alpha\text{–GTP} + GDP$

4. Subunit Dissociation
   – $G_s\alpha\text{–GTP}$ separates from the $\beta\gamma$ complex.

5. Adenylyl Cyclase Activation
   – Free $G_s\alpha\text{–GTP}$ binds AC → AC’s catalytic site switches “on.”

6. Second-Messenger Formation
   – AC converts ATP to cAMP.
   – Net reaction: $ATP \xrightarrow{AC} cAMP + PP_i$

7. Effector Kinase Activation
   – cAMP binds the regulatory subunits of protein kinase A (PKA) → catalytic subunits released and activated.

8. Target-Protein Phosphorylation
   – PKA transfers $PO_4^{3-}$ from ATP to serine/threonine residues on intracellular proteins.
   – Phosphorylation can activate or inhibit the substrate’s function, thereby altering cellular behavior.

Signal Termination Mechanisms

• Intrinsic GTPase of $G<em>s\alpha$
– Hydrolyzes GTP → GDP: $G</em>s\alpha\text{–GTP} \rightarrow G<em>s\alpha\text{–GDP} + P</em>i$
– $G_s\alpha$ re-associates with $\beta\gamma$, halting AC stimulation.
• Phosphodiesterase (PDE)
– Degrades cAMP to AMP: $cAMP \xrightarrow{PDE} AMP$ (AMP has no second-messenger activity).

Key Physiological Processes Regulated by cAMP/PKA

• Glycogen degradation (glycogenolysis) – mobilizes glucose units.
• Fatty-acid synthesis & metabolism.
• Heart-rate modulation (chronotropy) & blood-pressure control.
• Renal water reabsorption (e.g., vasopressin pathway).
• Bone resorption dynamics.

Opposing G-Protein Influence: $G<em>i$ vs. $G</em>s$

• Some receptors couple to the inhibitory G-protein $G<em>i$. – $G</em>i\alpha\text{–GTP}$ binds adenylyl cyclase and inhibits cAMP production.
• Cellular cAMP level = net result of concurrent $G<em>s$ (stimulatory) and $G</em>i$ (inhibitory) inputs.
– “Who wins” depends on which receptor types have more ligand bound at any moment.

Conceptual Summary

• cAMP pathway is a versatile regulatory module translating extracellular messages into rapid intracellular phosphorylation cascades.
• Activation requires three core events: ligand binds → $G<em>s$ activates → AC produces cAMP. • Deactivation relies on two off-switches: GTP hydrolysis on $G</em>s\alpha$ and PDE-mediated cAMP breakdown.
• Same pathway can yield diverse outcomes across tissues because the complement of PKA substrates differs from cell to cell.