Drugs & Toxins Affecting GPCR Signaling

Overview of GPCR Signaling

G protein-coupled receptors (GPCRs) are a major class of cell surface receptors that mediate cellular responses to hormones, neurotransmitters, and environmental stimulants. GPCRs activate intracellular G proteins, which in turn regulate various effectors such as adenylyl cyclase, influencing the production of second messengers like cyclic AMP (cAMP).

• GPCRs: Integral membrane proteins that transmit signals from extracellular ligands to intracellular effectors.

• G proteins: Heterotrimeric proteins (α, β, γ subunits) that relay signals from GPCRs to target enzymes.

• Adenylyl cyclase: An enzyme activated or inhibited by G proteins, catalyzing the conversion of ATP to cAMP.

Bacterial Toxins and GPCR Signaling

Certain bacterial toxins target G proteins, altering the activity of effector enzymes such as adenylyl cyclase. Two notable examples are cholera toxin and pertussis toxin.

• Cholera toxin: Increases the activity of Gs (stimulatory G protein) by ADP-ribosylation, causing persistent activation of adenylyl cyclase and elevated cAMP levels.

• Pertussis toxin: Inhibits Gi (inhibitory G protein) by ADP-ribosylation, preventing inhibition of adenylyl cyclase, also resulting in increased cAMP levels.

Key Mechanisms:

• Cholera toxin: Permanently activates Gs protein, leading to continuous stimulation of adenylyl cyclase.

• Pertussis toxin: Permanently inactivates Gi protein, preventing inhibition of adenylyl cyclase.

Illustration: Effects of Cholera and Pertussis Toxins

The provided diagram compares the effects of cholera and pertussis toxins on GPCR signaling:

• Cholera overstimulates Gs: "Pedal to the Metal"—leads to excessive cAMP production.

• Pertussis inhibits the inhibitor (Gi): "Broken Brakes"—removes inhibition, also increasing cAMP.

Example: Toxin Effects on cAMP

• Cholera toxin blocks GTP hydrolysis on Gs, causing persistent activation and increased cAMP.

• Pertussis toxin prevents Gi from inhibiting adenylyl cyclase, also increasing cAMP.

• Both toxins result in elevated cAMP concentrations, but by different mechanisms.

Practice: Mechanisms of Toxin Action

• Cholera toxin increases cAMP levels by activating Gs.

• Pertussis toxin increases cAMP levels by inhibiting Gi.

• Mechanisms include:

  • Activating or inhibiting GPCRs

  • Binding to and inhibiting adenylyl cyclase

  • Altering the activity of inhibitory G proteins

Agonists vs. Antagonists

Definitions and Mechanisms

Drugs and endogenous molecules can act as agonists or antagonists at various receptors, including GPCRs.

• Agonist: A molecule that binds to a receptor and mimics the effect of the endogenous ligand, activating the receptor.

• Antagonist: A molecule that binds to a receptor but does not activate it, blocking the action of agonists or endogenous ligands.

Example: Caffeine as an Antagonist

• Caffeine acts as an antagonist at adenosine receptors, blocking the inhibitory effects of adenosine and promoting alertness.

• By inhibiting adenosine signaling, caffeine increases neuronal activity and reduces drowsiness.

Practice: Identifying Agonists and Antagonists

• Caffeine is an antagonist at adenosine receptors.

• Antagonists block receptor activation; agonists mimic endogenous ligand effects.

Summary Table: Effects of Cholera and Pertussis Toxins on GPCR Signaling

Key Equations

• cAMP production by adenylyl cyclase:

• G protein activation:

Additional info: GPCR signaling is a central topic in biochemistry, with broad implications for pharmacology and physiology. Understanding the mechanisms of toxins and drugs that affect GPCRs is essential for interpreting cellular responses and drug actions.