Signal Transduction and G Protein-Coupled Receptors

Signal Transduction Pathway

• Signal interacts with receptor.
• Receptor transduces signal from extracellular to intracellular side.
• Transduction and amplification lead to cellular responses.
• Kinases phosphorylate proteins by adding phosphate groups from ATP hydrolysis to serine, threonine, or tyrosine residues.
• Phosphatases remove phosphate groups from phosphorylated amino acids, returning them to their original state.
• Kinases and phosphatases are essential for transduction and amplification.
• Kinases are specific to their target proteins.
• Some kinases are activated by phosphorylation from other kinases.
• Kinases bind ATP and have specificity for a target protein; otherwise, they would be called ATPases.

Signal Amplification

• One receptor activation can lead to the activation of hundreds or thousands of downstream molecules.
• This amplification is achieved through enzymes like kinases that can act repeatedly.
• Example: A priest calling everyone to church disseminates a message quickly to many people.

Fight or Flight Response in Liver Cells

• Epinephrine (adrenaline) is released into the bloodstream during the fight or flight response.
• Epinephrine binds to receptors on liver cells.
• The goal is to release glucose into the bloodstream for muscles to use for energy.

Steps in the Pathway:

1. Epinephrine binds to a G protein-coupled receptor (GPCR).
2. The receptor undergoes a conformational change and interacts with a G protein.
3. The G protein exchanges GDP for GTP, becoming active.
4. The active G protein interacts with adenylate cyclase.
5. Adenylate cyclase produces cyclic AMP (cAMP).
6. cAMP allosterically activates protein kinase A (PKA).
7. PKA phosphorylates phosphorylase kinase.
8. Phosphorylase kinase activates glycogen phosphorylase.
9. Glycogen phosphorylase breaks down glycogen into glucose, releasing it into the bloodstream.

G Protein-Coupled Receptors (GPCRs)

• GPCRs are a significant family of receptors with diverse functions.
• Many neurotransmitters, hormones, and sensory receptors are GPCRs.
• Examples: epinephrine, histamine, serotonin, dopamine, acetylcholine, opioid, cannabinoid, taste and smell receptor

GPCR Activation:

1. Inactive receptor is not in contact with the G protein.
2. Ligand binds to the receptor, causing a conformational change.
3. The receptor interacts with the G protein (alpha, beta, and gamma subunits).
4. The receptor activates the alpha subunit.
5. The alpha subunit exchanges GDP for GTP and separates from the beta/gamma subunits.
6. The active alpha subunit and beta/gamma subunits interact with other proteins.

GPCR Structure:

• The receptor crosses the membrane seven times.
• G proteins are peripheral membrane proteins (GPI-linked).
• The alpha subunit hydrolyzes GTP, which regulates the signaling duration.

Turning on the Switch:

• Ligand binding causes a conformational change in the receptor.
• The receptor interacts with the alpha subunit and facilitates the exchange of GDP for GTP.
• Guanine nucleotide exchange factors (GEFs) help exchange GDP for GTP.
• Active receptor acts as a GEF for the alpha subunit.

Mechanism of Activation of Protein

• GTP exchange (GDP for GTP).
• Protein-protein interaction.
• Allosteric regulation.
• Phosphorylation.

Termination of Signaling

1. GPCR kinase (GRK) phosphorylates the active receptor.
2. Arrestin binds to the phosphorylated receptor.
3. The receptor is internalized via endocytosis.
4. The receptor is either recycled or degraded.

Recycling Pathway:

• Endosomes acidify, changing the protonation state of amino acids in the receptor.
• This reduces the receptor's affinity for the ligand, causing it to dissociate.
• The receptor is then recycled back to the membrane.

Destruction Pathway:

• Occurs when the receptor is constantly activated.
• The cell downregulates the number of receptors on the membrane to maintain homeostasis.
• Example: Opioid addiction leads to receptor downregulation, requiring higher doses to achieve the same effect.