The mechanism of hormone action via cAMP involves cAMP acting as a second messenger to activate intracellular enzymes, ultimately leading to a cellular response.
Here's a breakdown of the cAMP mechanism:
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Hormone Binding: A water-soluble hormone (the first messenger) binds to its specific receptor on the surface of the target cell's plasma membrane.
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G-Protein Activation: The hormone-receptor complex activates a G protein located on the inner surface of the plasma membrane. G proteins are intermediary proteins that can activate or inhibit other enzymes or ion channels.
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Adenylate Cyclase Activation: The activated G protein, in turn, activates adenylate cyclase, an enzyme embedded in the plasma membrane.
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cAMP Production: Activated adenylate cyclase catalyzes the conversion of ATP (adenosine triphosphate) to cyclic AMP (cAMP). This significantly increases the intracellular concentration of cAMP. cAMP now acts as the second messenger.
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Protein Kinase A (PKA) Activation: cAMP then binds to the regulatory subunits of Protein Kinase A (PKA), an enzyme present in the cytoplasm. This binding causes the regulatory subunits to detach, activating the catalytic subunits of PKA.
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Protein Phosphorylation: Activated PKA catalyzes the phosphorylation of various intracellular proteins (enzymes, structural proteins, etc.). Phosphorylation involves the addition of a phosphate group to a protein.
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Cellular Response: The phosphorylation of specific proteins alters their activity, leading to a specific cellular response. This response can vary depending on the target cell and the proteins that are phosphorylated. Examples of cellular responses include:
- Enzyme activation or inactivation: Modifying metabolic pathways.
- Changes in membrane permeability: Altering ion channel activity.
- Gene transcription: Influencing the synthesis of new proteins.
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cAMP Degradation: The effects of cAMP are short-lived because phosphodiesterase, another enzyme in the cell, quickly degrades cAMP into AMP (adenosine monophosphate), which is inactive. This helps to terminate the signal and prevent overstimulation.
Summary Table:
| Step | Description |
|---|---|
| 1. Hormone Binding | Hormone binds to receptor on cell membrane. |
| 2. G-Protein Activation | Hormone-receptor complex activates G protein. |
| 3. Adenylate Cyclase Activation | G protein activates adenylate cyclase. |
| 4. cAMP Production | Adenylate cyclase converts ATP to cAMP (second messenger). |
| 5. PKA Activation | cAMP activates Protein Kinase A (PKA). |
| 6. Protein Phosphorylation | PKA phosphorylates intracellular proteins. |
| 7. Cellular Response | Phosphorylation alters protein activity, leading to a specific cellular effect. |
| 8. cAMP Degradation | Phosphodiesterase degrades cAMP, terminating the signal. |
In essence, the cAMP mechanism amplifies the original hormonal signal, allowing a small amount of hormone to elicit a large cellular response. This signal transduction pathway is crucial for regulating a wide variety of physiological processes.
