Mechanism of Hormone Action — Revision Notes

⚡ 30-Second Revision

Water-soluble hormones (Peptide, Protein, Catecholamine):

- Receptors: Cell surface (plasma membrane). - Mechanism: Second messenger system. - Key players: G-proteins, Adenylyl cyclase, Phospholipase C. - Second messengers: cAMP, IP3, DAG, Ca$^{2+}$. - Effect: Rapid, short-term, modifies existing proteins. - Examples: Insulin, Glucagon, Adrenaline.

Lipid-soluble hormones (Steroid, Thyroid):

- Receptors: Intracellular (cytoplasm or nucleus). - Mechanism: Gene expression modulation. - Key players: Hormone-receptor complex, Hormone Response Elements (HREs) on DNA. - Effect: Slower, long-lasting, synthesizes new proteins. - Examples: Cortisol, Estrogen, Thyroid hormones (T3, T4).

2-Minute Revision

Hormones act on target cells via two primary mechanisms. Water-soluble hormones (peptides, proteins, catecholamines) cannot cross the cell membrane, so they bind to specific receptors on the cell surface.

This binding activates G-proteins, which then activate effector enzymes like adenylyl cyclase (producing cAMP) or phospholipase C (producing IP3 and DAG). These molecules act as 'second messengers,' amplifying the signal and activating protein kinases (PKA, PKC) that phosphorylate existing cellular proteins, leading to rapid, short-term changes.

In contrast, lipid-soluble hormones (steroids, thyroid hormones) easily diffuse across the cell membrane. They bind to intracellular receptors in the cytoplasm or nucleus, forming a hormone-receptor complex.

This complex then binds to specific DNA sequences called Hormone Response Elements (HREs), directly regulating gene transcription. This leads to the synthesis of new proteins, resulting in slower but more prolonged cellular responses.

Understanding receptor location, second messengers, and the ultimate cellular outcome (protein modification vs. new protein synthesis) is key.

5-Minute Revision

The mechanism of hormone action is dictated by the hormone's chemical nature. Water-soluble hormones, such as insulin, glucagon, and adrenaline, are hydrophilic and cannot penetrate the lipid bilayer. Their action begins with binding to specific cell surface receptors, often G-protein coupled receptors (GPCRs). This binding activates an associated G-protein, which then activates an effector enzyme. Two major pathways emerge:

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cAMP Pathway: — G-protein activates adenylyl cyclase, which converts ATP to cyclic AMP (cAMP). cAMP acts as a second messenger, activating protein kinase A (PKA). PKA phosphorylates various intracellular proteins, altering their activity and leading to rapid cellular responses (e.g., glycogen breakdown by glucagon).
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IP3/DAG Pathway: — G-protein activates phospholipase C (PLC), which cleaves PIP2 into inositol trisphosphate (IP3) and diacylglycerol (DAG). IP3 causes **Ca$^{2+}$ release** from the ER (Ca$^{2+}$ is another second messenger), while DAG, along with Ca$^{2+}$, activates protein kinase C (PKC). PKC phosphorylates target proteins, leading to responses like muscle contraction.

Lipid-soluble hormones, including steroid hormones (e.g., cortisol, estrogen) and thyroid hormones (T3, T4), are lipophilic and readily diffuse across the cell membrane. Their receptors are located intracellularly, either in the cytoplasm or nucleus.

The hormone binds to its specific intracellular receptor, forming a hormone-receptor complex. This complex then translocates to the nucleus (if initially cytoplasmic) and binds to specific DNA sequences called Hormone Response Elements (HREs).

This binding directly regulates the transcription of target genes, leading to the synthesis of new messenger RNA (mRNA) and subsequently new proteins. These newly synthesized proteins mediate the cellular response, which is typically slower to develop but more prolonged (e.

g., growth, development, long-term metabolic changes). The key distinction lies in receptor location, the involvement of second messengers, and the ultimate impact on cellular machinery (modifying existing proteins vs.

synthesizing new ones).

Prelims Revision Notes

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Hormone Classification & Receptor Location:

* Water-soluble hormones: Peptides (Insulin, Glucagon), Proteins (Growth Hormone, FSH, LH), Catecholamines (Adrenaline, Noradrenaline). Receptors are on the cell surface (plasma membrane). * Lipid-soluble hormones: Steroids (Cortisol, Estrogen, Testosterone), Thyroid hormones (T3, T4). Receptors are intracellular (cytoplasm or nucleus).

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Water-soluble Hormone Mechanism (Second Messenger System):

* Binding: Hormone (first messenger) binds to specific cell surface receptor. * G-protein Activation: Receptor activates associated G-protein (e.g., Gs, Gq). * Effector Enzyme Activation: Activated G-protein activates an enzyme.

* Adenylyl Cyclase Pathway: Gs activates Adenylyl Cyclase $\rightarrow$ converts ATP to cAMP (second messenger) $\rightarrow$ cAMP activates Protein Kinase A (PKA) $\rightarrow$ PKA phosphorylates target proteins.

* Phospholipase C Pathway: Gq activates Phospholipase C (PLC) $\rightarrow$ cleaves PIP2 into IP3 and DAG (second messengers). * IP3 $\rightarrow$ binds to ER receptors $\rightarrow$ releases **Ca$^{2+}$** (second messenger).

* DAG (with Ca$^{2+}$) $\rightarrow$ activates Protein Kinase C (PKC) $\rightarrow$ PKC phosphorylates target proteins. * Cellular Response: Rapid, short-term changes by modifying existing proteins (e.

g., enzyme activation/inactivation, ion channel opening).

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Lipid-soluble Hormone Mechanism (Gene Expression Modulation):

* Diffusion: Hormone diffuses across the cell membrane into the cytoplasm. * Intracellular Receptor Binding: Binds to specific intracellular receptor (cytoplasmic or nuclear) $\rightarrow$ forms Hormone-Receptor Complex.

* Nuclear Translocation: Complex moves into the nucleus (if initially cytoplasmic). * DNA Binding: Complex binds to specific DNA sequences called Hormone Response Elements (HREs) in target gene promoters.

* Gene Regulation: Activates or represses gene transcription $\rightarrow$ leads to mRNA synthesis. * Protein Synthesis: mRNA translated into new proteins (enzymes, structural proteins).

* Cellular Response: Slower, long-lasting changes (e.g., growth, development, long-term metabolic adjustments).

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Key Terms: — Receptors, First Messenger, Second Messenger (cAMP, IP3, DAG, Ca$^{2+}$), Signal Transduction, Hormone-Receptor Complex, HREs, Protein Kinases (PKA, PKC).

Vyyuha Quick Recall

Water-soluble Stays Outside, Second Messengers Act. Lipid-soluble Goes Inside, Gene Expression Makes Changes.

WSO — Water-Soluble, Stays Outside (receptor on surface).
SMA — Second Messengers Act (cAMP, IP3, DAG).
LGI — Lipid-soluble, Goes Inside (receptor inside cell).
GEMC — Gene Expression Makes Changes (binds to DNA, new proteins).