Basics of Hormonal Signal Transduction
The transmission of a hormonal signal in the target cell always begins when the hormone (also known as the primary messenger) finds its specific receptor. As soon as they bind, an obligatory change in the spatial structure—the conformation of the receptor—occurs. The updated molecule begins to actively interact with other intracellular macromolecules. This process of molecular coupling is called signal transduction. As a result, a potent signal is generated that regulates the cellular response.
The outcome of such a cascade of events is always a change in the rate of metabolic reactions. The cell achieves this via two main pathways:
- The activity of existing enzymes changes (for example, through their chemical modification).
- The amount of enzymes changes (due to the upregulation or downregulation of their synthesis).
Which pathway the signal follows depends directly on the receptor's localization: on the membrane or inside the cell.
Signal Transmission via Membrane Receptors
Receptors embedded in the plasma membrane receive signals from hormones that are unable to penetrate inside the cell. This group of ligands includes peptide hormones, adrenaline, and locally acting hormones (cytokines, eicosanoids). Their mechanism of action is realized through two main pathways.
Pathway via G proteins and second messengers
- Binding of the hormone to the receptor on the membrane surface.
- Activation of a specific G protein.
- Engagement of effector enzymes: adenylate cyclase, guanylate cyclase, or phospholipase C.
- Generation of second messengers (mediators). These can include cAMP, cGMP, inositol trisphosphate (IP3), diacylglycerol (DAG), and Ca²⁺ ions.
- Activation of protein kinases under the action of second messengers.
- Phosphorylation of proteins inside the cell.
- Result: change in the functional activity of proteins, leading to an altered rate of metabolism.
Pathway via autophosphorylation This pathway is characteristic of receptors that possess intrinsic tyrosine kinase activity.
- The hormone binds to the receptor.
- Autophosphorylation of the receptor itself occurs.
- A potent phosphorylation cascade of various intracellular proteins is triggered.
- Enzymes and transcription factors are activated.
- Result: change in the amount of synthesized proteins (and their functional activity), which alters the rate of metabolism.
Action via Intracellular Receptors
If a hormone is able to cross the cell membrane, its receptors are localized inside—in the cytosol or cell nucleus. Typical ligands for this mechanism are steroid hormones and thyroxine. Their action unfolds in stages:
- Free penetration of the hormone into the cell.
- Binding to the receptor in the intracellular space.
- Formation of a stable, active hormone-receptor complex.
- Transport of the formed complex directly into the cell nucleus.
- Interaction of the complex with DNA (specific binding to regulatory regions: enhancers or silencers).
- Induction (activation) or repression (suppression) of protein synthesis at the transcription stage.
- Change in the actual amount of proteins and enzymes in the cell.
- Result: a sustained change in the metabolic rate.
Briefly: answers to topical questions
Which specific hormones act via receptors with tyrosine kinase activity?
The hormone insulin acts via receptors with tyrosine kinase activity.
This hormone interacts with catalytic receptors, which are integral proteins of the plasma membrane. The activation mechanism includes the following stages:
- Ligand binding — attachment of insulin to $\alpha$-subunits induces cooperative conformational changes.
- Receptor activation — $\beta$-subunits exhibit tyrosine kinase activity.
- Transautophosphorylation — one $\beta$-subunit phosphorylates the other at tyrosine residues.
- Signal transduction — phosphorylation of intracellular proteins occurs, altering the functional activity of enzymes.
To easily remember receptor localization, think of a hormone 'pass'. Peptides and adrenaline are large or hydrophilic 'guests' and are met strictly at the entrance (membrane receptors). Steroids and thyroxine are 'family', passing right through the lipid membrane directly into the house (intracellular receptors).
Class and exam questions
Which hormones act via intracellular receptors?
This group includes steroid hormones and thyroxine. They cross the cell membrane and bind to receptors located in the cytosol or cell nucleus.
What acts as the primary and secondary messenger?
The primary messenger is always the hormone itself. Second messengers in the membrane mechanism are molecules inside the cell: cAMP, cGMP, IP3, DAG, and calcium ions (Ca²⁺).
How exactly does cellular metabolism change upon hormonal stimulation?
The metabolic rate changes in two ways: either by altering the activity of pre-existing enzymes (e.g., via phosphorylation), or by changing their quantity (through transcription induction or repression).
What else to study on this topic
- Structural features of receptors with tyrosine kinase activity
- Mechanism of action of G proteins and their classification
- The role of DNA enhancers and silencers in hormone-receptor complex function
- Specifics of second messenger formation: IP3 and DAG
- Differences in the onset speed of effects between peptide and steroid hormones
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