21.3
Segnalazione di membrana
Gli ormoni, o qualsiasi molecola che si lega a un recettore, noto come un ligando, che sono lipo-insolubili (solubile in acqua) non sono in grado di d…
- [Istruttore] Alcuni ormoni come quelli
che sono lipidicamente insolubili come l'ossitocina,
non possono diffondersi attraverso la membrana cellulare.
Devono legarsi ai recettori sulla superficie della cellula.
Dopo l'associazione, l'ormone considerato il primo
messaggero attiva una cascata di segnalazione. Ad esempio,
quando un recettore, accoppiato a proteine G o GPCR,
è attivato da un primo messaggero all'esterno
della membrana cellulare, all'interno un enzima
come la fosfolipasi C idrolizza la membrana
fosfolipide PIP2 in IP3 e DAG,
due diversi messaggeri secondari.
Una volta formato, IP3 si sposta dalla membrana cellulare
alla membrana endoplasmatica dove si lega
a un canale di calcio IP3 che rilascia ioni di calcio,
un terzo messaggero secondario, nel citoplasma
e inducendo cambiamenti come la contrazione muscolare.
In questo modo, un ormone può colpire una cellula
che non entra, attivando un numero
di processi cellulari a seconda dei ligandi e dei recettori
che sono presenti.
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Q1: Why can't lipid-insoluble hormones cross the cell membrane directly?
Lipid-insoluble hormones are water-soluble molecules that cannot diffuse through the hydrophobic lipid bilayer of the cell membrane. Instead, these hormones bind to receptors on the cell surface, triggering a signaling cascade that produces cellular responses without the hormone entering the cell itself.
Q2: What happens when a first messenger binds to a G protein-coupled receptor?
When a hormone binds to a G protein-coupled receptor, the alpha subunit of the G protein dissociates from the receptor, activating an intracellular enzyme like phospholipase C. This enzyme hydrolyzes the membrane phospholipid PIP2 into IP3 and DAG, which are second messengers that trigger downstream cellular responses.
Q3: How do second messengers amplify hormonal signals inside the cell?
Second messengers like IP3 and calcium ions activate multiple proteins and pathways within the cell, amplifying the initial signal from a single hormone molecule. This multi-step process creates a cascade effect where each step depends on multiple reactions, verifying proper cellular activities occur and magnifying the response.
Q4: What are the three structural domains found in cell membrane receptors?
Cell membrane receptors contain an external ligand-binding domain that recognizes hormones, a transmembrane domain that spans the cell membrane, and an internal domain that initiates cellular responses. These three portions work together to translate extracellular signals into intracellular effects without requiring hormone entry.
Q5: How do ligand-gated ion channels differ from enzyme-linked receptors?
Ligand-gated ion channels undergo conformational changes when bound to a ligand, allowing ions to pass through a channel in the transmembrane region. Enzyme-linked receptors have an intracellular domain that is either associated with an enzyme or is an enzyme itself, directly activating other proteins when ligands bind.
Q6: What is signal transduction and why is it a multi-step process?
Signal transduction is the conversion of an extracellular stimulus into an intracellular response through multiple molecular interactions. Each step depends on other reactions, providing verification that proper cellular activities occur and allowing the response to be amplified before reaching the target outcome.
Q7: How can a hormone affect a cell without entering it?
Water-soluble hormones bind to cell surface receptors, triggering intracellular signaling cascades through second messengers like calcium ions. These messengers activate cellular processes such as muscle contraction depending on the ligands and receptors present, allowing hormones to produce effects without crossing the cell membrane. Understanding these mechanisms is central to the stress response and hypothalamic pituitary axis.