Hormone Transport, Receptors, Signal Transduction (3A) - Bio & Biochem

1. What is the typical downstream signaling consequence of RTK phosphotyrosines?

   Docking of SH2/PTB proteins to initiate kinase cascades. Phosphotyrosine residues on activated RTKs recruit SH2/PTB domain proteins, which propagate signals through pathways like MAPK for cell growth.

2. What is the major effect of plasma protein binding on steroid hormone half-life?

   Increases half-life by reducing filtration and metabolism. Binding to plasma proteins protects steroid hormones from rapid renal filtration and enzymatic degradation, thereby prolonging their presence in circulation.

3. What is the primary receptor location for peptide hormones in target cells?

   Cell-surface (plasma membrane) receptors. Peptide hormones are hydrophilic and cannot cross the membrane, so they bind extracellular receptors to initiate intracellular signaling cascades.

4. What is the core mechanism of receptor tyrosine kinase (RTK) activation?

   Ligand-induced dimerization and trans-autophosphorylation. Ligand binding promotes RTK dimerization, enabling cross-phosphorylation that creates docking sites for signaling molecules.

5. What is the defining feature of synaptic signaling?

   Neuron releases neurotransmitter into synaptic cleft. Synaptic signaling ensures rapid, targeted transmission across synapses, crucial for neural communication and reflex actions.

6. Which hormone class typically has a longer duration of action: peptide or steroid?

   Steroid hormones. Steroid hormones induce genomic effects via transcription, leading to sustained protein synthesis and longer-lasting physiological changes compared to rapid peptide actions.

7. What is the general mechanism by which steroid hormone receptors alter cell function?

   Ligand-bound receptor binds DNA to regulate transcription. Steroid receptors, upon ligand binding, translocate to the nucleus and act as transcription factors to modulate gene expression.

8. What is the defining feature of paracrine signaling?

   Local mediator acts on nearby cells. Paracrine signals diffuse locally to affect neighboring cells without entering the bloodstream, allowing targeted regulation in tissues.

9. What is the key transport difference between peptide hormones and steroid hormones in blood?

   Peptides circulate mostly free; steroids are carrier protein-bound. Peptide hormones are water-soluble and dissolve freely in plasma, while steroid hormones are lipid-soluble and require binding to proteins for solubility and transport.

10. What is the immediate product of adenylyl cyclase activity in GPCR signaling?

    cAMP from ATP. Adenylyl cyclase catalyzes the conversion of ATP to cAMP, which acts as a second messenger to amplify GPCR signals intracellularly.

11. Identify the receptor type: insulin receptor is best classified as what?

    Receptor tyrosine kinase (RTK). The insulin receptor undergoes ligand-induced autophosphorylation, initiating cascades for glucose uptake and metabolism regulation.

12. What is the main effect of DAG in the $PIP_2$ pathway?

    Activates protein kinase C (PKC). DAG remains membrane-bound and, with calcium, recruits and activates PKC to phosphorylate targets in signal transduction pathways.

13. Which G protein alpha subunit class inhibits adenylyl cyclase?

    $G_i$. $G_i$ suppresses adenylyl cyclase activity, decreasing cAMP and inhibiting processes like hormone secretion or ion channel opening.

14. What is the most common mechanism for terminating cAMP signaling?

    Phosphodiesterase degrades cAMP to AMP. Phosphodiesterase enzymes rapidly hydrolyze cAMP, preventing prolonged signaling and allowing precise control of cellular responses.

15. What is the defining feature of autocrine signaling?

    Cell signals itself via its own receptors. Autocrine signaling enables self-regulation where a cell produces and responds to its own signaling molecules, often in growth or immune responses.

16. What is the defining feature of juxtacrine signaling?

    Contact-dependent signaling between adjacent cells. Juxtacrine signaling requires direct physical contact, facilitating precise communication in developmental processes like cell fate determination.

17. Which G protein alpha subunit class activates adenylyl cyclase?

    $G_s$. $G_s$ stimulates adenylyl cyclase upon activation, increasing cAMP levels to promote downstream effects like glycogenolysis.

18. What is the defining feature of ligand-gated ion channel receptors?

    Ligand binding directly opens or closes an ion pore. These receptors function as ion channels where ligand binding induces conformational changes to regulate ion flux and membrane potential.

19. What is the primary receptor location for steroid hormones in target cells?

    Intracellular receptors in cytosol and/or nucleus. Steroid hormones are lipophilic and can diffuse through the plasma membrane to bind receptors inside the cell, enabling direct genomic regulation.

20. What is the defining feature of endocrine signaling compared with paracrine signaling?

    Endocrine signals travel via blood to distant targets. Endocrine signaling involves hormone release into the bloodstream for systemic distribution, unlike paracrine which is limited to diffusion over short distances.

21. What is the main effect of $IP_3$ in the $PIP_2$ pathway?

    Opens ER $Ca^{2+}$ channels to raise cytosolic $Ca^{2+}$. $IP_3$ binds to receptors on the endoplasmic reticulum, triggering calcium release that modulates various cellular processes like contraction or secretion.

22. Which G protein alpha subunit class activates phospholipase C?

    $G_q$. $G_q$ triggers phospholipase C to cleave $PIP_2$, initiating the $IP_3$/DAG pathway for calcium-mediated responses.

23. What second messenger is produced by phospholipase C from $PIP_2$?

    $IP_3$ and DAG. Phospholipase C hydrolyzes $PIP_2$ into $IP_3$ and DAG, generating dual second messengers for calcium release and kinase activation.

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