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1.Describe the difference between endocrine, paracrine and autocrine signaling.
A: In endocrine signaling, a ligand is released into the bloodstream and binds to distal target cell. In paracrine signaling, a ligand is released and binds to neighboring cells. In autocrine signaling, a signal is released and binds to receptors on the same cell.
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2.What is the difference between the ligand for an intracellular receptor and a cell-surface receptor?
A: Ligands for intracellular receptors are hydrophobic; for cell-surface receptors, ligands are hydrophilic.
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3.Does increasing the number of receptors on the cell surface increase or decrease its sensitivity to a given concentration of ligand?
A: It increases its sensitivity.
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4.Describe the 3-4 major classes of cell surface receptors.
A: 1. G-protein coupled, 2. ionotropic or ligand-gated ion channels, 3. receptors with intrinsic enzymatic activity; 4. receptor tyrosine kinase linked receptors (also called cytokine receptors)
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5.If a ligand binds to a receptor but does not activate the downstream signaling pathway, is it an agonist or an antagonist?
A. antagonist.
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6.What are the three subunits of G proteins?
A. alpha, beta and gamma
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7.What is the structure of G-protein coupled receptors?
Seven membrane spanning domains
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8.What second messenger is produced in cells upon binding of epinephrine to the -adrenergic receptor?
A. cAMP
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9.How does epinephrine binding to the -adrenergic receptor increase cAMP levels in the cell?
A. by transducing its signal to the enzyme adenylyl cyclase.
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10.Which subunit of the trimeric G protein activates adenylate cyclase?
A. G alpha S
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11.How is activation of adenylate cyclase by the G protein terminated?
A. by the intrinsic GTPase activity of G alpha S. This activity hydrolyzes GTP to GDP, and GDP bound G alpha S reassociates with G alpha beta and gamma.
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12.How does cAMP activate PKA?
A. By binding to the regulatory subunit leading to dissociation of the catalytic subunits.
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13.How can elevated cAMP, acting through PKA, alter transcription of specific genes in the nucleus?
A. The PKA catalytic subunit translocates into the nucleus and can phosphorylate and activate the transcription factor CREB.
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14.Describe two ways in which signal transduction through G-protein coupled receptors provides amplification of signaling.
A. 1. The adenylyl cyclase can make many many molecules of cAMP. 2. The ligand bound receptor can move in the membrane and bind to several G proteins, activating each sequentially. 3. cAMP activates PKA which phosphorylates many substrate proteins.
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15.When a ligand binds to a receptor linked to a Gq alpha subunit, what enzyme is activated? What second messengers are produced?
A. Phospholipase C is the activated enzyme. It produces the second messengers DAG and IP3. IP3 can bind to the IP3 receptor and lead to release of another second messenger, calcium, from the endoplasmic reticulum into the cytosol.
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16.How is PKC activated upon binding of a ligand to a Gq-coupled receptor?
A. Calcium binds to PKC and recruits it to the membrane where it interacts with DAG resulting in PKC activation.
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17.How is CamK activated upon binding of a ligand to a Gq-coupled receptor?
A. IP3 binds to the IP3 receptor and leads to release of calcium from the endoplasmic reticulum into the cytosol. Calcium binds to calmodulin and the calcium/calmodulin complex bind to the regulatory domain of CamK, leading to activation of the catalytic domain.
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18. What is the structure of RTKs?
A. Extracellular domain with ligand binding site, a single hydrophobic transmembrane alpha helix, and a cytosolic domain that includes a region with tyrosine kinase activity.
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19. What happens when ligand binds to an RTK?
A. Ligand binding triggers dimerization, leading to phosphorylation of activation lip tyrosines by one subunit. This leads to phosphorylation of additional tyrosine residues in cytoplasmic domain and to binding of adaptor proteins to phosphorylated residues.
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20. What happens when tyrosines are phosphorylated on the cytoplasmic domain of an RTK?
A. Adaptor proteins bind, including the ras GEF sos, which leads to activation of ras (by GTP binding), turning on MAP kinase signaling cascade.
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21. How does ras activation lead to downstream signaling in the cell?
A. Activated ras recruits Raf to the plasma membrane and activates it. Raf in turn phosphorylates and activates MEK, which phosphorylates and activates MAP kinase.
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22. What are the components of the MAP kinase signaling cascade?
A. Raf (MAPKKK), MEK (MAPKK) and MAP kinase (MAPK).
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23.What is the result of ligand binding to a ligand-gated ion channel?
A. The channel opens and allows ions to flow through.
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24. What five classes of taste do we perceive?
A. Sweet, sour, salty, bitter and umami.
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25. Describe two taste receptors.
A. The taste receptors for sweet, bitter and umami are G protein coupled receptors; the taste receptors for salty and sour are ionotropic receptors.
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26. What type of G-protein are sweet, umami and bitter taste receptors coupled to?
A. Gustducin, a G protein specific to taste receptors, and G alpha i
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27. What signaling cascades are linked to sweet, umami and bitter taste receptors?
A. Phospholipase C, which cleaves PIP2 to IP3 and DAG.
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28. What type of receptors are salty and sour taste receptors?
A. They are ionotropic receptors.
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