1. What common features are shared by most cell signaling systems?

2. Signaling by soluble extracellular molecules can be classified as endocrine, paracrine, or autocrine. Describe how these three types of cellular signaling differ. Growth hormone is secreted from the pituitary, which is located at the base of the brain and acts through growth hormone receptors located on the liver. Is this an example of endocrine, paracrine, or autocrine signaling? Why?

3. A ligand binds two different receptors with a K_d value of 10⁻⁷ M for receptor 1 and a K_d value of 10⁻⁹ M for receptor 2. For which receptor does the ligand show the greater affinity? Calculate the fraction of receptors that have a bound ligand ([RL]/R_T) in the case of receptor 1 and receptor 2 if the concentration of free ligand is 10⁻⁸ M.

4. To understand how a signaling pathway works, it is often useful to isolate the cell-surface receptor and to measure the activity of downstream effector proteins under different conditions. How could you use affinity chromatography to isolate a cell-surface receptor? With what technique could you measure the amount of activated G protein (the GTP-bound form) in ligand-stimulated cells? Describe the approach you would take.

5. How do seven–transmembrane domain G protein–coupled receptors transmit a signal across the plasma membrane? In your answer, include the conformational changes that occur in the receptor in response to ligand binding.

6. Signal-transducing heterotrimeric G proteins consist of three subunits designated α, β, and γ. The G_α subunit is a GTPase switch protein that cycles between active and inactive states depending on whether it is bound to GTP or to GDP. Review the steps for ligand-induced activation of effector proteins mediated by the heterotrimeric G proteins. Suppose that you have isolated a mutant G_α subunit that has an increased GTPase activity. What effect would this mutation have on the G protein and the effector protein?

7. Explain how FRET could be used to monitor the association of G_αs and adenylyl cyclase following activation of the epinephrine receptor.

8. Which of the following steps amplify the epinephrine signal response in cells: receptor activation of G protein, G protein activation of adenylyl cyclase, cAMP activation of PKA, or PKA phosphorylation of glycogen phosphorylase kinase (GPK)? Which change will have a greater effect on signal amplification: an increase in the number of epinephrine receptors or an increase in the number of G_αs proteins?

9. The cholera toxin, produced by the bacterium Vibrio cholerae, causes a watery diarrhea in infected individuals. What is the molecular basis for this effect of cholera toxin?

10. Both rhodopsin in vision and the muscarinic acetylcholine receptor in cardiac muscle are coupled to ion channels via G proteins. Describe the similarities and differences between these two systems.

11. Epinephrine binds to both β-adrenergic and α-adrenergic receptors. Describe the opposite actions on the effector protein, adenylyl cyclase, elicited by the binding of epinephrine to these two types of receptors. Describe the effect of adding an agonist or antagonist to a β-adrenergic receptor on the activity of adenylyl cyclase.

12. In liver and muscle, epinephrine stimulation of the cAMP pathway activates glycogen breakdown and inhibits glycogen synthesis, whereas in adipose tissue, epinephrine activates hydrolysis of triglycerides, and in other cells, it causes a diversity of other responses. What step in the cAMP signaling pathways in these cells specifies the cell response?

13. Continuous exposure of a G_αs protein–coupled receptor to its ligand leads to a phenomenon known as desensitization. Describe several molecular mechanisms for receptor desensitization. How can a receptor be reset to its original sensitized state? What effect would a mutant receptor lacking serine or threonine phosphorylation sites have on a cell?

14. What is the purpose of A kinase–associated proteins (AKAPs)? Describe how AKAPs work in heart muscle cells.

15. Inositol 1,4,5-trisphosphate (IP₃) and diacylglycerol (DAG) are second messenger molecules derived from the cleavage of phosphatidylinositol 4,5-bisphosphate [PI(4,5)P₂] by activated phospholipase C. Describe the role of IP₃ in causing a rise in cytosolic Ca²⁺ concentration. How do cells restore resting levels of cytosolic Ca²⁺? What is the principal function of DAG?

16. In Chapter 3, the K_d of calmodulin EF hands for binding Ca²⁺ is given as 10⁻⁶ M. Many proteins have much higher affinities for their respective ligands. Why is the specific affinity of calmodulin important for Ca²⁺ signaling processes such as that initiated by production of IP₃?

17. Most of the short-term physiological responses of cells to cAMP are mediated by activation of PKA. Another common second messenger is cGMP. What are the targets of cGMP in rod and smooth muscle cells?