1.  Like salt and pepper. Match the terms with the descriptions. ✓ 4

2.  Class differences. What are the three major classes of membrane receptors? ✓ 5

3.  Magnification. Explain how a small number of hormones binding to the extracellular surface of a cell can have a large biochemical effect inside the cell. ✓ 4

4.  Common properties. What are some of the structural features common to all membrane-bound receptors? ✓ 4 ✓ 5

5.  On–off. Why is the GTPase activity of G proteins crucial to the proper functioning of a cell? ✓ 4

6.  Specificity. Hormones affect the biochemistry of a distinct set of tissues. What accounts for the tissue specificity of hormone action? ✓ 4

7.  Like peanut butter and jelly. Match the terms with the descriptions.

8.  Chimeras. In an elegant experiment on the nature of receptor tyrosine kinase signaling, a gene was synthesized that encoded a chimeric receptor—the extracellular part came from the insulin receptor, and the membrane-spanning and cytoplasmic parts came from the EGF receptor. The striking result was that the binding of insulin induced tyrosine kinase activity, as evidenced by rapid autophosphorylation. What does this result tell you about the signaling mechanisms of the EGF and insulin receptors? ✓ 5

9.  Antibodies mimicking hormones. An antibody has two identical antigen-binding sites. Remarkably, antibodies to the extracellular parts of growth-factor receptors often lead to the same cellular effects as does exposure to growth factors. Explain this observation. ✓ 5

10.  Alike but different. What is the difference between heterotrimeric G proteins and small G proteins? ✓ 4

11.  Facile exchange. A mutated form of the a subunit of the heterotrimeric G protein has been identified; this form readily exchanges GDP for GTP even in the absence of an activated receptor. What would be the effect on a signaling pathway containing the mutated a subunit?

12.  Diffusion rates. Normally, rates of diffusion vary inversely with molecular weights; so smaller molecules diffuse faster than do larger ones. In cells, however, calcium ion diffuses more slowly than does cAMP. Propose a possible explanation.

13.  Awash with glucose. Glucose is mobilized for ATP generation in muscle in response to epinephrine, which activates G_αs. Cyclic AMP phosphodiesterase is an enzyme that converts cAMP into AMP. How would inhibitors of cAMP phosphodiesterase affect glucose mobilization in muscle? ✓ 4

14.  Many defects. Considerable effort has been directed toward determining the genes in which sequence variation contributes to the development of type 2 diabetes, a disease that results from a loss of sensitivity of cells to insulin. Approximately 800 genes have been implicated. Explain the significance of this observation.

15.  Growth-factor signaling. Human growth hormone binds to a cell-surface membrane protein that is not a receptor tyrosine kinase. The intracellular domain of the receptor can bind other proteins inside the cell. Furthermore, studies indicate that the receptor is monomeric in the absence of hormone but dimerizes on hormone binding. Propose a possible mechanism for growth-hormone signaling. ✓ 5

16.  Different genes. Differentiate among a proto-oncogene, an oncogene, and a tumor-repressor gene.

17.  Accelerator and brake. Only one copy of a proto-oncogene need be mutated to enhance the development of cancer, whereas both copies of a tumor-suppressor gene must be mutated to contribute to cancer development. Explain this distinction.

18.  Redundancy. Because of the high degree of genetic variability in tumors, typically no single anticancer therapy is universally effective for all patients, even within a given tumor type. Hence, it is often desirable to inhibit a particular pathway at more than one point in the signaling cascade. In addition to the EGFR-directed monoclonal antibody cetuximab, propose alternative strategies for targeting the EGF signaling pathway for antitumor drug development.

19.  A reappearance. Ligand-gated channels can be thought of as receptors. Explain.

20.  Molecular exposure. The binding of Ca²⁺ to calmodulin induces substantial conformational changes in its EF hands, exposing hydrophobic residues on the surface of the protein. How might this structural change help to propagate the calcium signal?

21.  Making connections. Suppose that you were investigating a newly discovered growth-factor signal-transduction pathway. You found that, if you added a GTP analog in which the terminal phosphate was replaced by sulfate, the duration of the hormonal response was increased. What can you conclude?

22.  Vive la difference. Why is the fact that a monomeric hormone binds to two identical receptor molecules, thus promoting the formation of a dimer of the receptor, considered remarkable? ✓ 5

23.  Active mutants. Some protein kinases are inactive unless they are phosphorylated on key serine or threonine residues. In some cases, active enzymes can be generated by mutating these serine or threonine residues to glutamate. Explain.

24.  Signal-to-noise ratio. At steady state, intracellular levels of Ca²⁺ must be kept low to prevent the precipitation of carboxylated and phosphorylated compounds, which form poorly soluble salts with Ca²⁺. The cytoplasmic level of Ca²⁺ is approximately 100 nM, several orders of magnitude lower than the concentration in the extracellular medium. How might the cell maintain such low levels of intracellular Ca²⁺? How does the cell take advantage of the difference in intracellular and extracellular Ca²⁺ concentrations?

25.  Receptor truncation. You prepare a cell line that overexpresses a mutant form of EGFR in which the entire intracellular region of the receptor has been deleted. Predict the effect of overexpression of this construct on EGF signaling in this cell line. ✓ 5

26.  Total amplification. Suppose that each β-adrenergic receptor bound to epinephrine converts 100 molecules of G_αs into their GTP-bound forms and that each molecule of activated adenylate cyclase produces 1000 molecules of cAMP per second. With the assumption of a full response, how many molecules of cAMP will be produced in 1 s after the formation of a single complex between epinephrine and the β-adrenergic receptor? ✓ 4

27.  Establishing specificity. You wish to determine the hormone-binding specificity of a newly identified membrane receptor. Three different hormones, X, Y, and Z, were mixed with the receptor in separate experiments, and the percentage of binding capacity of the receptor was determined as a function of hormone concentration, as shown in graph A.



(a) What concentrations of each hormone yield 50% maximal binding?

(b) Which hormone shows the highest binding affinity for the receptor?

You next wish to determine whether the hormone–receptor complex stimulates the adenylate cyclase cascade. To do so, you measure adenylate cyclase activity as a function of hormone concentration, as shown in graph B.



(a) What is the relation between the binding affinity of the hormone–receptor complex and the ability of the hormone to enhance adenylate cyclase activity? What can you conclude about the mechanism of action of the hormone–receptor complex?

(b) Suggest experiments that would determine whether a G_αs protein is a component of the signal-transduction pathway.

28.  Binding matters. A scientist wishes to determine the number of receptors specific for a ligand X, which he has in both radioactive and nonradioactive form. In one experiment, he adds increasing amounts of the radioactive X and measures how much of it is bound to the cells. The result is shown as total activity in the adjoining graph. Next, he performs the same experiment, except that he includes a several hundredfold excess of nonradioactive X. This result is shown as nonspecific binding. The difference between the two curves is the specific binding.



(a) Why is the total binding not an accurate representation of the number of receptors on the cell surface?

(b) What is the purpose of performing the experiment in the presence of excess nonradioactive ligand?

(c) What is the significance of the fact that specific binding attains a plateau?

29.  Counting receptors. With the use of experiments such as those described in problems 27 and 28, the number of receptors in the cell membrane can be calculated. Suppose that the specific activity of the radioactive ligand is 10¹² counts per minute (cpm) per millimole and that the maximal specific binding is 10⁴ cpm per milligram of membrane protein. There are 10¹⁰ cells per milligram of membrane protein. Assume that one ligand binds per receptor. Calculate the number of receptor molecules present per cell.