Problem 2
In principle, the physiological effects of epinephrine should be mimicked by addition of cAMP to the target cells. In practice, addition of cAMP to intact target cells elicits only a minimal physiological response. Why? When the structurally related derivative dibutyryl cAMP (shown) is added to intact cells, the expected physiological response is readily apparent. Explain the basis for the difference in cellular response to these two substances. Dibutyryl cAMP is widely used in studies of cAMP function.
Problem 3
The gram-negative bacterium Vibrio cholerae produces a protein, cholera toxin \(\left(M_{\mathrm{r}} 90,000\right)\), that is responsible for the characteristic symptoms of cholera: extensive loss of body water and \(\mathrm{Na}^{+}\)through continuous, debilitating diarrhea. If body fluids and \(\mathrm{Na}^{+}\)are not replaced, severe dehydration results; untreated, the disease is often fatal. When the cholera toxin gains access to the human intestinal tract, it binds tightly to specific sites in the plasma membrane of the epithelial cells lining the small intestine, causing adenylyl cyclase to undergo prolonged activation (hours or days). a. What is the expected effect of cholera toxin on [cAMP] in the intestinal cells? b. Based on the information above, suggest how cAMP normally functions in intestinal epithelial cells. c. Suggest a possible treatment for cholera.
Problem 4
Explain how mutations in the R or C subunit of cAMP-dependent protein kinase (PKA) might lead to (a) a constantly active PKA or (b) a constantly inactive PKA.
Problem 5
The respiratory symptoms of asthma result from constriction of the bronchi and bronchioles of the lungs, caused by contraction of the smooth muscle of their walls. Raising [cAMP] in the smooth muscle reverses the constriction of the bronchi and bronchioles. Explain the therapeutic effects of albuterol, an inhaled \(\beta\)-adrenergic agonist, in treating asthma. Would you expect this drug to have any side effects? If so, what design change could you make to the drug to minimize side effects?
Problem 6
Signals carried by hormones must eventually be terminated. Describe several mechanisms for signal termination.
Problem 9
In the \(\beta\)-adrenergic system, which of these contributes to the amplification of the signal (epinephrine) and which to the termination of the signal? Do any contribute to both amplification and termination of the signal? a. One \(\mathrm{G}_{\alpha}\) activates many adenylyl cyclase molecules. b. One protein kinase A (PKA) phosphorylates many target proteins. c. The intrinsic GTPase of G protein converts bound GTP to GDP. d. A phosphodiesterase acts on many molecules of cAMP. e. One epinephrine molecule activates many adrenergic receptors. f. One protein kinase phosphorylates many molecules of another protein kinase.
Problem 12
Compare the G protein G \(_{\text {s }}\), which acts in transducing the signal from \(\beta\)-adrenergic receptors, and the G protein Ras. What properties do they share? How do they differ? What is the functional difference between \(\mathrm{G}_{\mathrm{s}}\) and \(\mathrm{G}_{\mathrm{i}}\) ?
Problem 14
Many enzymes can hydrolyze GTP between the \(\beta\) and \(\gamma\) phosphates. The GTP analog \(\beta, \gamma\)-imidoguanosine \(5^{\prime}\)-triphosphate \((\mathrm{Gpp}(\mathrm{NH}) \mathrm{p})\), shown here, cannot be hydrolyzed between the \(\beta\) and \(\gamma\) phosphates.
Problem 16
An analog of cGMP, 8-Br-cGMP, will permeate cellular membranes, is only slowly degraded by a rod cell's PDE activity, and is as effective as cGMP in opening the gated channel in the cell's outer segment. If you suspended rod cells in a buffer containing a relatively high [8-Br-cGMP], then illuminated the cells while measuring their membrane potential, what would you expect to see?
Problem 17
Protein kinase B (PKB) inactivates glycogen synthase kinase (GSK3), and GSK3 inactivates glycogen synthase. Predict the effect of insulin on glycogen synthesis.
