Problem 16
Glucose Transporters A cell biologist working with cultured cells from intestinal epithelium finds that the cells take up glucose from the growth medium 10 times faster when the glucose concentration is \(5 \mathrm{~mm}\) than when it is \(0.2\) mo. She also finds that glucose uptake requires \(\mathrm{Na}^{+}\)in the growth medium. What can you say about the glucose transporter in these cells?
Problem 19
Electrogenic Transporters A single-cell organism, Paramecium, is large enough to allow the insertion of a microelectrode, permitting the measurement of the electrical potential between the inside of the cell and the surrounding medium (the membrane potential). The measured membrane potential is \(-50 \mathrm{mV}\) (inside negative) in a living cell. What would happen if you added valinomycin to the surrounding medium, which contains \(\mathrm{K}^{+}\)and \(\mathrm{Na}^{+}\)?
Problem 20
Energetics of the \(\mathrm{Na}^{+} \mathbf{K}^{+}\)ATPase For a typical vertebrate cell with a membrane potential of \(-0.070 \mathrm{~V}\) (inside negative), what is the free-energy change for transporting 1 mol of \(\mathrm{Na}^{+}\) from the cell into the blood at \(37^{\circ} \mathrm{C}\) ? Assume the \(\mathrm{Na}^{+}\) concentration is 12 mm inside the cell and 145 mm in blood plasma.
Problem 21
Action of Ouabain on Kidney Tissue Ouabain specifieally inhibits the \(\mathrm{Na}^{+} \mathrm{K}^{+}\)ATPase activity of animal tissues but is not known to inhibit any other enzyme. When ouabain is added to thin slices of living kidney tissue, it inhibits axygen consumption by 6696 . Why? What does this observation tell us about the use of respiratory energy by kidney tissue?
Problem 22
Digoxin to Inhibit \(\mathrm{Na}^{+} \mathrm{K}^{+}\)ATPase The \(\mathrm{Na}^{+} \mathrm{Ca}^{2+}\) exchanger expressed in cardiac myocytes is a bidirectional antiporter protein that removes calcium from the cytoplasm by exchanging it with sodium. Cardiac myocytes also express the \(\mathrm{Na}^{+} \mathrm{K}^{+}\)ATPase. Suppose that a \(\mathrm{Na}^{+} \mathrm{K}^{+}\)ATPase inhibitor (digoxin) is added to cardiac myocytes. Using your knowledge of the relative concentrations of ions (intracellular versus extracellular) and the important role of the \(\mathrm{Na}^{+} \mathrm{K}^{+}\)ATPase in maintaining the electrochemical gradient, what change would you expect in the intracellular \(\left[\mathrm{Ca}^{2+}\right] ?\) Why?
Problem 23
Energetics of Symport Suppose you determined experimentally that a cellular transport system for glucose, driven by symport of \(\mathrm{Na}^{+}\), could accumulate glucose to concentrations 25 times greater than in the external medium, while the external \(\left[\mathrm{Na}^{+}\right]\)was only 10 times greater than the intracellular \(\left[\mathrm{Na}^{+}\right]\). Would this violate the laws of thermodynamics? If not, how could you explain this observation?
Problem 25
Transport Types You have just discovered a new Lalsnine transporter in liver cells (hepatocytes). Poisoning hepatocytes with cyanide (which blocks ATP synthesis) reduces alanine transport by 909. Tenfold reduction in extracellular [Na \(^{+}\)] has no immediate effect on alanine transport. How would you use these observations to decide whether the alanine transporter is passive or active, primary or secondary?
Problem 27
Ion Channel Selectivity Potassium channels consist of four subunits that form a channel just wide enough for \(\mathrm{K}^{+}\) ions to pass through. Although \(\mathrm{Na}^{+}\)ions are smaller \(\left(M_{z} 23\right.\), radius \(0.95 \AA\) ) than \(K^{+}\)ions \(\left(M_{\mathrm{r}} 39\right.\), radius \(\left.1.33 \bar{A}\right)\), the potassium channels in the bacterium Streptomyces Lividans transport 104 times more \(\mathrm{K}^{+}\)ions than \(\mathrm{Na}^{+}\)ions. What prevents \(\mathrm{Na}^{+}\)ions from passing through potassium channels?
Problem 29
Predicting Membrane Protein Topology I Online bainformatics tools make hydropathy analysis easy if you know the amino acid sequence of a protein. At the Protein Data Bank (www?rosharg), the Protein Feature View displays additional information about a protein gleaned from other databases, such as Uniprot and SCOP2. A simple graphical view of a hydropathy plot created using a window of 15 residues shows hydrophobic regions in red and hydrophilic regions in blue. a. Looking only at the displayed hydropathy plots in the Protein Feature View, what predictions would you make about the membrane topology of these proteins: glycophorin A (PDB ID 1AFO), myoglobin (PDB ID \(1 \mathrm{MBO}\), and aquaporin (PDB ID 2B6O)? 1507 b. Now, refine your information using the ProtScale tools at the ExpASy bioinformatics resource portal. Each of the PDB Protein Feature Views was created with a UniProt Knowledgebsese ID. For glycophorin \(A\), the UniProtKB ID is P02724; for myoglobin, P02185; and for aquaporin, Q6J819. Go to the ExPASy portal (http://web.expasy orgLprotscale) and select the Kyte \& Doolittle hydropathy analysis option, with a window of 7 amino acids. Enter the UniProtKB ID for aquaporin (Q6JS19, which you can also get from the PDB's Protein Feature View page), then select the option to analyze the complete chain (residues 1 to 263). Use the default values for the other options and click Submit to get a hydropathy plot. Save a GIF image of this plot. Now repeat the analysis using a window of 15 amino acids. Compare the results for the 7 -residue and 15-residue window analyses. Which window size gives you a better signal-to-noise ratio? c Under what circumstances would it be important to use a narrower window?
