Quiz 3, BICH 410 (MWF 3-3:50P), Friday November 12, 1999

Write your name on each page. Clearly circle your response to multiple-choice questions. No partial credit for multiple-choice questions
gas constant R 8.315 J/mol-K
Faraday constant F 96.5 kJ/volt-mol

1. (3 pts) Which of the following segments is most likely to be part of the transmembrane alpha helix of an integral membrane protein?
Answer: C; in the transmembrane part, the amino acid sidechains are hydrophobic, which eliminates answers A and B. Answer D is not good because of the proline in the middle, that disrupts alpha helices.

2. (3 pts) What is the free energy change for the transport of calcium ions (Ca++) across a membrane from a region (left) where the concentration is 50 micromolar to a region (right) where the concentration is 50 micromolar? A membrane potential of 60 mV exists across the membrane where the right side is more negative than the left side. The temperature is 25 C.
Answer: B; deltaGt = RT ln [Ca++]right/[Ca++]left + ZF(membrane potential)
deltaGt = RT ln 1 + (2)(96.5 kJ/mole-volt)(-.06 volt) = 0 + (-11.58 kJ/mole) = -11.58 kJ/mole
The second term of the equation is negative because it would be favorable thermodynamically to move postively-charged calcium ions to a region that has a relatively more negative potential difference.

3. (3 pts) Which of the following is NOT involved in the translation of proteins that are bound to be secreted by the cell (signal hypothesis)?
Answer: E

4. (3 pts) The standard free energy change for the reaction catalyzed by aldolase is +22.8 kJ/mole. The concentrations for some intermediates inside the cell are: [fructose-1,6-bisphosphate] = 2.0 mM, [NAD+] = 0.9 mM, [NADH] = 3.0 mM, [dihydroxyacetone phosphate] = 0.5 mM, [glyceraldehyde-3-phosphate] = 0.2 mM. What is the actual physiological free energy change for the glycolytic reaction catalyzed by aldolase (37 C)?
Answer: E; reaction catalyzed by aldolase is FBP <-> DHAP + GAP
deltaG = deltaG0' + RT ln ([DHAP][GAP])/[FBP] = +22.8 kJ/mole + (8.315 J/mole-K)(310 K)/(1000 J/kJ) ln ((.0005 M)(.0002 M))/(.002 M)
deltaG = +22.8 kJ/mole + (2.578 kJ/mole)(ln .00005) = +22.8 + (-25.5) kJ/mole = -2.7 kJ/mole

5. (3 pts) If glucose molecules labeled with 14C in C-3 were fed to yeast growing anaerobically (alcoholic fermentation), where would the 14C label be in the products?
Answer: B; C-3 of glucose becomes the non-phosphorylated end-carbon of DHAP that is converted to the carbonyl carbon in GAP; the carbonyl carbon of GAP is oxidized to a carboxyl in the second phase of glycolysis, so the carboxyl of one (out of two) pyruvates is derived from the hexose C-3; in alcoholic fermentation the carboxyl of pyruvate is lost as CO2.

6. (2 pts) Which of the following is FALSE regarding the mechanism of glyceraldehyde-3-phosphate dehydrogenase?
Answer: C; the phosphohistidine is involved in the mechanism of phosphoglycerate mutase; see Fig. 14-9 for the mechanism of GAPDH

7. (3 pts) The catabolism of fructose to pyruvate in the liver does NOT involve the following enzyme:
Answer: C; see Fig. 14-26