Multiple choice answers

26.

      b. Between 10.07 and 12.48

      c. Residue 3's side chain becomes protonated → –COOH.

      d. You could say Glu's pK_a is probably lower, since it is close to the positively-charged
      Arg, or that Arg's pK_a is probably higher, since it is between two negatively charged
      groups.

28. The reaction catalyzed by isocitrate dehydrogenase has ΔG°′ = − 20.9 kJ/mol.
a. What is the ratio of [products]/[reactants] when ΔG = 0?
b. Write the reaction catalyzed by isocitrate dehydrogenase. (You do not need to draw any molecules.)
c. Describe one condition that would cause ΔG to increase to 0, and explain how that affects the activity of isocitrate dehydrogenase.

      a. ΔG = ΔG°′ + R T ln ([products]/[reactants])
      0 = − 20,900 J/mol + (8.314 J/mol-K) (298 K) ln ratio
      20,900 J/mol = 2477.572 J/mol (ln ratio)
      ln ratio = 8.44
      ratio = 4609 = 4600

      b. isocitrate + NAD⁺ → α-ketoglutarate + CO₂ + NADH + H⁺

      c. When O₂ is low, NADH can't be recycled using electron transport. [NAD⁺] could be
      quite low, while [NADH] could be high. At the same time, if pO₂ is low, [CO₂] might
      be higher. High [NADH] inhibits isocitrate dehydrogenase.

29. Electron transport is associated with pumping protons against their electrochemical gradient. Answer a and b; answer either c or d.
a. Fill in the table to indicate the number of protons pumped by each Complex as a
      result of oxidizing 1 NADH.

b. Given ΔV = − 0.20 V (P → N), [H⁺]_P = 1 x 10⁻⁷ M, and [H⁺]_N = 5.8 x 10⁻⁸ M,
       what is ΔG_T for moving H⁺ across the membrane by Complex IV?
c. How many ATP's can be made as a result of oxidizing 4 NADH? Show your labeled
       work for solving the problem.
d. Describe one condition that would change the answer to part c. You should be clear
      about whether the number of ATP's made would increase or decrease as a result of
      the change.

      b. ΔG_T = R T ln (c₂/c₁) + Z F ΔV
       = (8.314 J/mol-K) (298 K) ln (1 x 10⁻⁷ M/5.8 x 10⁻⁸ M) + (+2) (96480 J/V-mol) (0.20 V)
      = 2477.572 J/mol (ln 1.7) + 38592 J/mol
      = 2477.572 J/mol (0.54) + 38592 J/mol
      = 1338 J/mol + 38592 J/mol = 39930 J/mol = + 39.9 kJ/mol

      c. Oxidation of 4 NADH results in moving 40 H⁺ N → P. Since synthesis of 1 ATP requires
      moving 4 H⁺ from P to N, a total of 10 ATP can be synthesized.

      d. Changing the number of c subunits would change the number of ATP synthesized per
      revolution of the ring of c subunits. If the number of c subunits increased, more H⁺
      would have to move through F_ofor each ATP made, and the number of ATP would
      be lower. If the number of c subunits decreased, the number of ATP made would go
      up, but there's a thermodynamic limit to how much that could increase.

            Adding an uncoupler would decrease the number of ATP made, because some H⁺
      would move through the membrane without going through F_o.

Answer one number 30 or the other but not both.

30. PKA directly or indirectly regulates the activity of several enzymes involved in glycolysis, gluconeogenesis, and glycogen metabolism. (10)
a. Write the reaction PKA catalyzes.
b. How is PKA activated? Be specific. You do not need to describe the entire pathway,
      but you should name the initial cause and describe the final activation step.
c. In general, what is the relationship between activating PKA and regulation of
      carbohydrate metabolism?

30. Draw the reactions of the citric acid cycle and include the energy produced.
Write the name of each enzyme that catalyzes an oxidation-reduction reaction. (10)

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