Multiple Choice Answers:

21. G protein pathways can involve several different Gα's: G_sα, G_iα, G_qα, etc.
a. Describe three characteristics you would expect each Gα to have. (6)
b. Choose two of the Gα's and name one way that they are different. (3)

a. active with GTP bound, can convert GTP to GDP, inactive with GDP bound,
      require a receptor in order to release GDP and bind GTP, inactive when bg bind,
      bind and modify the activity of another enzyme
b.

22. The cell membrane has ΔV = − 0.070 V, [Na⁺]_out = 145 mM, and [Na⁺]_in = 12 mM. (9)
a. Calculate ΔG_T for moving Na⁺ into the cell.
b. The proposed model for function of the Na⁺–K⁺ ATPase involves moving Na⁺ from
       inside the cell to outside the cell in several steps.
      Write three steps that occur between binding Na⁺ inside the cell and releasing Na⁺.

a. − 12.9 kJ/mol
b. I. Bind ATP, transfer P to Asp side chain, release ADP, conformation change
b. II. Bind K⁺, bind H₂O, release P_i, conformation change, release K⁺

23. The Na⁺–Ca²⁺ exchanger moves 3 Na⁺ and 1 Ca²⁺ for each cycle. The exchanger is not an enzyme.
a. Explain why it is most reasonable for the exchanger to be an antiport. (2)
b. ΔG_T for moving Ca²⁺ into the cell is − 38.0 kJ/mol. Based upon your answers to 22 a
      and 23 a, what is the overall ΔG_T for moving 3 Na⁺ and 1 Ca²⁺ by the exchanger?
      Include the direction in which each type of ion is transported. (4)
c. Explain why this is an example of secondary active transport. (3)

a. Na⁺ and Ca²⁺ both have positive charges, so moving them in opposite directions has
      less effect on the electrochemical gradient (is more likely to be favorable).
b. Moving 3 Na⁺ out to in = − 38.8 kJ/mol, and moving 1 Ca²⁺ in to out = + 38.0 kJ/mol.
      Net = − 0.8 kJ/mol
c. No ATP was used directly for this, but ATP was required to create the Na⁺ gradient
      that is used by the exchanger to move Ca²⁺ in an unfavorable direction.

24. a. Write the overall reaction. (3)
b. The reaction occurs in two steps and involves a covalent intermediate. Name the
       product of the first step and draw the covalent intermediate in the active site. (3)
c. Name the substrate of the second step and draw the structure of the second product. (3)

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