Biochem 124 Problem Set

1. The standard-state enthalpy change for the oxidation of palmitic acid CH3(CH2)14COOH (s) + 24 O2 (g) -> 17 CO2 (g) + 16 H2O(l)

Is -9805 kJ a. Calculate ΔU b. Calculate the work done when 1 mole is oxidized at 1 atm pressure.

2. The equilibrium constant (Keq) for a binding reaction was seen to increase monotonically as the temperature (T) is increased from 290 K to 315 K (see table). Determine the ΔH°, ΔS°, and ΔG° for this reaction at room temperature (298 K).

T (K) Keq (M-1) 290 1.2 x 10-5 295 1.3 x 10-5 300 1.4 x 10-5 305 1.5 x 10-5 310 1.6 x 10-5 315 1.7 x 10-5

3. Assuming that a polypeptide chain has only one α-helical conformation and that there are three possible orientations for each amino acid residue in the random-coil state, calculate ΔS for the conformational change.

α-helix random coil for a polypeptide of 100 residues. What value of ΔH per residue would to make the melting point (the temperature at which the equilibrium constant equals 1) be 50°C? Compare this with the hydrogen-bond energy, estimated to be 0 to 12 kJ/mol

4. A simplified version of the Hexamer Toy Model is the Tetramer Toy Model (TTM), which involves only four residues. For the tetramer sequence H-P-P-H (where H’s are hydrophobic residues and P’s are polar residues),

a. Draw all the possible conformations of the tetramer. Hint: the folded form is, where the dotted line indicates a stabilizing H-H interaction:

b. Assuming that H-H interaction is stabilizing, how many energy

levels are there in the TTM? c. Calculate the ΔS for each energy level relative to the lowest

conformation.

5. Consider the oxidation of ethanol by NAD+ to form acetaldehyde: NAD+ + C2H5OH NADH + CH3CHO + H+

Assuming standard half-cell reduction potentials E°’ NAD+/NADH = -0.32V and E°’ CH3CHO/EtOH = -0.18 V, calculate the standard cell potential E°’, the standard free energy change ΔG°’, and the equilibrium constant K’. Based on your answer, does the system favor the formation of acetaldehyde or ethanol at equilibrium? Assume T=310K.

6. Calculate the membrane potentials across a membrane at 37°C, if the NaCl concentration on the right is 0.1M and on the left is 0.01M, given each of the following conditions.

a. Membrane is permeable only to Na+ b. Membrane is permeable only to Cl- c. Membrane is permeable only to both

7. Using the Gibbs-Duhem equation, obtain an expression for

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µ2 −µ2° from

theequation

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µ1 −µ1O = −RTV1

OC21M2

+ BC2

8. The data below show the binding of oxygen to squid hemocyanin. Determine from a Hill plot whether the binding is cooperative and estimate the minimum number of subunits in the molecule.

pO2 (mm)

Percent Saturation

pO2 (mm)

Percent Saturation

1.13 0.30 136.7 55.7 5.55 1.33 166.8 67.3 7.72 1.92 203.2 73.4 10.72 3.51 262.2 79.4 31.71 8.37 327.0 83.4 71.87 18.96 452.8 87.5 100.5 32.90 566.9 89.2 123.3 47.80 736.7 91.3