Question

4. Calculate the standard reduction potential and the standard free energy change for each of the following oxidation-reduction reactions:

a. Ubiquinol (QH2) + 2 Cytochrome C(Fe +3) <--------> Ubiquinone (Q) + 2 Cytochrome C(Fe+2) + 2H+

b. Succinate + ½ O2 <--------> Fumarate + H2O

c. Explain how odd chain fatty acids can serve as gluconeogenic precursors.

Answer 1

(a)

Ubiquinol (QH₂) + 2 Cytochrome C(Fe⁺³) <--------> Ubiquinone (Q) + 2 Cytochrome C(Fe⁺²) + 2H⁺

Ubiquinol (QH₂)  --------> Ubiquinone (Q) + 2e^- + 2H⁺ E^o = -0.04 V

2 Cytochrome C(Fe⁺³) + 2e^- --------> 2 Cytochrome C(Fe⁺²)   E^o = 0.23 V

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Add (i) + (ii) ===> E^o_cell = -0.04 V + 0.23 V = 0.19 V

ΔG^o = -nFE^o_cell

ΔG^o = -2 mole x 96500 J/V.mole x 0.19 V

ΔG^o = -36670 J

ΔG^o = -36.67 kJ

(b)

Succinate + ½ O₂ <--------> Fumarate + H₂O

½ O₂ + 2H⁺ + 2e^-  ------------> H₂O E^o = 0.82 V

Succinate^- --------> Fumarate + 2H⁺ + 2e   E^o = -0.03 V

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Add (i) + (ii) ===> E^o_cell = 0.82 V -0.03 V = 0.79 V

ΔG^o = -nFE^o_cell

ΔG^o = -2 mole x 96500 J/V.mole x 0.79 V

ΔG^o = -152570 J

ΔG^o = -152.47 kJ