Solution to PS 4 #3

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From the difference in the standard reduction potential, Eo' and DG0',   can be calculated as follows:

For Succinate Dehydrogenase using enzyme bound FAD:

succinate -> fumarate +2e^- + 2H⁺     has E^o'=-0.03v
FAD + 2e^- + 2H⁺ -> FADH₂           has E^o'=+0.05v

So net  if FAD is electron acceptor,  E^o'= 0.05-0.03=0.02 v and DG=-nFE^o'=-2(96.48)(0.02v)= -3.9 kJ/mol

using NAD⁺ a electron acceptor instead would mean;

succinate -> fumarate +2e^- + 2H⁺      has Eo'= -.03 v
NAD⁺ + 2e^- +2H⁺ -> NADH          has Eo'=-0.32v

So net Eo'= -0.03 -0.32v=-0.35 v and DG=-2(96.48 v)(-0.35v)= + 68 kJ/mol.


So the oxidation of succinate by FAD is favored by the negative free energy change (DG^o'= -3.9 kJ/mol).  The oxidation of NAD⁺ would require a large, positive free energy change (DG^o'= 68 kJ/mol).

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