C4H4O42â ââââ> C4H2O42â + 2H+ + 2eâ
FAD + 2H+ + 2eâ ââââ> FADH2
C4H4O4 2â + FAD ââââ> C4H2O42â + FADH2
Oxidant Reductant n E0â (V)
Acetate Acetaldehyde 2 -0.67
NAD+ NADH+H+ 2 -0.32
FAD FADH2 2 -0.22
Pyruvate Lactate 2 -0.19
Fumarate Sucinate 2 +0.03
Dehydroascorbate Ascorbate 2 +0.08
O2+4H+ H2O 4 +0.82
1. The zero for the table of Eoâ values is the same as that for the standard potentialsâthe normal hydrogen electrode where the H+ concentration is 1 M. The Eoâ for 2H+ + 2eâ ââââ> H2, however, is no longer 0.00. Why not? Using the Nernst equation, determine the value of Eoâ for 2H+ + 2eâ ââââ> H2.
2. Rewrite the entries for FAD and fumarate so they look more like the tables that are in your CHEM 250 textbookâthat is, instead of âoxidantâ and âreductantâ columns, write the balanced half reactions.
3. Calculate the Eoâ for the overall reaction that is presented at the beginning of this reading/exercise. Should this be a spontaneous reaction? What is âG Ìâ? (The âprimeâ in âG Ìâ designates pH 7, just as it does in Eoâ.)
4. Does Eoâ for this reaction depend on the pH? Why or why not?
One biochemistry text (Berg, Tymoczko & Stryer, Biochemistry, 6th Edition) lists the
following âG Ìâ:
Succinate + FAD(enzyme-bound) ââ> Fumarate + FADH2(enzyme-bound)
âG Ìâ = 0 kJ/mol
This suggests that the Eoâ values we measure in dilute aqueous solution may be substantially different when enzymesâprotein catalystsâare involved. But it still presents us with a puzzle: how does the reaction go forward when âG Ìâ = 0 kJ/mol?
5. Propose a reason why this reaction might go forward despite the tabulated âG Ìâ being zero.
Another reaction we looked at in the previous exercise is the reduction of pyruvate to lactate by NADH:
Pyruvate + NADH + H+ âââââ> Lactate + NAD+
6. What is the âEoâ and âG Ìâ for this reaction? Is this reaction expected to be spontaneous for this reaction at pH 7? Is the Eoâ for this reaction dependent on the pH? Why or why not?