In the mitochondrial electron transfer chain, one of the steps that mediates transfer of electrons from nadh to o2 is the transfer of electrons from the reduced (ferrous) heme of cytochrome b to the oxidized (ferric) heme of cytochrome c1: fe ii (cyt b) + fe iii (cyt c1) → fe iii (cyt b) + fe ii (cyt c1) the standard state reduction potentials of fe iii (cyt c1) and fe iii (cyt b) are 0.25 v and 0.08 v, respectively, relative to the standard hydrogen electrode.(a) express the above redox reaction as two half reactions, one as an oxidation and one as a reduction. (b) determine the equilibrium constant for the reaction as written. is this reaction spontaneous, favoring products?

(a)  Oxidation:  Fe(II)(cyt b) ⇌ Fe(III)(cyt b) + e⁻

     Reduction: Fe(III)(cyt c1) + e⁻ ⇌ Fe(II)(cyt c1)

(b) K = 750; spontaneous

Explanation:

(a) Half-reactions

Oxidation:  Fe(II)(cyt b) ⇌ Fe(III)(cyt b) + e⁻

Reduction: Fe(III)(cyt c1) + e⁻ ⇌ Fe(II)(cyt c1)

Overall:      Fe(II)(cyt b) + Fe(III)(cyt c1) ⇌ Fe(III)(cyt b) + Fe(II)(cyt c1)

(b) Equilibrium constant

(i) Standard cell potential

The standard reduction potentials are

                                                       E°/V

Fe(III)(cyt c1)  + e⁻ ⇌ Fe(II)(cyt c1); 0.25

Fe(III)(cyt b) + e- ⇌ Fe(II)(cyt b);    0.08

For the reaction in Part (a),                                                                    

                                                                                           E°/V

Fe(II)(cyt b) ⇌ Fe(III)(cyt b)                                                -0.08

Fe(III)(cyt c1) ⇌ Fe(II)(cyt c1)                                               0.25

Fe(II)(cyt b) + Fe(III)(cyt c1) ⇌ Fe(III)(cyt b) + Fe(II)(cyt c1)  0.17

The standard cell potential is 0.17 V.

(ii) Equilibrium constant

E° and K are positive, so the reaction is spontaneous.

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