Redox Chemistry and Acid−Base Equilibria of Mitochondrial Plant Cytochromes c

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Abstract
Mitochondrial cytochromes c from spinach, cucumber, and sweet potato have been investigated through direct electrochemical measurements and electronic and 1H NMR spectroscopies, under conditions of varying temperature and pH. The solution behaviors of these plant cytochromes closely resemble, but do not fully reproduce, those of homologous eukaryotic species. The reduction potentials (E°‘) at pH 7 and 25 °C are +0.268 V (spinach), +0.271 V (cucumber), and +0.274 V (sweet potato) vs SHE. Three acid−base equilibria have been determined for the oxidized proteins with apparent pKa values of 2.5, 4.8, and 8.3−8.9, which are related to disruption of axial heme ligation, deprotonation of the solvent-exposed heme propionate-7 and replacement of the methionine axially bound to the heme iron with a stronger ligand, respectively. The most significant peculiarities with respect to the mammalian analogues include: (i) less negative reduction enthalpies and entropies (ΔS°‘rc and ΔH°‘rc) for the various protein conformers [low- and high-T native (N1 and N2) and alkaline (A)], whose effects at pH 7 and 25 °C largely compensate to produce E°‘ values very similar to those of the mammalian proteins; (ii) the N1 → N2 transition that occurs at a lower temperature (e.g., 30−35 °C vs 50 °C at pH 7.5) and at a lower pH (7 vs 7.5); and (iii) a more pronounced temperature-induced decrease in the pKa for the alkaline transition which allows observation of the alkaline conformer(s) at pH values as low as 7 upon increasing the temperature above 40 °C. Regarding the pH and the temperature ranges of existence of the various protein conformers, these plant cytochromes c are closer to bacterial cytochromes c2.

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