Equilibrium thermodynamics of a physiologically-relevant heme-protein complex

We used isothermal titration calorimetry to study the equilibrium thermodyn amics for formation of the physiologically-relevant redox protein complex b etween yeast ferricytochrome c and yeast ferricytochrome c peroxidase. A 1: 1 binding stoichiometry was observed, and the binding free energies agree w ith results from other techniques. The binding is either enthalpy- or entro py-driven depending on the conditions, and the heat capacity change upon bi nding is negative. Increasing the ionic strength destabilizes the complex, and both the binding enthalpy and entropy increase. Increasing the temperat ure stabilizes the complex, indicating a positive van't Hoff binding enthal py, yet the calorimetric binding enthalpy is negative (-1.4 to -6.2 kcal mo l(-1)). We suggest that this discrepancy is caused by solvent reorganizatio n in an intermediate state. The measured enthalpy and heat capacity changes are in reasonable agreement with the values estimated from the surface are a change upon complex formation. These results are compared to those for fo rmation of the horse ferricytochrome c/yeast ferricytochrome c peroxidase c omplex. The results suggest that the crystal and solution structures for th e yeast complex are the same, while the crystal and solution structures for horse cytochrome c/yeast cytochrome c peroxidase are different.