To gain insight into the folding mechanism of the cytochrome c complex, we prepared a complete set of homologous and hybrid two-fragment ferric complexes of four different types and related complexes from horse, tuna, yeast iso-1, and Candida cytochromes c. The complexes were characterized for structural properties. Apparent equilibrium constants of the complexes were determined to calculate ΔG0for binding. The results have allowed us to assign four core domains of the complex. A core domain is a structural region containing a hydrophobic core and the surrounding shell which folds and unfolds as a unit. Core domain 1 folds by itself and consists essentially of the right channel structure, found by R. E. Dickerson and colleagues, and a part of the heme. Core domains 2, 3, and 4, respectively, are assigned based on the cores located on the left (the FeS bond) and right sides and at the bottom of heme. Evidence of the core domain-domain interaction to stabilize the FeS bond, combined with the kinetic studies by G. R. Parr and H. Taniuchi, has led to a model of two alternative folding orders of the core domains for the horse type I complex: domain 1 → 3 → 2 → 4 or 1 → 2 → 3 → 4. Furthermore, ΔG0variation between the complexes has shown nonadditive behavior, indicating the existence of a residue-residue interaction between the heme- and apofragments in the complex. Evidence suggests that this interaction in most cases occurs within or through the core groups of the ordered interface between the heme- and the apofragments formed by folding of core domains 1, 2, and 3. Evidence also suggests that such core group interaction manifests itself in the interaction to stabilize the FeS bond and may be manifested in the core domain-domain interaction. © 1992.
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