Intramolecular flip between two alternative forms of complex formed from a heme fragment and apoprotein of horse cytochrome c

The previous studies have shown that (a) noncovalent interactions of the ferro-heme fragment of residues 1-38 and apoprotein (1-104) of horse cytochrome c simultaneously and specifically form two isomeric complexes, types I and II resembling the native protein (the redundant residues flexibly protruding from the ordered structure); (b) the type II form but not type I appears to bind to CO; and (c) residues 39-55 are more flexible for type II form than type I (Parr, G. R., and Taniuchi, H. (1981) J. Biol. Chem. 256, 125-132). In the present study, we investigated 1) kinetics and thermodynamics of interconversion between type I and II forms of complex ferro-(1-38)-H.(1-104); 2) the properties of the CO binding population; 3) the rate of dissociation of complexes ferri- and ferro-(1-38)-H.(39-104) (mimicking type II form); and 4) thermal transition of the 695-nm absorption band and biological activity of complexes. The results indicate (a) interconversion between type I and II forms of complex ferro-(1-38)-H.(1-104) occurs without going through dissociation (t1/2 less than or equal to 12 min at 10 degrees C) and is associated with delta H (= -7.2 +/- 3.7 kcal/mol at 10 degrees C) favoring type I form and delta S (= 23 +/- 13 e.u. at 10 degrees C) favoring type II; (b) the CO-binding population correlates with type II; and (c) change from the ferrous to the ferric state of heme appears to perturb the thermodynamic relationship between type I and II forms. Interpreting the results and available evidence, we suggest that "intramolecular" flip between ferro-type I and ferro-type II forms would establish the Boltzmann distribution of these two distinctly different energy states, type I form having more strengthened interatomic interactions and type II more pronounced internal motion.