Conformational transitions of frog heart ferricytochrome c.

A monomeric cytochrome c containing an intramolecular disulfide bond linking residues 20 and 102 was obtained from bullfrog hearts, as previously reported by Chan et al. [Chan, S. K., Walasek, O. F., Barlow, G. H., & Margoliask, E. (1967) Fed. Proc., Fed. Am. Soc. Exp. Biol. 26, 723]. The stability of the native globular conformation of this protein is enhanced relative to that of other cytochromes c; e.g., it is stable to 0.01 N HCl in the absence of neutral salts at 25 degrees C, and its denaturation transition in guanidine at neutral pH and 25 degrees C is centered at 3.8 M. Guanidine-denatured from cytochrome c renatures in two kinetic phases whose time constants differ by about 2 orders of magnitude as observed when using other cytochromes c. However, the absolute values for the time constants of the frog protein are notably smaller. We note that the time constants for cytochromes c are inversely related to the midpoints of their guanidine transitions, suggesting that within a homologous series of proteins the more stable the conformation the faster it folds.