Kinetics of unfolding and folding of horse heart ferricytochrome c with urea

Kinetic data for the reversible folding and unfolding by urea of horse heart ferricytochrome c, in 0.05 M phosphate + 0.25 M Na2SO4 buffer, pH 7.0, in the region of the main denaturation transition, 4-9 M, are reported. Stopped flow technique and absorptivity at 695, 528, and 361 nm as the monitoring probes were used. The decay profiles in the region of the transition 6-7.2 M urea are adequately described by a rate law with two exponential decay terms, but a rate law with only a single term is found to be applicable at the lower and higher limits of the transition. The apparent rate constant for the fast phase exhibits urea dependence with a minimum value at about 6.5 M urea, while the apparent rate constant of the slow phase is found to be independent of urea and has a value of 0.04 +/- 0.02 s-1. The assignment of the two apparent rate constants to the respective steps and the characterization of the processes involved were carried out through correlation of the kinetic data to the results from equilibrium studies for urea denaturation ( Myer, Y. P., MacDonald, L. H., Verma, B. C., and Pande, A. (1980) Biochemistry 19, 199-207). A mechanism X1 in equilibrium X2 in equilibrium D, where the first step is the urea-dependent unfolding and folding, and the second, an apparent urea-independent process involving possibly reorganization of the unfolded form X2, has been proposed to account for the above findings. The X2 in equilibrium D process is further considered in light of various possibilities: the incorrect folding of the unfolded form, the alteration of heme iron coordination, and the cis-trans isomerization of proline.