Defining folding and unfolding reactions of apocytochrome b5 using equilibrium and kinetic fluorescence measurements

The refolding and unfolding kinetics of a soluble domain of apocytochrome b(5) extending from residue 1 to 104 have been characterized using stopped flow and equilibrium-based fluorescence methods. The isolated apoprotein unfolds reversibly in the presence of GuHCl. From cooperative unfolding curves, the conformational stability (Delta G(uw)), in the absence of denaturant, is estimated to be 11.6 +/- 1.5 kJ mol(-1) at 10 degrees C. The stability of apocytochrome bs is lower than that of the corresponding form of the holoprotein (Delta G similar to 25 kJ mol(-1)) and exhibits a transition midpoint at 1.6 M GuHCl. Kinetic studies support the concept of a two-state model with both unfolding and refolding rates showing an exponential dependence on denaturant concentration with no evidence of the formation of transient intermediates in either limb of the chevron plot. Apocytochrome bs is therefore an example of a protein in which both kinetics and equilibria associated with folding are described by a two-state model. The values of m(ku) and m(kf) obtained from kinetic analysis are an indication of a transition state (m(ku)/m(eq) of 0.29) that resembles the native form by retaining similar solvent accessibility and many of the noncovalent interactions found in the apoprotein. The changes in heat capacity support a transition state that resembles the apoprotein with a value for Delta C-pf of -3.6 kJ mol(-1) K-1 estimated for the refolding reaction. From these measurements, a model of refolding that involves the rapid nucleation of hydrophobic residues around Trp26 is suggested as a major event in the formation of the native apoprotein.