Effect of pH on formation of a nativelike intermediate on the unfolding pathway of a Lys 73 --> His variant of yeast iso-1-cytochrome c.

Previous work on a Lys 73 --> His (H73) variant of iso-1-cytochrome c at pH 7.5 [Godbole et al. (1997) Biochemistry 36, 119-126] showed that this variant unfolds through a nativelike intermediate that has properties consistent with replacement of the Met 80 heme ligand by His 73. Here, the pH dependence of the equilibrium unfolding of the wild type (WT) and H73 proteins have been investigated, since a characteristic pH dependence is expected for the stability of an intermediate stabilized by histidine-heme ligation. Stability has been evaluated using guanidine hydrochloride and pH denaturation methods. Above pH 5, the m-values from guanidine hydrochloride denaturation of the WT and H73 variants remain significantly different, consistent with continued population of this intermediate. At pH 4.5 the m-values for the two proteins are within error the same. To assess stability at lower pH, acid denaturation was carried out. The midpoint is about 3.3 for both proteins but the transition is broader for the H73 protein, suggestive of intermediates again being populated during the unfolding of the H73 protein at this lower pH. Heme ligation by Met 80 was monitored (695 nm absorbance) during gdnHCl (pH 4.5 and 5.0) and acid denaturation, confirming, respectively, the absence and presence of intermediates. A thermodynamic analysis demonstrates that this complex pH dependence for the presence of histidine ligation induced intermediates is expected and implicates a titratable group with a pKa of approximately 6.6. The analysis also demonstrates when the pH dependences of global stability and stability of an intermediate differ significantly, population of folding intermediates as a function of pH will show novel behavior.