Three-state analysis of sperm whale apomyoglobin folding.

We give a quantitative description of the urea- and acid-induced transitions of apomyoglobin at 0 degree C and 2 mM sodium citrate. Our data consist of two series of unfolding curves: (1) acid-induced unfolding carried out in the presence of various concentrations of urea and (2) urea-induced unfolding at various pH values. A three-state equation is derived which relates the stability of three different conformations of apomyoglobin (native [N], unfolded [U], and intermediate [I]) as a function of urea and of pH. This equation fits our data reasonably well. The parameters which give the best fit have both thermodynamic and structural implications for N, I, and U. Specifically, I is closer in Gibbs energy to U than to N, indicating that side-chain packing results in much of the stability of native protein structure. The equilibria between N and I and between I and U are equally sensitive to urea, suggesting that much of the surface of I is inaccessible to solvent. The acid-induced transition in which N unfolds can be described as the result of titration of approximately two histidines with low pKaS in N. Under physiological conditions (neutral pH, no urea) I is the most stable non-native conformation.