Protein stability: Electrostatics and compact denatured states

Globular proteins can be denatured by changing pH and ionic strength. Much recent evidence has led to the surprising conclusion that there are two acid-denatured states: one highly unfolded and the other more compact, sometimes called the "molten globule." Here we describe a molecular theory for electrostatic stability of globular proteins based on the properties of the constituent amino acids: oil/water partition coefficients, pK values of the titratable groups, and their temperature dependences. Predicted denaturation temperatures vs. pH are in good agreement with experiments of other workers on myoglobin. The theory also predicts two populations of denatured species, one open and the other more compact, with densities in the range found experimentally for molten globular states. In addition, it predicts a phase diagram (stability vs. pH, ionic strength) in good agreement with experiments of Goto and Fink [Goto, Y. & Fink, A. L. (1989) Biochemistry 28, 945-952; and Goto, Y. & Fink, A. L. (1990) J. Mol. Biol. 214, 803-805]. The well-known salt destabilization of myoglobin has been generally considered evidence for ion pairing, but the present theory, based on smeared charge repulsion, explains the salt destabilization at low pH without ion pairing. In addition, for myoglobin the theory predicts salt stabilization at high pH, as observed for β-lactamase by Goto and Fink.