Unfolding and Hydrogen Exchange of Proteins: the Three-dimensional Ising Lattice as a Model

Abstract

WE have noted a discrepancy between the models which are used to explain two kinds of experimental data related to protein unfolding. On the one hand, recent experiments on the reversible unfolding of the proteins ribonuclease, chymotrypsinogen and myoglobin, as revealed by changes in optical rotation and light absorption as a function of temperature or pH, demonstrate that such transitions can be described by a two state or all-or-none model^1–3, in which partly folded molecules are not observed. On the other hand, there are the results on isotopic hydrogen exchange of proteins^4,5. This exchange (for example, >NH + D₂O→>ND + DHO) is possible only for hydrogen atoms which are exposed to the solvent. Hence a portion of the hydrogen atoms of a protein molecule exchanges more slowly than do exposed hydrogens, and the rate of exchange is equal to the product of the exchange rate for free hydrogens and the degree of unfolding (k_obs=k_o·θ_unf). Experimentally one observes an enormous variety of exchange rates among buried peptide group hydrogens, whereas the two state model predicts a single degree of unfolding as well as a single rate of unfolding and hence a single rate of exchange characteristic of all buried hydrogens.