The role of hydration in protein stability: comparison of the cold and heat unfolded states of Yfh1

Protein unfolding occurs at both low and high temperatures, although in most cases, only the high-temperature transition can be experimentally studied. A pressing question is how much the low- and high-temperature denatured states, although thermodynamically equivalent, are structurally and kinetically similar. We have combined experimental and computational approaches to compare the high- and low-temperature unfolded states of Yfh1, a natural protein that, at physiologic pH, undergoes cold and heat denaturation around 0 °C and 40 °C without the help of ad hoc destabilization. We observe that the two denatured states have similar but not identical residual secondary structures, different kinetics and compactness and a remarkably different degree of hydration. We use molecular dynamics simulations to rationalize the role of solvation and its effect on protein stability.