Protein denaturation by urea: Slash and bond

The stability of the native folds of globular proteins is rather remarkable, in that this stability is marginal and restricted to a relatively narrow window of thermodynamic and solution composition conditions (1). The development of a deep understanding of the balance of forces that tip the scales between native and denatured states in terms of the individual roles of electrostatics, hydrophobic interactions, polymer entropy, temperature, and pressure would have a profound impact on our ability to understand native structures and abnormal aggregated states and aid in development of bio-mimetic systems. Determining how unfolding occurs, i.e., the dynamic pathway by which the denatured state is established, is even more demanding but may provide insight into the landscape governing protein folding (2). Earlier simulations combining stress from increased temperature and denaturant cosolvent, urea, have followed this pathway for short times (3).

In this issue of PNAS, Hua et al. (4) present the results of a tour de force simulation comprising several microsecond-long simulations of the dynamics of ambient temperature lysozyme in concentrated urea solution, revealing a mechanistic pathway isolating the impact of urea on protein unfolding for the first time. In particular, the simulations reveal a stepwise process, starting from a state manifesting preferential solvation of the globular state by urea, compared with water, driven at least in part by the greater Van der Waals attraction of the protein for urea. The loss of native structure occurs with an initial intrusion into the tertiary structure predominantly by urea, followed only later by substantial hydration, in contrast to evidence for initial hydration during denaturation of chymotrypsin inhibitor 2 at elevated temperatures (3). Appreciating the lessons provided by these observations requires some reconciliation with related studies on the interactions of urea with simpler solutes in aqueous media.

The principles behind the destabilization of …

¹E-mail: rossky{at}mail.utexas.edu