A partially buried site in homologous HPr proteins is not optimized for stability

Characterizing the denatured state of human prion 121-230

Misfolding and aggregation of the prion protein (PrP) are responsible for the development of fatal transmissible neurodegenerative diseases. PrP undergoes structural conversion from a natively folded state into a misfolded state, resulting in insoluble amyloid fibrils. Partial unfolding has been recognized as an essential step in fibrillation. The strong correlation of unfolding and fibrillation emphasizes the importance of denatured states. To gain insight into possible aggregation-prone denatured states, we characterized the denatured state of human prion (huPrP) 121-230 near extended conformation by self-guided Langevin dynamics simulations. Our results revealed that denatured huPrP is partially folded with alpha-helical structure.

Non-linear effects of temperature and urea on the thermodynamics and kinetics of folding and unfolding of hisactophilin

Extensive measurements and analysis of thermodynamic stability and kinetics of urea-induced unfolding and folding of hisactophilin are reported for 5-50 degrees C, at pH 6.7. Under these conditions hisactophilin has moderate thermodynamic stability, and equilibrium and kinetic data are well fit by a two-state transition between the native and the denatured states. Equilibrium and kinetic m values decrease with increasing temperature, and decrease with increasing denaturant concentration. The betaF values at different temperatures and urea concentrations are quite constant, however, at about 0.7. This suggests that the transition state for hisactophilin unfolding is native-like and changes little with changing solution conditions, consistent with a narrow free energy profile for the transition state. The activation enthalpy and entropy of unfolding are unusually low for hisactophilin, as is also the case for the corresponding equilibrium parameters. Conventional Arrhenius and Eyring plots for both folding and unfolding are markedly non-linear, but these plots become linear for constant DeltaG/T contours. The Gibbs free energy changes for structural changes in hisactophilin have a non-linear denaturant dependence that is comparable to non-linearities observed for many other proteins. These non-linearities can be fit for many proteins using a variation of the Tanford model, incorporating empirical quadratic denaturant dependencies for Gibbs free energies of transfer of amino acid constituents from water to urea, and changes in fractional solvent accessible surface area of protein constituents based on the known protein structures. Noteworthy exceptions that are not well fit include amyloidogenic proteins and large proteins, which may form intermediates. The model is easily implemented and should be widely applicable to analysis of urea-induced structural transitions in proteins.

The Side Chain of Aspartic Acid 69 Dictates the Folding Mechanism of Bacillus subtilis HPr

Many small, single-domain proteins show equilibrium and kinetic folding mechanisms that appear to be adequately described as two state. The two-state model makes several predictions that can be tested experimentally. First, the conformational stability determined at or extrapolated to a set of reference conditions should be independent of the measurement method (thermal or solvent denaturation or hydrogen exchange). Second, model-independent measures of the cardinal thermodynamic parameters (T(m), DeltaH) as determined from direct calorimetric means should be identical to those determined from the two-state analysis of thermal unfolding data. Third, the ratio of the kinetic folding and unfolding rate constants should be equal to K(eq) determined from an equilibrium measurement under the same conditions. Here, we show that the wild-type HPr protein from Bacillus subtilis does not meet all of these criteria under our standard conditions. However, if we replace the side chain of Asp69, or add moderate concentrations of salt, we find excellent two-state behavior in both equilibrium and kinetic folding. Thus, for this protein and possibly others, very subtle changes in the primary structure or in the solution conditions can dramatically alter the relative stabilities of the native intermediate, and unfolded ensembles can cause an observable change in the nature of the folding mechanism.