Protein unfolding denaturant induced

Turning now to the chapters in this volume, a variety of complementary techniques and approaches have been used to characterize peptide and protein unfolding induced by temperature, pressure, and solvent. Our goal has been to assemble these complementary views within a single volume in order to develop a more complete picture of denatured peptides and proteins. The unifying observation in common to all chapters is the detection of preferred backbone conformations in experimentally accessible unfolded states. 

That which we call a rose, by any other name would smell as sweet. The essence of the denatured state, in contrast, is rather more elusive. The difficulty stems, at least in part, from the wide variety of methods of inducing a protein to unfold. Some proteins appear to be natively unfolded that is, they remain unfolded in the cell under conditions in which they retain their biological activity (Plaxco and Gross, 1997 Wright and Dyson 1999 Dunker, 2002). Other proteins unfold only under the influence of changes in pH, high or low temperatures, or... 

Figure 16.5 (a) The native folded state of the protein and the unfolded, denatured state following the thermally-induced structural change (b) the duplex state of nucleic acids, stable at low temperatures, in which the bases are paired and stacked, and the monomer states following the thermal disruption in which the bases are unpaired and randomly arranged along the backbone. 

As mentioned earlier, proteins are subject to cold denaturation because they exhibit maximal stability at temperatures greater than 0°C. The basis of this effect is the reduction in the stabilizing influence of hydrophobic interactions as temperature is reduced. Recall that the burial of hydrophobic side-chains in the folded protein is favored by entropy considerations (AS is positive), but that the enthalpy change associated with these burials is unfavorable (AH, too, is positive). Thus, as temperature decreases, there is less energy available to remove water from around hydrophobic groups in contact with the solvent. Furthermore, as temperature is reduced, the term [— TAS] takes on a smaller absolute value. For these reasons, the contribution of the hydrophobic effect to the net free energy of stabilization of a protein is reduced at low temperatures, and cold-induced unfolding of proteins (cold denaturation) may occur. 

The practical solution to the protein stability dilemma is to remove the water. Lyophilization (freeze-drying) is most commonly used to prepare dehydrated proteins, which, theoretically, should have the desired long-term stability at ambient temperatures. However, as will be described in this review, recent infrared spectroscopic studies have documented that the acute freezing and dehydration stresses of lyophilization can induce protein unfolding [8-11]. Unfolding not only can lead to irreversible protein denaturation, even if the sample is rehydrated immediately, but can also reduce storage stability in the dried solid [12,13]. 

Structural peculiarities of the different proteins should be therefore taken into account when a relationship between the degree of succinylation and emulsification is derived. These include conformational changes and the contribution of hydrophobic regions exposed by the unfolding of the globular protein structure. Moreover, the technological treatment of the raw material or even the protein, which can induce denaturation or interaction with nonprotein compounds, is to be considered for understanding the possibly different behavior of proteins from the same source. 

In Equation 10.8, AG is the folding free energy of the protein, is the first-order rate constant of the slow hydrogen exchange reaction at the C-2 position in the imidazole side chain of an unprotected histidine, m is 54G, / 5[denaturant], T is the temperature in Kelvin, R is the ideal gas constant, and [P] is the protein concentration expressed in n-ma- equivalents. Equation 10.8 can be daived from Equation 10.9, which is commonly used in the linear extrapolation method (LEM) to analyze denaturant-induced equilibrium unfolding curves [34] ... 

Sasahara, K., M. Sakurai, and K. Nitta. 2001. Pressure effect on denaturant-induced unfolding of hen egg white lysozyme. Proteins Structure, Function, and Genetics. 44, 180. 

The biophysical characterization of globular proteins will almost always include some type of study of the unfolding of protein to obtain thermodynamic parameters. The basic idea is that a transition between a native and unfolded state, induced by temperature, pH, or denaturant concentration, can serve as a standard reaction for obtaining a thermodynamic measure of the stability of the native state. For example, the free energy change for the unfolding reaction can be used to compare the stability of a set of mutant forms of a protein (1-4). 

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