Protein Folding and Stability

Most physical properties of a protein change substantially when it unfolds. Consequently, many techniques can be used to follow unfolding those used most often are ultraviolet difference spectroscopy, circular dichroism, optical rotation, fluorescence, and NMR spectroscopies. One of the most popular methods of estimating protein stability is based on monitoring urea and guanidine hydrochloride denaturation curves of the protein.  

The conformational stability of a globular protein may be defined as the free energy change for the equilibrium 

In general, AG varies linearly with denaturant concentration. The method of least-squares analysis is used to fit the data from the transition region to the equation 

An interesting consequence of the relationship in Eq. [75] is that relative maxima and minima in protein stability occur at pHs for which the net charge of the denatured and native states are equal. Thus, the isoionic point is the pH of maximal stability only when this pH happens to be isoionic for the denatured state as well. 

Upon biosynthesis, a polypeptide folds into its native conformation, which is structurally stable and functionally active. The conformation adopted ultimately depends upon the polypeptide s amino acid sequence, explaining why different polypeptide types have different characteristic conformations. We have previously noted that stretches of secondary structure are stabilized by short-range interactions between adjacent amino acid residues. Tertiary structure, on the other hand, is stabilized by interactions between amino acid residues that may be far apart from each other in terms of amino acid sequence, but which are brought into close proximity by protein folding. The major stabilizing forces of a polypeptide s overall conformation are  

Hydrophobic interactions are the single most important stabilizing influence of protein native structure. The hydrophobic effect refers to the tendency of non-polar substances to minimize contact with a polar solvent such as water. Non-polar amino acid residues constitute a significant proportion of the primary sequence of virtually all polypeptides. These polypeptides will fold in such a way as to maximize the number of such non-polar residue side chains buried in the polypeptide s interior, i.e. away from the surrounding aqueous environment. This situation is most energetically favourable. 

The description of protein structure as presented thus far may lead to the conclusion that proteins are static, rigid structures. This is not the case. A protein s constituent atoms are constantly in motion, and groups ranging from individual amino acid side chains to entire domains can be displaced via random motion by anything up to approximately 0.2 nm. A protein s conformation, therefore, displays a limited degree of flexibility, and such movement is termed breathing . 

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Mutations of Cytochrome c that Primarily Affect Protein Stability and Folding.. 146... 

Shirley, B. A., ed., (1995). Protein Stability and Folding Theory and Practice. Humana Press, Totowa, NJ. 

Freire E. Differential scanning calorimetry. In Shirley BA, ed. Protein Stability and Folding— Theory and Practice. Totowa, NJ Humana Press, 1995 191-218. 

Xia, Y., Levitt, M. Funnel-Kke organization in sequence space determines the distributions of protein stability and folding rate preferred by evolution. Proteins 2004, 55(1), 107-14. 

W. Pfeil, Protein Stability and Folding A Collection of Thermodynamic Data. Springer, Berlin,... 

The interfacial water [1-10], which is usually regarded as the key in understanding various processes, such as solvation process of particles/molecules in water [11-16], adsorption/desorption at surfaces [17-22], electrochemical reactions [23,24], protein stability and folding [25], molecular... 

Roca, M., Messer, B., 8c Warshel, A. (2007). Electrostatic contributions to protein stability and folding energy. FEES Letters, 581, 2065. 

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