Thermal denaturation, unfolded proteins

The van t Hoff plots for thermal denaturation of proteins are linear in the transition region, thus allowing the enthalpy change (AHm) of unfolding at the transition temperature (Tm) to be estimated. Because of the change in free energy in (AG) = 0 at Tm (reversible process), the entropy of unfolding (ASm) at the transition midpoint can be calculated from ... 

Commonly, the thermal denaturation of proteins is often discussed in terms of the Lumry-Eyring model, which involves two steps reversible unfolding and irreversible alteration of the unfolded state to produce the final denatured state, which is unable to fold back to the native protein. 

The stabilizing (destabilizing) effect of pressure on the thermal denaturation of proteins has been associated with the presence (absence) of aggregation of the unfolded protein [79]. Intermolecular aggregation is indeed one of the most commonly observed effects of thermal denaturation. The temperature of unfolding may however be lowered considerably by a... 

Heat treatment is one of the most commonly used techniques in food processing, and is frequently responsible for the transition of proteins from the native folded state to the denatured unfolded state. Thermal denaturation of proteins involves a structural change which affects the nutritional quality of foods (Plum, 2009). 

Folded proteins can be caused to spontaneously unfold upon being exposed to chaotropic agents, such as urea or guanidine hydrochloride (Gdn), or to elevated temperature (thermal denaturation). As solution conditions are changed by addition of denaturant, the mole fraction of denatured protein increases from a minimum of zero to a maximum of 1.0 in a characteristic unfolding isotherm (Fig. 7a). From a plot such as Figure 7a one can determine the concentration of denaturant, or the temperature in the case of thermal denaturation, required to achieve half maximal unfolding, ie, where... 

Privalov et al. (1989) also reported the temperature dependence of the ellipticity at 222 nm for the proteins studied at various pH values (Fig. 28). At the highest temperature studied (80°C), the 222 nm ellipticity value for the thermally unfolded, acid-unfolded, and Gdm-HCl-unfolded proteins appear to be converging, but show a range of 2000 deg cm2/dmol out of a total of 5000 deg cm2/dmol. (ApoMb is an exception in that, as noted before, the thermally denatured protein is apparently an associated /1-sheet. However, the acid- and Gdm HC1-unfolded forms of apoMb have similar [0] 222 values at 80°C.)... 

Studies of thermally denatured proteins remain technically challenging owing to the propensity of thermally unfolded proteins to aggregate. Despite this potential difficulty, small-angle scattering techniques have been employed in the characterization of a number of thermally unfolded states. 

An excluded-volume random-coil conformation will be achieved when the solvent quality exceeds the theta point, the temperature or denatu-rant concentration at which the solvent-monomer interactions exactly balance the monomer—monomer interactions that cause the polymer to collapse into a globule under more benign solvent conditions. A number of lines of small-angle scattering—based evidence are consistent with the suggestion that typical chemical or thermal denaturation conditions are good solvents (i.e., are beyond the theta point) and thus that chemically or thermally unfolded proteins adopt a near random-coil conformation. 

Results from thermal denaturation and heat capacity studies have shown that the proteins are not necessarily completely unfolded in this process. The volume observations also suggest that the denatured state is not one in which all hydrophobic groups are exposed to water. But the results can also be understood from the effect of close polar and electrostatic groups interacting with the water structure surrounding the hydrophobic groups. The volume change is heavily... 

Another common indication of protein stability is the concentration of either urea or GnHCl required to unfold half of the protein available. This concentration, given the symbol [D]i/2, is analogous to the Tm value from thermal denaturation curves. Increase or decrease in [D]i/2 is presumed to indicate a corresponding increase or decrease in protein stability, respectively. Analysis of these curves can also provide thermodynamic information [123-126]. As these experiments can be done at any temperature, they are more useful in that they can provide information regarding stability at or near room temperature. 

Although less frequently discussed, heat processes often influence the textures and chemistries of the intermediate and end products, and thermal treatments are not without consequences on milk proteins that are denatured. Denaturation of proteins occurs under precise conditions of pH, temperature and ionic strength leading to their unfolding. Denaturation is significantly slower when proteins are near their isoelectric point. Only (3-lactoglobulin is irreversibly denatured at pH 7 and 70°C a-lactalbumin is denatured at pH 6.7 and 65°C. Aggregation of these proteins, besides hydrophobic... 

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