Enthalpy, folded/denatured proteins

We cannot answer the question posed by Anfin-sen s hypothesis. Does the native state have a minimum value of the Gibbs energy Nevertheless, it is observed that proteins usually behave as if folded, unfolded forms are in a true thermodynamic equilibrium, and that this equilibrium is attained rapidly. The difference AG between a folded and a denatured protein is only 21-63 kj mol-1, which shows that folded proteins are only marginally more stable than are unfolded polypeptide chains.645 The value of AG of unfolding as a function of temperature T is given by Eq. 29-13, where AH(T) and ACp are the changes in enthalpy and heat capacity upon unfolding.645 646... 

The native state of a protein has many of its hydrophobic side chains shielded from water because they are packed in hydrophobic cores. Conversely, the denatured state has many of its hydrophobic side chains exposed to solvent. The water molecules stack around these in icebergs as they maximize their hydrogen bonds with one another (Chapter 11). This lowers the entropy of water, because the individual molecules have less freedom of movement, and lowers the enthalpy because more hydrogen bonds are made.2 Similarly, the hydrogen bond donors and acceptors in the polypeptide backbone of the denatured protein are largely exposed to solvent and tie down more water molecules.3 These water molecules are released as the protein folds, and the gain in entropy of water compensates considerably for the loss of conformational entropy. 

Another phase change is the denaturation of a protein from a normal, folded state to a disrupted, unfolded state-just like what happens when an egg is cooked. The Clausius-Clapeyron equation is applicable, but here the "pressure" of the phase is equal to the fraction that is denatured at a particular temperature. Lysozyme, an enzyme that breaks down bacterial cell walls, has an enthalpy of denaturation, AdenW, that is 160.8 kJ/mol. If the denatured fraction of lysozyme, fdeiv is 0.113 at 45.0°C, what is fjen 75.0°C ... 

When a folded protein in solution is heated to a high enough temperature, its polypeptide chain will unfold to become the denatured protein—a process known as denaturation. The temperature at which most of the protein unfolds is called the melting temperature. The melting temperature of a certain protein is found to be 63°C, and the enthalpy of denaturation is 510 kJ/mol. 

Even extremophilic organisms and their proteins contain the same 20 amino acids with bonds similar to those in mesophiles. As the difference in free enthalpy between folded and unfolded states of globular proteins AG N >G is only about 45 15 kj mol-1 the sequence and structure of extremophilic proteins should differ from those of ordinary species. However, the main question, namely which properties cause the increase in denaturation temperature of thermostable proteins, is still debated (Rehaber, 1992). Theoretical and experimental analyses have shown that thermal stability is largely achieved by small but relevant changes at different locations in the structure involving electrostatic interactions and hydro-phobic effects (Karshikoff, 2001). There is no evidence for a common determinant or for just one effect causing thermostability. 

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. 

An interesting aspect of the photoreaction of PYP is the similarity to the protein folding/unfolding reaction. Hellingwerf and his coworkers applied the transition state theory to the photoreaction of PYP and estimated the thermodynamic parameters, the entropy, enthalpy, and heat capacity changes of activation [29]. They also carried out thermodynamic analysis on the thermal denaturation of PYP. Consequently, they found that the heat capacity changes in the photoreaction are comparable to those in the unfolding... 

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