Heat capacity of protein

Kholodenko, V. and E. Freire. 1999. A simple method to measure the absolute heat capacity of proteins. Anal Biochem 270 336-338. 

Brandts, 1967 Privalov and Khechinashvili, 1974 Sturtevant, 1977). It is assumed that water ordering increases in the vicinity of nonpolar groups (Kauzmann, 1959). If the order of the water molecules surrounding nonpolar groups decreases faster than that of bulk water as the temperature rises, one will observe the gradual melting of ordered water as the increment of the partial heat capacity of protein in water media. 

Several measurements have been made of the low-temperature heat capacities of proteins. 

Makhatadze GI, Privalov PL. Heat capacity of proteins. I. Partial 33. molar heat capacity of individual amino acid residues in aqueous solution hydration effect. J. Mol. Biol. 1990 213 375-384. 

Enthalpy, Entropy, and Heat Capacity of Protein—Water Systems Below 0°C. A number of investigators have reported the apparent enthalpy of fusion as a function of temperature and composition for several hydrated proteins. MacKenzie and coworkers (10) determined absorption isotherms at low temperatures and found that 1) these absorption isotherms have essentially the same sigmoidal shapes as those observed above zero degrees 2) the magnitudes of the values for partial molal enthalpy and entropy increase as the content of unfrozen water decreases 3) the heat of fusion decreases as the content of unfrozen water decreases and 4) the heat capacity of the system increases as the content of unfrozen water increases. Taking these findings all together, the thermodynamic properties of unfrozen water are not very different from those of supercooled water at comparable temperatures. 

Heat Capacity. Measurements of the heat capacity of protein systems are particularly interesting for several reasons 1) they reflect solvation of nonpolar elements in addition to other parts of the protein surface and thus can be viewed as the most complete thermodynamic probe for water-protein interactions. 2) Heat capacity can be measured conveniently for both solution and solid samples thus the two categories of protein hydration studies, those on solutions and those on solid samples, can be correlated. 3) There is a substantial literature on the heat capacities of small molecules and on additivity relationships, which appear to be more accurate for heat capacity than for other thermodynamic functions. 

In the presence of the solvent water, the heat capacity of proteins increases. The partial molar heat capacity of proteins in the native state, Cp(T), appears to be a linear... 

The heat capacity of protein unfolding. Figure 30.1.3 shows the enthalpy of folding T4 lysozyme. 

The average properties of the transition state may be estimated from a variety of criteria. The Tanford j8t value, measured from the relative sensitivities of the folding kinetics and equilibrium constants to [GdmCl] (Chapter 18, section Bl, equation 18.9), is 0.6, indicating that about 60% of the surface area of the protein is buried in the transition state for folding, relative to that buried in the native structure. The change in heat capacity of the transition state relative to that... 

With additional information, including the heat capacity of the buffer solvent, the partial specific volumes (volume per gram of the solute), and the specific volume of the solvent, one can extract the partial specific heat capacity (J K 1g I) of the solute. Privalov has summarized these calculations.8 Because the solutions are studied at very low concentrations, it is assumed that the contribution to the total heat capacity from the solvent cancels out when one calculates the excess heat capacity. With only minor exceptions, the procedures used to calculate parameters associated with the transformations in nucleic acids and in proteins are the same and yield quantities that are interpreted in similar ways, although researchers in these two fields may use a different notation for the same quantity. 

The populations of other intermediate states are very small and can be neglected. For larger more complex proteins made up of multiple subunits, and in many fibrous proteins, this conclusion cannot be supported. Complex globular proteins appear to melt cooperatively in domains in which the smaller units melt independently, and the melting in fibrous proteins is even more complex. While the molar quantities for the heat capacity are dependent upon the size of the protein, the partial specific heat capacities of many proteins are very nearly the same. 

In all globular proteins studied, a significant increase in the heat capacity of the denatured protein relative to the native state has been observed in the vicinity of the denaturation transition. (This quantity is represented in... 

Fig. 1. Partial specific heat capacity of sperm whale metmyoglobin in aqueous solutions with different pH values in the temperature range in which heat denaturation takes place. The observed heat capacity peak corresponds to the heat absorption upon protein denaturation that also results in a significant heat capacity increase A°CP [for details see Privalov et al. (1986)].

The transition of a protein or a single cooperative domain from the native to the denatured state is always accompanied by a significant increase of its partial heat capacity (see, for reviews, Sturtevant, 1977 Privalov, 1979). The denaturationaJ increment of heat capacity A JCP = C° Cp amounts to 25-50% of the partial heat capacity of the native protein and does not depend noticeably on the environmental conditions under which denaturation proceeds (Fig. 1) or on the method of denaturation. However, it is different foi different proteins and seems to correlate with the number of contacts between nonpolar groups in native proteins (Table I). On the other hand, the partial specific heat capacities of denatured states of different proteins appear to be rather similar (Tiktopulo et... 

Different biopolymers are used when the texture-building capacity of protein alone, for instance, is insufficiently capable to create the target structure. However, phase separation effects can occur, as described in theory and technical detail in other chapters of this publication. These phenomena are also dependent on processing factors such as heat or mechanical treatment, the latter effects being rarely investigated so far. 

Fig. 4. The apparent specific heat capacity of lysozyme from 0 to 0.45 g of water per gram of protein. The curve is calculated. The heat capacity measurements were made with lyophilized powders of lysozyme, appropriately hydrated, except for the four measurements indicated by the square symbols, for which the sample was a film formed by slowly drying a concentrated solution of lysozyme. From Yang and Rupley (1979).

Hutchens et al. (1969) determined the heat capacities of zinc insulin at 0 and 0.04 h and of chymotrypsinogen A at 0 and 0.107 h, from 10 to 310 K. For all samples the data were a smooth function of temperature, with no indication of a glass or phase transition at any temperature. The absence of a phase transition corresponding to the ice-liquid water transition is expected for low hydrations. These appear to be the only data in the literature that have been used to determine the entropy of a protein sample. Hutchens et al. (1969) calculated the standard entropy of formation of a peptide bond as 9.0—9.3 cal K mol" . 

Results such as those shown in Fig. 7 have been interpreted as reflecting, in the higher hydration range II, a secondary hydration phase, corresponding perhaps to the B shell of solvent about ions. In this view the water of range II would be a shell of solvent serving to interface the bulk solvent with the water ordered in the monolayer about the native protein surface. This molecular interpretation of the physics of the 0.35 to 0.75 h range conflicts with the heat capacity isotherms (see Section II,A,3), which show that the heat capacity of a native protein is invariant to hydration above about 0.4 h. 

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