Protein-solvent preferential interactions, protein hydration, and the modulation of biochemical reactions by solvent components

Abstract

Solvent additives (cosolvents, osmolytes) modulate biochemical reactions if, during the course of the reaction, there is a change in preferential interactions of solvent components with the reacting system. Preferential interactions can be expressed in terms of preferential binding of the cosolvent or its preferential exclusion (preferential hydration). The driving force is the perturbation by the protein of the chemical potential of the cosolvent. It is shown that the measured change of the amount of water in contact with protein during the course of the reaction modulated by an osmolyte is a change in preferential hydration that is strictly a measure of the cosolvent chemical potential perturbation by the protein in the ternary water-protein-cosolvent system. It is not equal to the change in water of hydration, because water of hydration is a reflection strictly of protein-water forces in a binary system. There is no direct relation between water of preferential hydration and water of hydration. For the better part of a century, it has been common practice to modulate biochemical (biological) reactions by the addition to the aqueous solvent (dilute buffer) of compounds that were required at high concentration (ϳ0.5 M or higher) to exert their effect. For example, sucrose and glycerol were used to stabilize biological systems, whereas urea and guanidine hydrochloride were used to solubilize coagulated systems and to unfold (denature) proteins. The aim in the use of these additives, referred to as cosolvents, was to displace to the right or left the chemical equilibrium, Reactant º Product. Although this practice was widespread, a theoretical underpinning was lacking until the discovery by Wyman in 1948 of the phenomenon of linked functions (1) and his development of the linkage relationship equations (2, 3), which showed the necessary thermodynamic interdependence between the displacement of a chemical equilibrium and the change in binding of a ligand to the system during the course of the reaction. Complete understanding of the action of cosolvents in such modulation required the combination of the Wyman linkage relations with the multicomponent solution thermodynamics theory developed by Kirkwood and Goldberg (4), Stockmayer (5), and Scatchard (6).The basic Wyman linkage equation states that, at any ligand concentration, m L , the gradient of the equilibrium constant with respect to ligand activity is equal to the change in the binding of the ligand to the biological system during the course of the reaction (at constant temperature and pressure that will be maintained throughout):where K is the equilibrium constant of the reaction, a L is the activity of the ligand (a L ϭ m L ␥ L ), L Prod and L React are the bindings of the ligand to the two end states of the reaction, and m L and ␥ L are the molal concentration and activity coefficient of the ligand. [In our notation, the subscripts W, L, and P refer to water, ligand (cosolvent), and protein (macromolecule), respectively.] Tanford, in 1969 (7), showed that ...

Quaternary Diffusion Coefficients in a Protein−Polymer−Salt−Water System Determined by Rayleigh Interferometry

“…33 Figure 1 also shows that the behavior of the quaternary data is consistent with the ternary data. However, since electrostatic screening is the dominant effect, the quaternary (D 11 ) V values are closer to the corresponding (D 11 ) V (13) values than to the (D 11 ) V (12) values within our experimental concentration domain. Thus, it is expected that more accurate predictions of (D 11 ) V should be based on (D 11 ) V (13) and not (D 11 ) V (12) .…”

“…However, since electrostatic screening is the dominant effect, the quaternary (D 11 ) V values are closer to the corresponding (D 11 ) V (13) values than to the (D 11 ) V (12) values within our experimental concentration domain. Thus, it is expected that more accurate predictions of (D 11 ) V should be based on (D 11 ) V (13) and not (D 11 ) V (12) . Since lysozyme-chloride electrostatic interaction in the presence of NaCl is screened, dielectric effects due to PEG2000 become much less significant.…”

“…Our results are shown in Figure 3. From the error bar comparison along the C 3 coordinate, we observe that (D 21 ) V and (D 21 ) V (12) are equal to each other within the experimental error. We therefore conclude that the addition of salt has negligible effect on both lysozyme-PEG2000 cross-diffusion coefficients.…”

“…Finally, the values of the two cross terms obtained at C 2 ) 0.050 M and C 3 ) 0.50 M were also compared with those obtained for the lysozyme-TEG-NaCl-water system in the same experimental conditions (see Table 2 We next compare (D 21 ) V for the flux of PEG2000 due to a gradient of lysozyme with the corresponding ternary data, (D 21 ) V (12) . Our results are shown in Figure 3.…”

“…[10][11][12] These additives are usually used to induce phase transformations such as protein crystallization, [13][14][15][16] liquid-liquid phase separation, aggregation, and gelation. [17][18][19][20] They are also used to stabilize protein native structure 12 and modulate enzymatic activity in aqueous and nonaqueous media. 21 For a system with N + 1 components, isothermal diffusion can be described using the generalized Fick's first law: 22 Here, C i is the molar concentration of solute i, and J i is its corresponding flux.…”