Protein hydration layer association

Water molecules in the protein hydration layer have a finite residence time. No single water molecule stays in the layer forever, as it makes sojourn between the layer and the bulk. This residence time can play a critical role in protein association because it offers a quantitative estimate of the rigidity of the layer. The final act of association of two proteins may require partial desolvation around the necessary amino acid residue sites. This is only possible if the residence time of water around these sites is sufficiently short. The residence time is determined by the dynamics in the hydration layer. This correlation between hydration layer and protein association is an important problem that deserves careful attention. 

Here A//adS is the enthalpy of adsorption, T is the temperature, and AAads is the entropy change associated with the adsorption of the protein onto the surface. Protein adsorption will take place if AGads < 0. Considering a complex system, where proteins are dissolved in an aqueous environment, and are brought into contact with an artificial interface, there are a vast number of parameters that impact AGads due to their small size (i.e., large diffusion coefficient), water molecules are the first to reach the surface when a solid substrate is placed in an aqueous biological environment. Hence, a hydrate layer is formed. With some delay, proteins diffuse to the interface and competition for a suitable spot for adsorption starts. This competition... 

Fig. 3. Preferential protein hydration in the presence of precipitating agents used in crystallization experiments. When high concentrations of salts are used as precipitants, a precipitant-poor layer forms near the protein (P) surface due to a higher affinity of the protein for water than for the precipitant. Other precipitants (e.g., polyethylene glycol polymers) induce formation of a similar precipitant-depleted region near the protein by solvent exclusion effects. In either case formation of the precipitant-depleted layer is energetically unfavorable. Consequently, the overall effect of precipitants is to promote molecular associations that decrease the total protein surface area exposed to solvent. After Timasheff and Arakawa (1988).

As ionic strength, in Figure 2.3, is increased, the solution again reaches a point where the solute molecules begin to separate from solvent and preferentially form self-interactions among themselves that result in crystals or precipitate. The explanation for this salting-out phenomenon is that the salt ions and macromolecules compete for the attention of solvent molecules, that is, water. Both the salt ions and the protein molecules require hydration layers to maintain their solubility. When competition between ions and proteins becomes sufficiently intense, the protein molecules begin to self-associate in order to satisfy, by intermolecular interactions, their electrostatic requirements. Thus dehydration, or the elimination and perturbation of solvent layers around protein molecules, induces insolubility. 

Abbreviations are LY, hen egg-white lysozyme CON A, demetallized concanavalin A TP, demetallized porcine trypsin tRNA, nonspecific yeast transfer ribonucleic acid CA, human erythrocyte carbonic anhydrase B Hb, human adult carbonmonoxyhemo-globin AP, E. coli alkaline phosphatase TF, demetallized human transferrin IG, human nonspecific -/-immunoglobulin AD, alcohol dehydrogenase from yeast CP, human ceruloplasmin HC1 / 20, l/IO(L), 1/10(0, 1/2, 1/1, various states of association of Helix pomatia hemocyanin. Dashed line m>calculated using Equation 3 with no adjustable parameteis, using the viscosity of pure water to compute v . The proteins were assumed spherical, and a 3J-A hydration layer was included in computing the hydrodynamic radii. After Ref. 7. 

Figure 8.1. Schematic representation of the old view of a protein molecule in aqueous environment, with a layer of strongly associated water (the hydration layer is an iceberg), suspended in aqueous solution. The hydration layer moves with the protein molecule (as proposed by the iceberg model), and beyond this layer the water molecules adapt to the normal tetrahedral geometry. Adapted with permission from Chem. Rev., 104 (2004), 2099-2123. Copyright (2004) American Chemical Society.

The mechanism and the rate of hydrogen-bond breaking in the hydration layer surrounding an aqueous protein have been studied by using a time correlation function technique to understand these aspects in the hydration layer of lysozyme. Water molecules in Ihe layer are found to exhibit three distinct bond-breaking mechanisms compared to bulk reorientation [4]. The reorientation processes are associated wilh the hydrogen-bond breakin switching events (HBSE). Three important characteristics that are common for almost aU of the reorientation processes... 

Understanding the relationship between the properties of proteins [4] and their associated water [5-7] is an ongoing challenge. Many biological functions [3] can be understood only if we know the structure and function of the first hydration layer. When a protein is in solution, there are two categories of water molecule in close proximity to it, (i) internal bound molecules and (ii) hydration water molecules. The internal bound molecules, located in cavities of the protein, play a structural role in protein folding. 

A prominent component of cytoplasm consists of microtubules which appear under the electron microscope to have a diameter of 24 2 nm and a 13 - to 15-nm hollow core.307-310 However, the true diameter of a hydrated microtubule is about 30 nm and the microtubule may be further surrounded by a 5-20 nm low density layer of associated proteins. Microtubules are present in the most striking form in the flagella and cilia of eukaryotic cells (Fig. 1-8). The stable microtubules of cilia are integral components of the machinery causing their motion (Chapter 19). Labile microtubules, which form and then disappear, are often found in cytoplasm in which motion is taking place, for example, in the pseudopodia of the ameba. The mitotic spindle... 

The present article was stimulated by the recent experimental data on protein-covered latex colloidal systems immersed in various electrolyte solutions NaCl, NaNC>3, NaSCN and Ca(NOg)2, which showed strong specific anionic effects on the restabilization curves.1 In the opinion of Lopez-Leon et al.,1 the above polarization model for double layer/hydration forces could explain only some of their experiments, but not all of them. However, they assumed that at pH = 10 the adsorption of anions was negligible hence specific anion effects could not be predicted by their association with the positive sites of the surface. Furthermore, at pH = 4 they assumed the... 

It is clear that a perfect agreement with experiment cannot be provided by a theory which ignores the additional interactions between ions, and ions and surfaces, not included in the mean field potential (such as image forces,14 excluded volume effects,15 and ion-dispersion16 or ion-hydration forces17). However, it will be shown that the experimental results reported by Lopez-Leon et al.1 can be more than qualitatively reproduced for uniunivalent electrolytes by the present polarization model for hydration/double layer forces, if one accounts for the association equilibria with the surface sites for all the ions present in the electrolyte (H+, OH , anions, and cations).11 Some additional reasons for the quantitative disagreements, involving the structural modifications of the adsorbed protein layer and the nonuniformity of the colloidal particles, will be also noted. 

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