Water-protein interactions

Pethig, R. 1992. Protein-Water Interactions Determined by Dielectric Methods. Annu. Rev. Phys. Chem. 43, 177. 

Water binding varies with the number and type of polar groups (5 ). Other factors that affect the mechanism of protein-water interactions include protein conformation and environmental factors that affect protein polarity and/or conformation. Conformational changes in the protein molecules can affect the nature and availability of the hydration sites. Transition from globular to random coil conformation may expose previously buried amino acid side chains, thereby making them available to interact with aqueous medium. Consequently, an unfolded conformation may permit the protein to bind more water than was possible in the globular form ( ). 

Another example of (ii) above can be found in the conclusion of Kuhn et al. (1992), who proposed that the deep grooves in protein surfaces are formed by protein-water interactions. 

Regenstein, J.M. 1984. Protein Water Interactions in Muscle Foods. In Proceedings 37th Annual Reciprocal Meat Conference, pp. 44-51. American Meat Science Association, Savoy, 111. 

The addition of polyhydroxyl compounds to enzyme solutions have been shown to increase the stabilities of enzymes, (13,16,19,20). This is thought to be due to the interaction of the polyhydroxyl compound, (e.g. sucrose, polyethylene glycols, sugar alcohols, etc), with water in the system. This effectively reduces the protein - water interactions as the polyhydroxy compounds become preferentially hydrated and thus die hydrophobic interactions of the protein structure are effectively strengthened. This leads to an increased resistance to thermal denaturadon of the protein structure, and in the case of enzymes, an increase in the stability of the enzyme, shown by retention of enzymic activity at temperatures at which unmodified aqueous enzyme solutions are deactivated. 

We recently developed a systematic method that uses the intrinsic tryptophan residue (Trp or W) as a local optical probe [49, 50]. Using site-directed mutagenesis, tryptophan can be mutated into different positions one at a time to scan protein surfaces. With femtosecond temporal and single-residue spatial resolution, the fluorescence Stokes shift of the local excited Trp can be followed in real time, and thus, the location, dynamics, and functional roles of protein-water interactions can be studied directly. With MD simulations, the solvation by water and protein (residues) is differentiated carefully to determine the hydration dynamics. Here, we focus our own work and review our recent systematic studies on hydration dynamics and protein-water fluctuations in a series of biological systems using the powerful intrinsic tryptophan as a local optical probe, and thus reveal the dynamic role of hydrating water molecules around proteins, which is a longstanding unresolved problem and a topic central to protein science. 

Micrographs of the freeze-dried RDP preparations are shown in Figure 6. The unheated and microwave treated samples are clearly differentiated from those treated with hot water. The former consist of ragged fragments containing numerous but small pores while the latter appeared more aggregated and exhibits larger orifices. A consideration of bulk density (Table IV) and microstructure may help to explain some aspects of protein/water interaction properties. Porosity and particle size could be important parameters, however they are difficult to control and are rarely measured in studies of functional properties. 

Protein—Water Interactions in Human and Tortoise Lysozymes ... 

E2 is the protein-water interaction energy, which is generally site specific but we consider an average value of 4140 X 10 erg. m and cr are the mass and diameter of the water molecule, respectively. The calculated frequency is quite close to the peak value of the density of state of hydration water of a dendrimer (Lin et al., 2005). The mean square displacement in the bound state is given by drt P(r,t), where P(i,t) is the time and position... 

Figure 7.11 A schematic split of the protein-water interaction into a sum of "group" interactions, similar to the split depicted in figure 7.4.

The soft part. The second term in (7.249) corresponds to the soft part of the protein-water interaction potential. This is given by (see section 7.7)... 

An adhesive-cohesive model for protein compressibility has been proposed by Dadarlat and Post [57]. This model assumes that the compressibility is a competition between adhesive protein-water interactions and cohesive protein-protein interactions. Computer simulations suggest that the intrinsic compressibility largely accounts for the experimental compressibilities indicating that the contribution of hydration water is small. The model also accounts for the correlation between the compressibility of the native state and the change in heat capacity upon unfolding for nine single chain proteins. 

Protein-Water Interactions. There appears a general agreement that all thermodynamic measurements, such as heat capacity results of Rupley et al. [6] and Hoeve [7] or the NMR data on frozen materials of Bryant [8], indicate that 0.3-0.4 g H 0/g protein form an unfrozen boundary layer at subzero temperature. 

Carerl, G.C. Gratton, E. Yang, P.-H. Rupley, J.A. "Protein-Water Interactions. Correlation of Infrared Spectroscopic, Heat Capacity, Diamagnetic Susceptibility and Enzymatic Measurements on Lysozyme Powders," submitted for publication, 1979. 

We have made the following approximate calculation to estimate protein-water interactions by a less cumbersome procedure it is assumed that the protein molecule has a unique fixed structure determined by x-ray crystallography and interactions are calculated between the protein and a single water molecule in the absence of other solvent molecules. Using this simple system, one may consider all positions and orientations of the single water molecule relative to the protein in a step-wise manner. We present here the result of this calculation for the crystal of bovine pancreatic trypsin inhibitor (BPTI). The calculated energy, mapped in three dimensions, is a highly informative description of the crystal s solvent space. 

Application to the BPTI Crystal. The system to be simulated consisted of the protein atoms of one BPTI molecule (5) and 140 water molecules. The required number of water molecules could be calculated both from the volume of the crystal for which protein-water energy is zero or negative (solvent space) (9) and from unit cell volume and density of protein and water. Protein-water interactions were calculated as in the first part of this article, protein-protein interactions as described elsewhere ( ). Interactions between water molecules were calculated using the ST2 model, introduced by Rahman and Stillinger in a molecular dynamics simulation of liquid water (11). 

In a dilute protein solution, the nano length scale or the molecular structure of protein molecules determines the thermodynamic equilibrium between protein-protein and protein-water interactions. The consequent surface and hydrodynamic properties of proteins are resulted from the proportion of hydrophobic, hydrophilic, and charged amino acid residues. For example, caseins could adopt a random coil structure due to their flexible structure as a result of phosphorylated serine residues caseins indeed lack the ordered structures of a-helix, 3-sheet, and 3-turn found in globular proteins. This gives rise to better multifunctionality of caseins over globular proteins. 

The isoelectric point of a protein is defined as that pH at which the net charge is zero (Wismer-Pedersen, 1971). Since protein-protein ionic interactions are promoted at this point, it would be expected that the protein matrix would shrink and WHC would be at a minimum (Kapsalis, 1975). It follows that increasing the pH away from isoelectric point would also result in a higher WHC, since protein-water interactions are favored (Hamm, 1960). Bouton et al. (1971) were able to increase the ultimate pH and WHC of meat by preslaughter injection of epinephrine and showed that tenderness increased directly with pH values. Further work by Bouton et al. (1972) and Bouton and Harris (1972) showed that as pH increased from normal values of 5.5 to 7.0, tenderness of the tissue increased and became independent of the contracture state. 

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