Solubility of proteins

Solubility of protein in water varies within wide limits. While some proteins dissolve easily in salt-free water, others dissolve only in the presence of certain concentrations of salts, a third group is soluble in mixtures of water and ethanol, and insoluble in any solvent. 

Texturization is not measured directly but is inferred from the degree of denaturation or decrease of solubility of proteins. The quantities are determined by the difference in rates of moisture uptake between the native protein and the texturized protein (Kilara, 1984), or by a dyebinding assay (Bradford, 1976). Protein denaturation may be measured by determining changes in heat capacity, but it is more practical to measure the amount of insoluble fractions and differences in solubility after physical treatment (Kilara, 1984). The different rates of water absorption are presumed to relate to the degree of texturization as texturized proteins absorb water at different rates. The insolubility test for denaturation is therefore sometimes used as substitute for direct measurement of texturization. Protein solubility is affected by surface hydrophobicity, which is directly related to the extent of protein-protein interactions, an intrinsic property of the denatured state of the proteins (Damodaran, 1989 Vojdani, 1996). 

Laurent, T. C., The interaction between polysaccharides and other macromolecules. 5. The solubility of proteins in the presence of dextran, Biochem.., 89, 253, 1963. 

Although the feasibility of the proteomics or bioinformatics approach has been demonstrated, considerable room remains for improved methods for selective solublization of protein biomarkers and for rapid cleavage to produce peptides. There is also demand for advanced instrumentation to collect, process, and analyze microorganisms. 

Work on the solubility was continued by Osborne to produce the classifications of vegetable proteins that are still used. Studies of the solubility of proteins remains important both in characterising and purifying proteins. 

Surfactants are well-known protein denaturants. However, when sufficiently dilute, some surfactants (e.g. polysorbate) exert a stabilizing influence on some protein types. Proteins display a tendency to aggregate at interfaces (air—liquid or liquid—liquid), a process that often promotes their denaturation. Addition of surfactant reduces surface tension of aqueous solutions and often increases the solubility of proteins dissolved therein. This helps reduce the rate of protein... 

An empirically derived relationships that describes the systematic effects of different neutral salts on the solubility of proteins. Collins and Washabaugh indicate that the order of ionic species eluding from a Sephadex G-10 column corresponds to the known order of effectiveness ions in the Hofmeister series on protein solubility ... 

Salt (ionic strength) gradients in lEC discussed in Section 5.4.3.3 are frequently used in the separation of complex peptides, proteins, and other biopolymer samples as a complementary technique to RP solvent gradient separations, often in a 2D setup [99,100]. The gradients usually start at a low salt (chloride, sulfate, etc.) concentration and typically run from 0.005 to 0.5 M. A buffer is used to control the pH acetonitrile and methanol may be added to improve the resolution and urea to improve the solubility of proteins that are difficult to dissolve. Ion exchangers with not strongly hydrophobic matrices usually prevent protein denaturation in aqueous mobile phases. 

Organic solvents. Addition of organic solvents decreases the solubility of proteins by reducing the dielectric constant of the medium. For the precipitation of enzymes, methanol, ethanol or propanol are mostly used, but acetone and diethyl ether can also be employed. The principal disadvantage of organic solvents is their tendency to cause stmctural damage of enzyme molecule. 

Lowers solubility of protein at or near isoelectric point or pi, where the protein is in net unionized form some protein may become denatured under these conditions Heat-sensitive proteins expose hydrophobic domains, which exhibit decreased solubility. This process must be reversible if original conformation is required for protein activity... 

Microscopic structure of texturized water-extracted soy flour and texturized soy concentrate were quite similar to that of texturized soy flour. Scanning electron microgrpahs showed that water extraction of soy flours had little effect on morphological characteristics of texturized soy products (Figure 10). Solubility of soluble sugars was not affected by texturization, whereas solubility of proteins decreased sharply when soy flour was texturized (Table VII). It appears that soluble sugars did not interact with proteins during texturization. Based upon results of microscopy and solubility studies, it is reasonable to speculate that natural soluble carbohydrates are not required (do not play an important role) in development of texture or stabilization of structure. 

Recently, Melander and Horvath (7, 10) have proposed a single theory to account for the effects of neutral salts on the electrostatic and the hydrophobic interactions in the salting out and the chromatography of proteins. In simplified terms, the theory accounts for the solubility of proteins in terras of two contributions, electrostatic and hydrophobic in nature. 

Solubility The reported relationship between water absorption and solubility of proteins has not been consistent. Water absorption capacity of sunflower concentrates increased slightly as the solubility index of the protein decreased (17). Hermansson (2 ) reported that a highly soluble protein exhibits poor water binding, but a reverse relationship between water absorption, evidenced by swelling and solubility, was not observed. In a later report, Hermansson (27) stated that solubility measurements give no information as to whether or not a protein will bind water. 

In what way do the solubilities of proteins usually vary with pH Why ... 

The solubility of proteins from parasitic organisms can often be enhanced by the use of hot sodium dodecyl sulphate (SDS) before solubilization in SD buffers. The ability of SDS to denature proteins also helps in allowing access to hydrophobic proteins, not normally seen after standard preparation procedures. SDS treatment in whole F. hepatica preparations has been shown to yield more protein spots visualized on gel analysis than other methods (Jefferies et a/., 2000). 

The dependence of the solubility of proteins on the nature and concentration of salts had already been found by Hofmeister (1888) and by Setschenow (1889) in the 19th century. Hofmeister, in fact, based the series that now bears his name (and which we have already encountered in Chapter 3, Section 3.4 Figure 8.6) on the ease of precipitation of hen egg-white lysozyme. 

Fig. 8). These results indicate that the solubility of protein in the reaction mixture may be an indicator of foldedness. ... 

The solubility of proteins decreases as the concentration of ammonium sulfate in the solution is increased. The concentration of ammonium sulfate at which a particular protein comes out of solution and precipitates may be sufficiently different from other proteins in the mixture to effect a separation. 

Neutral salts have a pronounced effect on the solubility of proteins, especially if they are globular. At low concentration, salts increase the solubility of many proteins in a phenomenon known as salting-in. This solubilization is a function of the solvent ionic strength, which depends on the concentration and on the electrical charge of the cations and anions that constitute the salt. These effects are caused by changes in the ionization of dissociating groups of the protein. 

All these approaches have been used to alter protein function, to increase the activity or solubility of proteins, or to adapt enzymes for industrial applications. The goal of artificial man-made proteins with tailor-made activities is, however, still far away and none of the currently existing approaches provides the ultimate solution to the directed evolution of proteins. Nevertheless, numerous examples of successfully altered and improved proteins clearly show the power of directed evolution for protein design. 

Proteolytic modification has special importance for the improvement of solubility of proteins. This effect becomes significant even after very limited proteolysis. Hydrolysis of casein to DH of 2 and 6.7% with Staphylococcus aureus V8 protease increased the isoelectric solubility to 25 and 50%, respectively (Chobert et al., 1988a). However, it should be noted that the solubility profiles were not identical, due to a shift of the isoelectric point of the modified proteins. Solubility of a protein hydrolysate depends on the enzyme used (Adler-Nissen, 1986a). Protamex (a Bacillus proteinase complex) hydrolysates of sodium caseinate (DH 9 and 15%) displayed 85-90% solubility between pH 4 and 5 (Slattery and FitzGerald, 1998). 

To change the molecular structure of globular proteins by bringing a protein solution to its boiling point and exposing it to acids or alkalies or various detergents. The process reduces the solubility of proteins and prevents crystallization. 

Neutral salts have a significant effect on the solubility of proteins. The effects of salts can be expressed in terms of their ionic strength (u) when they are in solution. Ionic strength can be calculated as... 

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