Protein salt effect

Protein conformation is also markedly affected by the concentration and type of ionic species present. In solution, individual salt effects can be either stabilizing or denaturing [26,27], These effects correspond to the Hofmeister lyotropic series ... 

Lee K, Fitch CA, Lecomte JT, Garcfa-Moreno EB (2002) Electrostatic effects in highly charged proteins Salt sensitivity of pKa values of histidines in staphylococcal nuclease. Biochemistry 41 5656-5667. 

Srinivasan J, Trevathan MW, Beroza P, Case DA (1999) Application of a pairwise generalized Born model to proteins and nucleic acids Inclusion of salt effects. Theor ChemAcc 101 426-434. 

The studies of Pauli he. cit.) and his co-workers, however, have revealed the fact that isohydric solutions of different acids do not effect equal combination with the isoelectric protein relatively more acetic acid for example being combined than hydrochloric acid in isohydric solutions. Again, both the actual position of these maxima as well as the magnitudes of the viscosities observed vary much with the nature of the acid employed. Thus the relatively weak oxalic acid appears to be a much stronger acid than sulphuric acid, whilst trichloracetic acid does not differ appreciably from acetic acid in its effect on the viscosity of albumin. It is probable that the degree of solvation of the protein molecules and of the protein salts must not be regarded as constant but that they vary both with the nature of the salt and in the presence of neutral salts which exert like alcohol a desolvating action more or less complete on the solvated isoelectric protein as well as on the undissociated protein salts. 

Maltais, A., Remondetto, G.E., Gonzales, R., Subirade, M. (2005). Formation of soy protein isolate cold-set gels protein and salt effects. Journal of Food Science, 70, 67-73. 

For comparison with these data, the types of salt effects that have been observed for model protein and polypeptide systems that achieve true thermodynamic equilibrium in solution can be summarized into three classes ... 

Melander, W. and Horvath, C. 1977. Salt effects on hydrophobic interactions in precipitation and chromatography of proteins An interpretation of the lyotropic series. Arch. Biochem. Biophys. 183 200-215. 

Recent structural comparisons between enzymes from mesophiles and thermo-philes have validated numerous protein-stablizing effects, including hydrophobic interactions, packing efficiency, salt bridges, hydrogen bonds, loop stabilization... 

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... 

The differences between the gel structures are striking and the addition of salt gave rise to a coarse aggregated gel structure. Similar salt effects have been observed for other protein gels such as soy protein and serum albumin gels. 

For the process in vivo flavodoxin is the external electron donor. For isolated nitrogenase, dithionite 8204 is traditionally used as electron donor. If dithionite is the external donor, the oxidized iron protein with 2MgADP (after the electron is transferred) then dissociates from the MoFe protein. This dissociation, which initially seemed necessary for enzyme function, is, however, apparently a result of the salt effect of dithionite, the concentration of which for effective electron transfer must be sufficiently large. 

Aimar et al. [19] noted that in the UF of complex liquids, such as cheese whey, which contains proteins, salts and casein fragments, concentration polarization, and adsorption and cake formation play a role in flux behavior during crossflow filtration. They may induce osmotic pressure in the retentate side since the chemical potential of the solute-rich polarized layer is lower than that of the permeate, and therefore at equilibrium, a positive osmotic pressure develops in the retentate to equal that of the permeate. The smaller the solute, the greater is its contribution to the osmotic pressure of the liquid, so that in milk, lactose and the minerals have the biggest contribution to osmotic pressure. In skim milk or whey, the osmotic pressure is around 7 bar (700 kPa) and this must be exceeded in RO to commence permeation in UF, only the proteins contribute to the osmotic pressure, which increases exponentially with protein concentration [56]. In any case, a TMP greater than the osmotic pressure is required for solvent to flow from the retentate side to the permeate side. This leads to the reduction in the effectiveness of applied TMP as driving force to permeation. 

The Poisson-Boltzmann model is the. other major model for protein electrostatics, which has been widely used, especially because of its ability to treat salt effects. In applications, where the protein is not highly charged the Boltzmann term in Eqn. (15) can be linearized since e< >/kT < 1, giving... 

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