Globular proteins hydration

Hanafusa [1.36] showed with this method, how the amount of unfrozen water in a 0.57 % solution of ovalbumin reaches practically zero at -20 °C, if 0.01 M sucrose is added (Fig. 1.51). For globular proteins Hanafusa described the freezing process as follows between 0 °C and -20 °C, water molecules from the multilayer hydrate shell are decomposed. Be-... 

A recent report by Gekko and Noguchi (1979) confirmed that of 14 globular proteins studied, all showed positive compressibilities. The results revealed that a large negative compression of the void compensates a positive compression due to the hydration of the proteins, resulting in a small positive value for /3S. 

Kinsella (16), in his recent review on texturized proteins, described the texturization process as follows the globular proteins (glycinins) in the aleurone granules become hydrated within the extruder barrel, are gradually unravelled, and are stretched by the shearing action of the rotating screw flites. 

Both internal structure and overall size and shape of proteins vary enormously. Globular proteins vary considerably in the tightness of packing and the amount of internal water of hydration. However, a density of 1.4 g cm-3 is typical. 

Future work will naturally extend to study complex systems, such as hydration dynamics around different secondary-structure globular proteins at interfaces of protein-DNA, RNA, or protein complexes and at the active sites in enzymes. On the theoretical side, significant efforts are needed to solve the serious discrepancies of total solvation energy, ultrafast inertial motion, as well as protein flexibility and induced solvation. 

Kharakoz, D.P., Sarvazyan, A.P. 1993. Hydrational and intrinsic compressibilities of globular-proteins. Biopolymers 33, 11-26. 

Amino acids of the general form, 1, are the monomeric molecules which are condensed to form the polypeptide chains of the fibrous and globular proteins. The naturally occurring molecules are the L-enantiomers, shown in 1 for chemical formulae see Fig. 19.1. D-amino acids can be synthesized and the individual L- or D-amino acids or the D,Lrracemates can be crystallized. All the common amino acids have been studied by neutron or X-ray crystal structure analysis (see Thble 14.1), in the anhydrous or hydrate forms, as hydrochlorides or hydrochloride hydrates. 

The most important side-chains for hydration are Asp and Glu which bind, on average, 2 water molecules per carboxylate group (see Thble 23.4 b). Whereas Asp and Glu are preferentially located at the outside of the globular proteins and therefore in contact with solvent, the other amino acids are more buried in the interior. They hydrogen-bond not only with internal water molecules but also with protein main-chain and side-chain atoms. Consequently their functional groups are less accessible for water molecules, with only 0.34 water molecules bound per hydrogen-bond site on average... 

Figure 3.3 illustrates the idea of excluded volume. It shows two protein molecules as two adjacent spheres of the same radius R. Because molecules are not penetrable by each other, the volume of a solution occupied by a macromolecule is not accessible to other macromolecules. A minimal distance between two adjacent spherical molecules of a globular protein equals the sum of their radii, or the diameter of one of them. This means that around each protein molecule there is an excluded volume U), which is 8-fold larger than that of protein molecule itself and is not accessible for centres of other protein molecules. The excluded volume is still larger for non-spherical macromolecules and depends on the flexibility of the macromolecular chain, and its configurational, rotational, vibrational properties and hydration (Tanford 1961). 

Hydrophobic effects are thus of practical interest. If we accept the goal of a simple, physical, molecularly valid explanation, then hydrophobic effects have also proved conceptually subtle. The reason is that hydrophobic phenomena are not tied directly to a simple dominating interaction as is the case for hydrophilic hydration of Na+, as an example. Instead hydrophobic effects are built up more collectively. In concert with this indirectness, hydrophobic effects are viewed as entropic interactions and exhibit counterintuitive temperature dependencies. An example is the cold denaturation of globular proteins. Though it is believed that hydrophobic effects stabilize compact protein structures and proteins denature when heated sufficiently, it now appears common for protein structures to unfold upon appropriate cooling. This entropic character of hydrophobic effects makes them more fascinating and more difficult. 

The mass rado h is commonly used in describing protein hydration. Hydration, however, should depend more closely on protein surface than on volume or mass. Most of the data described in this review are for small globular proteins, for which weight and surface-based measures should be similar. Comparisons of proteins of much different size may need to take into account surface area, compactness, and domain size. 

Claesson et al. (1989) measured the forces between hydrophobized mica surfaces, with and without adsorbed insulin. They concluded that the range of the hydration force for this globular protein is less than 10 A and that the hydration layer is not more than one or two water molecules thick. 

From the difference between the free energy and enthalpy of hydration. The two small globular proteins, lysozyme and ribonuclease, perhaps can be considered compatible at this level of comparison. 

This behavior can be seen as complementary to another aspect of protein folding the withdrawal of hydrophobic side chains from solvent. The latter minimizes perturbation by burying those portions of the polypeptide for which water is the poorest solvent. The former minimizes perturbation of solvent by what remains exposed. Not all biological macromolecules show so small an effect. Nucleic acids require for their hydration about twice the amount of water required by globular proteins (for heat capacity measurements comparing protein and tRNA, see Rupley and Siemankowski, 1986). It may be signihcant that DNA, with an extensive hydration shell, undergoes facile hydration-dependent conformational transitions, which are not found for proteins. 

T Tydrogels are a class of synthetic polymers of diverse chemical nature distinguished from other polymers by the capacity to imbibe relatively large amounts of water in their structure. The water content of these materials varies from about 30 to 90 wt % depending on both the chemical nature and physical structure of the polymer. Many natural or biocompatible polymers are also highly hydrated, e.g. 30-50 wt % water is bound by globular proteins (I). Partly for this reason, hydrogels... 

In the field of biology, the effects of hydration on equilibrium protein structure and dynamics are fundamental to the relationship between structure and biological function [21-27]. In particular, the assessment of perturbation of liquid water structure and dynamics by hydrophilic and hydrophobic molecular surfaces is fundamental to the quantitative understanding of the stability and enzymatic activity of globular proteins and functions of membranes. Examples of structures that impose spatial restriction on water molecules include polymer gels, micelles, vesicles, and microemulsions. In the last three cases since the hydrophobic effect is the primary cause for the self-organization of these structures, obviously the configuration of water molecules near the hydrophilic-hydrophobic interfaces is of considerable relevance. 

Arakawa, T., Bhat, R., and Timasheff, S.N. Preferential hydration does not always stabilize the native structure of globular proteins. Biochemistry, 29, 1924, 1990. 

One source of information, hydrodynamic measurements, has recently been reviewed by Squire and Himmel ( ) Their analysis suggests large amounts of water associated with globular proteins, but even their restricted data set shows large variations between the amount of hydration calculated from sedimentation or diffusion results. Because the cube of the friction factor enters into the... 

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