Water interactions with proteins

This chapter does not purport to be the final word on water and fat absorption of plant proteins. Rather, it is designed to summarize information on the mechanism of protein interaction with water and fat, to pull together the various terms used to describe and methods used to assess water and fat absorption, and to encourage a more uniform and quantitative approach to the study of protein functionality and performance in food. The major protein products to be examined in this review are of soy origin other products will be reviewed briefly in comparisons with soy products. 

The pH has a great influence on the ionisation of polar functional groups in the protein-bound amino acids, and thus on the abihty of proteins to interact with water (bind water). Ionised functional groups of proteins interact with water in a similar way to salt ions. The ionised basic side chains of lysine and histidine bind about four molecules of water by hydrogen bonds, the acidic side chains of glutamate and aspartate bind about six water molecules, while the neutral carboxyl groups of amino acids (interaction of dipole-dipole type) bind two water molecules of as well as the polar non-ionised side chains of serine and other amino acids. 

In addition to studying the interaction with antifreeze protein with various ice surfaces we have also performed several simulations of these proteins in boxes of water. This was done for two main reasons. The first was to investigate the behavior and stability of antifreeze proteins in water and secondly to study protein interactions with water. In this section we summarize the molecular dynamics simulations of antifreeze proteins in water. 

Among the common amino acids, eleven have side chains that contain polar functional groups that can form hydrogen bonds, such as —OH, —NH2, and — CO2 H. These hydrophilic amino acids are commonly found on the outside of a protein, where their interactions with water molecules increase the solubility of the protein. The other nine amino acids have nonpolar hydrophobic side chains containing mostly carbon and hydrogen atoms. These amino acids are often tucked into the inside of a protein, away from the aqueous environment of the cell. 

The intracellular and plasma membranes have a complex structure. The main components of a membrane are lipids (or phospholipids) and different proteins. Lipids are fatlike substances representing the esters of one di- or trivalent alcohol and two aliphatic fatty acid molecules (with 14 to 24 carbon atoms). In phospholipids, phosphoric acid residues, -0-P0(0 )-O-, are located close to the ester links, -C0-0-. The lipid or phospholipid molecules have the form of a compact polar head (the ester and phosphate groups) and two parallel, long nonpolar tails (the hydrocarbon chains of the fatty acids). The polar head is hydrophihc and readily interacts with water the hydrocarbon tails to the... 

Consider what happens when a nonoptimal ligand binds to the protein. The binding of this modified ligand is much weaker not because it s not the right size to fit into the protein-binding site, but because the complementary group on the protein loses a favorable interaction with water that is not replaced by an equally favorable interaction with the ligand (Fig. 2-6). 

To move through the membrane (change sides or transverse diffusion), a molecule must be able to pass through the hydrophobic portion of the lipid bilayer. For ions and proteins, this means that they must lose their interactions with water (desolvation). Because this is extremely difficult, ions and proteins do not move through membranes by themselves. Small molecules such as C02, NH3 (but not NH ). and water can diffuse through membranes however, most other small molecules pass through the lipid bilayer very slowly, if at all. This permeability barrier means that cells must develop mechanisms to move molecules from one side of the membrane to the other. 

The amino acids can be classified as either hydrophobic or hydrophilic, depending on the ease ith which their side chains interact with water. In general, proteins fold so that amino adds ith hydrophobic side chains are in the interior of the molecule where they are protected from water and those with hydrophilic side chains are on the surface. 

Water absorption or hydration is considered by some as the first and the critical step in imparting desired functional properties to proteins. Most additives are in dehydrated form the interaction with water is important to properties such as hydration, swelling, solubility, viscosity, and gelation. Protein has been reported to be primarily responsible for water absorption,... 

Symons, M.C. (2000) Spectroscopy of aqueous solutions protein and DNA interactions with water. Cell. Mol. Life Sci. 57,... 

Figure 12-5 (A) Stereoscopic view of the structure of the catalytic site of phosphorylase b in the inhibited T-state with the inhibitor nojirimycin tetra-zole bound into the active site. Inorganic phosphate (P ) as well as the coenzyme pyridoxal 5 -phosphate (PLP) are also shown. (B) Details of interactions of the inhibitor, P , and PLP with the protein and with water molecules (small circles). This is a weak-binding state but the P has displaced the negatively charged side chain carboxylate of Asp 283 (visible at the lower right in A).

In a mixed solvent system a macromolecule may display an overall preferential interaction for one of the solvent components, but this does not eliminate interactions with the other solvent component as well. For example, in the water-2-chloroethanol system, particular regions of the protein molecule, such as ionized side chains, must be interacting with water molecules. Therefore, the extent of preferential interaction observed must be related to the absolute interactions of the protein with the solvent components. In fact, it can be shown (40) that ... 

In the analysis of fibrous protein structures little mention was made of the importance of their interaction with water in determining the final folded structures of the proteins. This is because most of the side chains in fibrous proteins are exposed to water except when they interact with each other to form multimolecular aggregates. Then the relative affinity between other similar side chains and water becomes a major issue. In the case of globular proteins the interaction of amino acid side chains with water is a major issue from the start because globular proteins have many of their amino acid side chains buried in the interior of their folded structures. Hence, in our analysis of the structure of globular proteins we must be aware of the structural considerations that are important in the determination of fibrous proteins but also of additional considerations, first raised in chapter 1, that relate to the interaction of the amino acid side chains with water. 

The structures of the various lipoproteins appear to be similar (figs. 20.11 and 20.12). Each of the lipoprotein classes contains a neutral lipid core composed of triacylglycerol and/or cholesteryl ester. Around this core is a coat of protein, phospholipid, and cholesterol, with the polar portions oriented toward the surface of the lipoprotein and the hydro-phobic parts associated with the neutral lipid core. The hydrophilic surface interacts with water in plasma, promoting the solubility of the lipoprotein. 

We can make some generalizations about how proteins fold. For example, it is a stabilizing feature to get hydrogen-bonding portions of the chain in close proximity. Proteins typically fold with nonpolar side chains on the interior of the protein, away from water, and polar side chains on the outside of the protein, where they can interact with water molecules. In spite of these (gross) generalizations, however, the problem of how and why polypeptide chains fold into functional proteins remains one of the fundamental unsolved problems in physical biochemistry. 

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