Stabilization, compact protein structures

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 following example may serve as an indication of the contribution from hydro-phobic interaction to the stabilization of a compact protein structure. 

The folding of proteins into their characteristic three-dimensional shape is governed primarily by noncovalent interactions. Hydrogen bonding governs the formation of a helices and [) sheets and bends, while hydrophobic effects tend to drive the association of nonpolar side chains. Hydrophobicity also helps to stabilize the overall compact native structure of a protein over its extended conformation in the denatured state, because of the release of water from the chain s hydration sheath as the protein... 

The stability of proteins toward covalent degradation pathways can often depend on the protein s folded state. In each pathway, solvent accessibility and varying degrees of structural freedom of the peptide backbone and/or side chains around the labile residue are required for reactions to take place. Accordingly, stabilization of the protein s folded state (i.e., its compact structure) that minimizes solvent accessibility can lower the reaction rate of some covalent protein modifications, extending the shelf life of the protein product. Therefore, the selection of formulation excipients depends on their direct and indirect influence on the rates of covalent protein degradation. 

The unique three-dimensional structure of each polypeptide is determined by its amino acid sequence. Interactions between the amino acid side chains guide the folding of the polypeptide to form a compact structure. Four types of interactions cooperate in stabilizing the tertiary structures of globular proteins. 

Some foods require an adjustment of pH to enhance, control, or mask taste or flavor. pH can affect emulsification since protein solubility and charge repulsion can be reduced at isoelectric pH. However, the cohesiveness of protein films tends to be maximal near the isoelectric pH (Mangino, 1994). Near the isoelectric pH, proteins are able to form close-packed dense films, thus increasing the protein concentration at an interface. Waniska and Kinsella (1988) demonstrated that at isoelectric pH, electrostatic repulsion was minimized, allowing hydrophobic residues to stabilize a compact tertiary structure of 3-lg. Klemaszewski et al. (1992) reported that the coalescence stability of emulsion stabilized by P-lg was dependent on pH. However, the coalescence stabilities of a-la- or SCN-stabilized emulsions were unaffected by pH. 

Proteins that are not soluble at their isoelectric point cannot readily participate in emulsion formation. Thus proteins such as casein and soy are not suitable for emulsified products with a pH near 4.6. For proteins that are soluble at their isoelectric points, this may be the pH of maximal protein adsorption [12]. Near the isoelectric pH, electrostatic repulsion is minimized, allowing hydrophobic residues to stabilize a more compact tertiary structure. There are data to support that for some proteins, absorption is increased at pH values on either side of the isoelectric point. If enough work can be expended to obtain protein adsorption, charge repulsion due to the residual charge increases emulsion stability. 

Some of the interactions that determine the three-dimensional structure of a protein molecule support a compact conformation, whereas others tend to expand the molecule. In aqueous solution hydrophobic parts of the protein are buried as much as possible in the interior of the molecule but in the adsorbed state the hydrophobic residues may be exposed to the sorbent surface, still shielded from water. Therefore, an expanded structure will be promoted upon adsorption if the compact structure in solution is stabilized by intramolecular hydrophobic bonding. More precisely, whether or not adsorbing protein molecules change their structure depends on the contribution from intramolecular hydrophobic bonding, relative to those from other interactions, to the overall stabilization of the structure in solution. In reference ( ) such an analysis of the structure determining factors has been made for HPA and RNase. It leads to the conclusion that HPA, more than RNase, is able to adapt its structure at sorbent surfaces. 

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