Globular proteins, denaturation

The tertiary structure of a globular protein is determined by its amino acid sequence. The most important proof of this came from experiments showing that de-naturation of some proteins is reversible. Certain globular proteins denatured by heat, extremes of pH, or denaturing reagents will regain their native structure and their biological activity if returned to conditions in which the native conformation is stable. This process is called renaturation. 

Hydrogen bonds and intramolecular interactions (electrostatic, van der Waals) stabilize the native structure of the protein in a co-operative manner. Upon denaturation, the co-operative effect is lost, resulting in tmfolding of the molecule and exposure of the inner hydrophobic core to the hydrophilic aqueous environment. For small globular proteins, denaturation is an almost all-or-none... 

Protein tertiar-y structure is also influenced by the environment. In water a globular- protein usually adopts a shape that places its hydrophobic groups toward the interior, with its polar- groups on the surface, where they are solvated by water molecules. About 65% of the mass of most cells is water, and the proteins present in cells are said to be in their native state—the tertiary structure in which they express their biological activity. When the tertiar-y structure of a protein is disrupted by adding substances that cause the protein chain to unfold, the protein becomes denatured and loses most, if not all, of its activity. Evidence that supports the view that the tertiary structure is dictated by the primary structure includes experiments in which proteins are denatured and allowed to stand, whereupon they are observed to spontaneously readopt then native-state conformation with full recovery of biological activity. 

Because the tertiary structure of a globular protein is delicately held together by weak intramolecular attractions, a modest change in temperature or pH is often enough to disrupt that structure and cause the protein to become denatured. Denaturation occurs under such mild conditions that the primary structure remains intact but the tertiary structure unfolds from a specific globular shape to a randomly looped chain (Figure 26.7). 

Figure 26.7 A representation of protein denaturation. A globular protein loses its specific three-dimensional shape and becomes randomly looped.

There is a continuing interest to improve and extend the fimctional properties range of dairy proteins to provide both health benefits and their characteristic physical behaviors under different temperature, moisture, and pH conditions so that they may be included in foods that ordinarily do not contain them. One such research area is the extrusion texturization of whey proteins, which have resulted in dairy proteins with new characteristics imparted by a controlled texturization process, depending on the application desired (Hale et al., 2002 Manoi and Rizvi, 2008 Onwulata, 2009 Onwulata et al., 1998). Protein texturization is a two-step process that involves, first, the unfolding of the globular structure (denaturation) and, second, the alignments of the partially unfolded structures in the direction of mass flow in the extruder. The surface characteristics are imparted at the extruder die as the molten mass exits (Onwulata et al., 2003a). 

Le Bon, C., Nicolai, T., and Durand, D., Kinetics of Aggregation and gelation of globular proteins after heat-induced denaturation, Macromolecules, 32,6120, 1999. 

Salting-out salts (Na2S04, NaCl, MgS04) Surface tension increase Weak binding Stabilize globular proteins and precipitants of native and denatured proteins... 

E. Zerovnik, R. Jerala, L. Rroon-Zitko, R. H. Pain, and V. Turk, Intermediates in denaturation of a small globular protein, recombinant human Stefin B, J. Biol. Chem, 13, 9041 (1992). 

The NMR spectrum given by a globular protein with a well-defined tertiary structure differs from that of the same protein under denaturing conditions in two respects. First, the reduction in mobility of residues when the protein folds into a stable tertiary structure produces a broadening of resonances. Second, alterations in resonances caused by chemical shifts arise due to the stable placement of specific protons in unique chemical environments which leads to the appearance of resonances in new positions. 

Soluble proteins have a more complex structure than the fibrous, completely insoluble structural proteins. The shape of soluble proteins is more or less spherical (globular). In their biologically active form, globular proteins have a defined spatial structure (the native conformation). If this structure is destroyed (denaturation see p. 74), not only does the biological effect disappear, but the protein also usually precipitates in insoluble form. This happens, for example, when eggs are boiled the proteins dissolved in the egg white are denatured by the heat and produce the solid egg white. 

Chemical reactions Polymerization of casein and whey proteins are due to some kind of chemical reactions. The different proteins as found in the supernatant of milk after precipitation at pH 4.6 are collectively called whey proteins. These globular proteins are more water soluble than caseins and are subject to heat dena-turation. Denaturation increases their water-binding capacity. The principal fractions are P-lactoglobulin, a-lactalbumin, bovine serum albumin (BSA), and immunoglobulins (Ig). 

Durand, D., Gimel, J.Ch., Nicolai, T. (2002). Aggregation, gelation and phase separation of heat denatured globular proteins. PhysicaA, 304,253-265. 

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