Protein , denaturation shape

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

A number of different molecular mechanisms can underpin the loss of biological activity of any protein. These include both covalent and non-covalent modification of the protein molecule, as summarized in Table 6.5. Protein denaturation, for example, entails a partial or complete alteration of the protein s three-dimensional shape. This is underlined by the disruption of the intramolecular forces that stabilize a protein s native conformation, namely hydrogen bonding, ionic attractions and hydrophobic interactions (Chapter 2). Covalent modifications of protein structure that can adversely affect its biological activity are summarized below. 

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. 

The pH of a solution can affect lipase activity in a number of ways. Like all proteins, enzymes have a tertiary stmcture that is sensitive to pH. In general, denaturation of enzymes occurs at extreme low and high pH values. At extremes of pH, the tertiary structure of the protein may be disrupted and the protein denatured. Many proteins aggregate on pH-induced denaturation and this behavior can be observed by visual inspection. If the activity of an enzyme is plotted against the pH, a bell-shaped curve usually results, with either a sharp or broad pH optimum. 

What happens when the three-dimensional structure of a protein is disrupted Think of the difference between the consistency of a raw egg white and that of a hard-boiled egg. When the forces holding a polypeptide chain in its three-dimensional shape are broken, the protein is unfolded in a process called denaturation. High temperatures can denature proteins, which is why cooking foods that contain proteins results in the proteins denaturation. Denatured proteins form the solid white of a hard-boiled egg. In addition, proteins can be denatured by extremes in pH, mechanical agitation, and chemical treatments. When egg whites are beaten, the proteins are denatured, as shown in Figure 19.4. Because the folded shape of a protein is essential for its function, denaturation of a protein results in loss of its function. This is one reason why organisms can live only in a narrow temperature and pH range. 

Precipitation Simple and convenient operation high yield Poor particle size control irregular-shaped particles broad size distribution protein denaturation difficult to handle multicomponents... 

Rehydration will usually not lead to recovery of the initial product, but to a different product. Drying creates irreversible transformations such as protein denaturation (insoluble), modified aroma and color, loss of firmness and shape. In order to comparethe rehydration capacities, different criteria and standards, like temperature or stirring, have been defined according to product specificity and final use (Lewicki, 1998 Pisecky, 1997). 

The coil-globule transition has attracted much attention in polymer science, initially because it was believed to be a model for the denaturing of proteins. Denaturation of proteins occurs when they are heated or subjected to strong solvents. The shape of the molecules then changes dramatically. In their compact form, many biopolymers such as proteins and DNA do indeed adopt a globular conformation (they have to, to be able to fit into small volumes such as a cell nucleus however, sometimes leaking out occurs as shown in Fig. 2.14). It was thought that... 

Figure 3 Comparison of 9.5 and 94.4 GHz EPR spectra of iso-1-cythochrome c from Saccharomyces cerevisiae labeled at native cysteine 102 with MTSSL illustrates an enhanced sensitivity of W-band EPR line shape to changes in local protein dynamics upon protein denaturing. The spectra at each of the frequencies were taken at room temperature and are normalized by the value of the double integral. Protein was denatured by addition of 2 M of guanidine hydrochloride (Pierce, Illinois). Protein concentration was 0.2 mM in a HEPES buffer at pH 6.5 (Smirnov et al, in preparation)...

Protein tertiary structure is also influenced by the environment In water a globu lar 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 m cells are said to be m their native state—the tertiary structure m which they express their biological activ ity When the tertiary 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 m which proteins are denatured and allowed to stand whereupon they are observed to spontaneously readopt their native state confer matron with full recovery of biological activity... 

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