Factors Affecting Protein Denaturation

Figure 4. Two-state equilibrium model of the denaturation process.

irreversible denaturation are both initiated by reversible conformational change, eg, by unfolding. This initial conformational change is rate-limiting (41) and, in the case of irreversible denaturation, is followed by such secondary processes as aggregation or covalent modification (eg, proteolysis) resulting in a change in primary structure. If irreversible denaturation takes place, analysis of protein denaturation using the two-state model is precluded, because the two-state equilibrium is accompanied by an irreversible transition from the D state to an alternate structure(s) that cannot be readily characterized. Analysis of protein denaturation using the two-state model, therefore, requires that three criteria be met: (1) the N -> D transition must be thermodynamically reversible; (2) an experimental technique must be used such that the transition be detected in the presence of the perturbing influence or dénaturant; and (3) a reference conformational state, usually taken to be the N state, must be identified such that the stabilities of different proteins are compared. The limiting case for thermodynamic reversibility is an infinitely dilute solution. However, if thermodynamic reversibility can be assumed for the protein system in question, good approximation of the thermodynamic parameters AG0, AH°, AS", and zlC° can normally be obtained from

where z)G°pp is the apparent change in standard free energy, AHlpp is the apparent change in enthalpy, AS°pp is the apparent change in entropy, ACpapp is the apparent change in heat capacity, kD is the equilibrium or denaturation rate constant, R is the gas constant, and Tis absolute temperature (Kelvin). Protein denaturation is normally accompanied by increases in (1) enthalpy, indicating that the N state possesses a lower free energy than the D state; (2) entropy, associated with the disorder that results from unfolding of the polypeptide chain; and (3) heat capacity,

As previously stated, many food processing operations to which protein-containing foods are subjected, can result in protein denaturation. Such factors as temperature, pH, pressure, shear, irradiation, and the presence of salts and oxidizing/reducing agents can be used by the processor to produce desirable and high-quality products. However, these factors may also serve as processing hazards that must be avoided in order to maintain native protein structure and associated functional properties. In the broadest sense, processes that result in protein denaturation can be divided into two major categories: physical and chemical.

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